makemakemakemake to Update Archive Filesmakemake
make
MAKEFILES
MAKEFILE_LIST
make Reads a Makefile
override Directive
foreach Function
call Function
value Function
eval Function
origin Function
flavor Function
shell Function
make
make to Update Archive Files
make
makeThis file documents the GNU make utility, which determines
automatically which pieces of a large program need to be recompiled,
and issues the commands to recompile them.
This is Edition 0.70, last updated 1 April 2006,
of The GNU Make Manual, for GNU make version 3.81.
Copyright © 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”(a) The FSF's Back-Cover Text is: “You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.”
--- The Detailed Node Listing ---
Overview of make
An Introduction to Makefiles
Writing Makefiles
Writing Rules
Using Wildcard Characters in File Names
Searching Directories for Prerequisites
Static Pattern Rules
Writing the Commands in Rules
Command Syntax
Command Execution
Recursive Use of make
How to Use Variables
Advanced Features for Reference to Variables
Conditional Parts of Makefiles
Functions for Transforming Text
How to Run make
Using Implicit Rules
Defining and Redefining Pattern Rules
Using make to Update Archive Files
Implicit Rule for Archive Member Targets
makeThe make utility automatically determines which pieces of a large
program need to be recompiled, and issues commands to recompile them.
This manual describes GNU make, which was implemented by Richard
Stallman and Roland McGrath. Development since Version 3.76 has been
handled by Paul D. Smith.
GNU make conforms to section 6.2 of IEEE Standard
1003.2-1992 (POSIX.2).
Our examples show C programs, since they are most common, but you can use
make with any programming language whose compiler can be run with a
shell command. Indeed, make is not limited to programs. You can
use it to describe any task where some files must be updated automatically
from others whenever the others change.
To prepare to use make, you must write a file called
the makefile that describes the relationships among files
in your program and provides commands for updating each file.
In a program, typically, the executable file is updated from object
files, which are in turn made by compiling source files.
Once a suitable makefile exists, each time you change some source files, this simple shell command:
make
suffices to perform all necessary recompilations. The make program
uses the makefile data base and the last-modification times of the files to
decide which of the files need to be updated. For each of those files, it
issues the commands recorded in the data base.
You can provide command line arguments to make to control which
files should be recompiled, or how. See How to Run make.
If you are new to make, or are looking for a general
introduction, read the first few sections of each chapter, skipping the
later sections. In each chapter, the first few sections contain
introductory or general information and the later sections contain
specialized or technical information.
The exception is the second chapter, An Introduction to Makefiles, all of which is introductory.
If you are familiar with other make programs, see Features of GNU make, which lists the enhancements GNU
make has, and Incompatibilities and Missing Features, which explains the few things GNU make lacks that
others have.
For a quick summary, see Options Summary, Quick Reference, and Special Targets.
If you have problems with GNU make or think you've found a bug,
please report it to the developers; we cannot promise to do anything but
we might well want to fix it.
Before reporting a bug, make sure you've actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation!
Before reporting a bug or trying to fix it yourself, try to isolate it
to the smallest possible makefile that reproduces the problem. Then
send us the makefile and the exact results make gave you,
including any error or warning messages. Please don't paraphrase
these messages: it's best to cut and paste them into your report.
When generating this small makefile, be sure to not use any non-free
or unusual tools in your commands: you can almost always emulate what
such a tool would do with simple shell commands. Finally, be sure to
explain what you expected to occur; this will help us decide whether
the problem was really in the documentation.
Once you have a precise problem you can report it in one of two ways. Either send electronic mail to:
bug-make@gnu.org
or use our Web-based project management tool, at:
http://savannah.gnu.org/projects/make/
In addition to the information above, please be careful to include the
version number of make you are using. You can get this
information with the command `make --version'. Be sure also to
include the type of machine and operating system you are using. One
way to obtain this information is by looking at the final lines of
output from the command `make --help'.
You need a file called a makefile to tell make what to do.
Most often, the makefile tells make how to compile and link a
program.
In this chapter, we will discuss a simple makefile that describes how to
compile and link a text editor which consists of eight C source files
and three header files. The makefile can also tell make how to
run miscellaneous commands when explicitly asked (for example, to remove
certain files as a clean-up operation). To see a more complex example
of a makefile, see Complex Makefile.
When make recompiles the editor, each changed C source file
must be recompiled. If a header file has changed, each C source file
that includes the header file must be recompiled to be safe. Each
compilation produces an object file corresponding to the source file.
Finally, if any source file has been recompiled, all the object files,
whether newly made or saved from previous compilations, must be linked
together to produce the new executable editor.
A simple makefile consists of “rules” with the following shape:
target ... : prerequisites ...
command
...
...
A target is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as `clean' (see Phony Targets).
A prerequisite is a file that is used as input to create the target. A target often depends on several files.
A command is an action that make carries out.
A rule may have more than one command, each on its own line.
Please note: you need to put a tab character at the beginning of
every command line! This is an obscurity that catches the unwary.
Usually a command is in a rule with prerequisites and serves to create a target file if any of the prerequisites change. However, the rule that specifies commands for the target need not have prerequisites. For example, the rule containing the delete command associated with the target `clean' does not have prerequisites.
A rule, then, explains how and when to remake certain files
which are the targets of the particular rule. make carries out
the commands on the prerequisites to create or update the target. A
rule can also explain how and when to carry out an action.
See Writing Rules.
A makefile may contain other text besides rules, but a simple makefile need only contain rules. Rules may look somewhat more complicated than shown in this template, but all fit the pattern more or less.
Here is a straightforward makefile that describes the way an
executable file called edit depends on eight object files
which, in turn, depend on eight C source and three header files.
In this example, all the C files include defs.h, but only those defining editing commands include command.h, and only low level files that change the editor buffer include buffer.h.
edit : main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
cc -o edit main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
main.o : main.c defs.h
cc -c main.c
kbd.o : kbd.c defs.h command.h
cc -c kbd.c
command.o : command.c defs.h command.h
cc -c command.c
display.o : display.c defs.h buffer.h
cc -c display.c
insert.o : insert.c defs.h buffer.h
cc -c insert.c
search.o : search.c defs.h buffer.h
cc -c search.c
files.o : files.c defs.h buffer.h command.h
cc -c files.c
utils.o : utils.c defs.h
cc -c utils.c
clean :
rm edit main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
We split each long line into two lines using backslash-newline; this is like using one long line, but is easier to read. To use this makefile to create the executable file called edit, type:
make
To use this makefile to delete the executable file and all the object files from the directory, type:
make clean
In the example makefile, the targets include the executable file `edit', and the object files `main.o' and `kbd.o'. The prerequisites are files such as `main.c' and `defs.h'. In fact, each `.o' file is both a target and a prerequisite. Commands include `cc -c main.c' and `cc -c kbd.c'.
When a target is a file, it needs to be recompiled or relinked if any of its prerequisites change. In addition, any prerequisites that are themselves automatically generated should be updated first. In this example, edit depends on each of the eight object files; the object file main.o depends on the source file main.c and on the header file defs.h.
A shell command follows each line that contains a target and
prerequisites. These shell commands say how to update the target file.
A tab character must come at the beginning of every command line to
distinguish command lines from other lines in the makefile. (Bear in
mind that make does not know anything about how the commands
work. It is up to you to supply commands that will update the target
file properly. All make does is execute the commands in the rule
you have specified when the target file needs to be updated.)
The target `clean' is not a file, but merely the name of an
action. Since you
normally
do not want to carry out the actions in this rule, `clean' is not a prerequisite of any other rule.
Consequently, make never does anything with it unless you tell
it specifically. Note that this rule not only is not a prerequisite, it
also does not have any prerequisites, so the only purpose of the rule
is to run the specified commands. Targets that do not refer to files
but are just actions are called phony targets. See Phony Targets, for information about this kind of target. See Errors in Commands, to see how to cause make to ignore errors
from rm or any other command.
make Processes a Makefile
By default, make starts with the first target (not targets whose
names start with `.'). This is called the default goal.
(Goals are the targets that make strives ultimately to
update. You can override this behavior using the command line
(see Arguments to Specify the Goals) or with the
.DEFAULT_GOAL special variable (see Other Special Variables).
In the simple example of the previous section, the default goal is to
update the executable program edit; therefore, we put that rule
first.
Thus, when you give the command:
make
make reads the makefile in the current directory and begins by
processing the first rule. In the example, this rule is for relinking
edit; but before make can fully process this rule, it
must process the rules for the files that edit depends on,
which in this case are the object files. Each of these files is
processed according to its own rule. These rules say to update each
`.o' file by compiling its source file. The recompilation must
be done if the source file, or any of the header files named as
prerequisites, is more recent than the object file, or if the object
file does not exist.
The other rules are processed because their targets appear as
prerequisites of the goal. If some other rule is not depended on by the
goal (or anything it depends on, etc.), that rule is not processed,
unless you tell make to do so (with a command such as
make clean).
Before recompiling an object file, make considers updating its
prerequisites, the source file and header files. This makefile does not
specify anything to be done for them—the `.c' and `.h' files
are not the targets of any rules—so make does nothing for these
files. But make would update automatically generated C programs,
such as those made by Bison or Yacc, by their own rules at this time.
After recompiling whichever object files need it, make decides
whether to relink edit. This must be done if the file
edit does not exist, or if any of the object files are newer than
it. If an object file was just recompiled, it is now newer than
edit, so edit is relinked.
Thus, if we change the file insert.c and run make,
make will compile that file to update insert.o, and then
link edit. If we change the file command.h and run
make, make will recompile the object files kbd.o,
command.o and files.o and then link the file edit.
In our example, we had to list all the object files twice in the rule for edit (repeated here):
edit : main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
cc -o edit main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. Variables allow a text string to be defined once and substituted in multiple places later (see How to Use Variables).
It is standard practice for every makefile to have a variable named
objects, OBJECTS, objs, OBJS, obj,
or OBJ which is a list of all object file names. We would
define such a variable objects with a line like this in the
makefile:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing `$(objects)' (see How to Use Variables).
Here is how the complete simple makefile looks when you use a variable for the object files:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
edit : $(objects)
cc -o edit $(objects)
main.o : main.c defs.h
cc -c main.c
kbd.o : kbd.c defs.h command.h
cc -c kbd.c
command.o : command.c defs.h command.h
cc -c command.c
display.o : display.c defs.h buffer.h
cc -c display.c
insert.o : insert.c defs.h buffer.h
cc -c insert.c
search.o : search.c defs.h buffer.h
cc -c search.c
files.o : files.c defs.h buffer.h command.h
cc -c files.c
utils.o : utils.c defs.h
cc -c utils.c
clean :
rm edit $(objects)
make Deduce the Commands
It is not necessary to spell out the commands for compiling the individual
C source files, because make can figure them out: it has an
implicit rule for updating a `.o' file from a correspondingly
named `.c' file using a `cc -c' command. For example, it will
use the command `cc -c main.c -o main.o' to compile main.c into
main.o. We can therefore omit the commands from the rules for the
object files. See Using Implicit Rules.
When a `.c' file is used automatically in this way, it is also automatically added to the list of prerequisites. We can therefore omit the `.c' files from the prerequisites, provided we omit the commands.
Here is the entire example, with both of these changes, and a variable
objects as suggested above:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
edit : $(objects)
cc -o edit $(objects)
main.o : defs.h
kbd.o : defs.h command.h
command.o : defs.h command.h
display.o : defs.h buffer.h
insert.o : defs.h buffer.h
search.o : defs.h buffer.h
files.o : defs.h buffer.h command.h
utils.o : defs.h
.PHONY : clean
clean :
rm edit $(objects)
This is how we would write the makefile in actual practice. (The complications associated with `clean' are described elsewhere. See Phony Targets, and Errors in Commands.)
Because implicit rules are so convenient, they are important. You will see them used frequently.
When the objects of a makefile are created only by implicit rules, an alternative style of makefile is possible. In this style of makefile, you group entries by their prerequisites instead of by their targets. Here is what one looks like:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
edit : $(objects)
cc -o edit $(objects)
$(objects) : defs.h
kbd.o command.o files.o : command.h
display.o insert.o search.o files.o : buffer.h
Here defs.h is given as a prerequisite of all the object files; command.h and buffer.h are prerequisites of the specific object files listed for them.
Whether this is better is a matter of taste: it is more compact, but some people dislike it because they find it clearer to put all the information about each target in one place.
Compiling a program is not the only thing you might want to write rules for. Makefiles commonly tell how to do a few other things besides compiling a program: for example, how to delete all the object files and executables so that the directory is `clean'.
Here is how we
could write a make rule for cleaning our example editor:
clean:
rm edit $(objects)
In practice, we might want to write the rule in a somewhat more complicated manner to handle unanticipated situations. We would do this:
.PHONY : clean
clean :
-rm edit $(objects)
This prevents make from getting confused by an actual file
called clean and causes it to continue in spite of errors from
rm. (See Phony Targets, and Errors in Commands.)
A rule such as this should not be placed at the beginning of the
makefile, because we do not want it to run by default! Thus, in the
example makefile, we want the rule for edit, which recompiles
the editor, to remain the default goal.
Since clean is not a prerequisite of edit, this rule will not
run at all if we give the command `make' with no arguments. In
order to make the rule run, we have to type `make clean'.
See How to Run make.
The information that tells make how to recompile a system comes from
reading a data base called the makefile.
Makefiles contain five kinds of things: explicit rules, implicit rules, variable definitions, directives, and comments. Rules, variables, and directives are described at length in later chapters.
objects
as a list of all object files (see Variables Make Makefiles Simpler).
make to do something special while
reading the makefile. These include:
#, escape it with a backslash (e.g., \#). Comments may
appear on any line in the makefile, although they are treated
specially in certain situations.
Within a command script (if the line begins with a TAB character) the entire line is passed to the shell, just as with any other line that begins with a TAB. The shell decides how to interpret the text: whether or not this is a comment is up to the shell.
Within a define directive, comments are not ignored during the
definition of the variable, but rather kept intact in the value of the
variable. When the variable is expanded they will either be treated
as make comments or as command script text, depending on the
context in which the variable is evaluated.
By default, when make looks for the makefile, it tries the
following names, in order: GNUmakefile, makefile
and Makefile.
Normally you should call your makefile either makefile or
Makefile. (We recommend Makefile because it appears
prominently near the beginning of a directory listing, right near other
important files such as README.) The first name checked,
GNUmakefile, is not recommended for most makefiles. You should
use this name if you have a makefile that is specific to GNU
make, and will not be understood by other versions of
make. Other make programs look for makefile and
Makefile, but not GNUmakefile.
If make finds none of these names, it does not use any makefile.
Then you must specify a goal with a command argument, and make
will attempt to figure out how to remake it using only its built-in
implicit rules. See Using Implicit Rules.
If you want to use a nonstandard name for your makefile, you can specify
the makefile name with the `-f' or `--file' option. The
arguments `-f name' or `--file=name' tell
make to read the file name as the makefile. If you use
more than one `-f' or `--file' option, you can specify several
makefiles. All the makefiles are effectively concatenated in the order
specified. The default makefile names GNUmakefile,
makefile and Makefile are not checked automatically if you
specify `-f' or `--file'.
The include directive tells make to suspend reading the
current makefile and read one or more other makefiles before continuing.
The directive is a line in the makefile that looks like this:
include filenames...
filenames can contain shell file name patterns. If
filenames is empty, nothing is included and no error is printed.
Extra spaces are allowed and ignored at the beginning of the line, but
a tab is not allowed. (If the line begins with a tab, it will be
considered a command line.) Whitespace is required between
include and the file names, and between file names; extra
whitespace is ignored there and at the end of the directive. A
comment starting with `#' is allowed at the end of the line. If
the file names contain any variable or function references, they are
expanded. See How to Use Variables.
For example, if you have three .mk files, a.mk,
b.mk, and c.mk, and $(bar) expands to
bish bash, then the following expression
include foo *.mk $(bar)
is equivalent to
include foo a.mk b.mk c.mk bish bash
When make processes an include directive, it suspends
reading of the containing makefile and reads from each listed file in
turn. When that is finished, make resumes reading the
makefile in which the directive appears.
One occasion for using include directives is when several programs,
handled by individual makefiles in various directories, need to use a
common set of variable definitions
(see Setting Variables) or pattern rules
(see Defining and Redefining Pattern Rules).
Another such occasion is when you want to generate prerequisites from
source files automatically; the prerequisites can be put in a file that
is included by the main makefile. This practice is generally cleaner
than that of somehow appending the prerequisites to the end of the main
makefile as has been traditionally done with other versions of
make. See Automatic Prerequisites.
If the specified name does not start with a slash, and the file is not
found in the current directory, several other directories are searched.
First, any directories you have specified with the `-I' or
`--include-dir' option are searched
(see Summary of Options).
Then the following directories (if they exist)
are searched, in this order:
prefix/include (normally /usr/local/include
1)
/usr/gnu/include,
/usr/local/include, /usr/include.
If an included makefile cannot be found in any of these directories, a
warning message is generated, but it is not an immediately fatal error;
processing of the makefile containing the include continues.
Once it has finished reading makefiles, make will try to remake
any that are out of date or don't exist.
See How Makefiles Are Remade.
Only after it has tried to find a way to remake a makefile and failed,
will make diagnose the missing makefile as a fatal error.
If you want make to simply ignore a makefile which does not exist
and cannot be remade, with no error message, use the -include
directive instead of include, like this:
-include filenames...
This acts like include in every way except that there is no
error (not even a warning) if any of the filenames do not exist.
For compatibility with some other make implementations,
sinclude is another name for -include.
MAKEFILES
If the environment variable MAKEFILES is defined, make
considers its value as a list of names (separated by whitespace) of
additional makefiles to be read before the others. This works much like
the include directive: various directories are searched for those
files (see Including Other Makefiles). In addition, the
default goal is never taken from one of these makefiles and it is not an
error if the files listed in MAKEFILES are not found.
The main use of MAKEFILES is in communication between recursive
invocations of make (see Recursive Use of make). It usually is not desirable to set the environment
variable before a top-level invocation of make, because it is
usually better not to mess with a makefile from outside. However, if
you are running make without a specific makefile, a makefile in
MAKEFILES can do useful things to help the built-in implicit
rules work better, such as defining search paths (see Directory Search).
Some users are tempted to set MAKEFILES in the environment
automatically on login, and program makefiles to expect this to be done.
This is a very bad idea, because such makefiles will fail to work if run by
anyone else. It is much better to write explicit include directives
in the makefiles. See Including Other Makefiles.
MAKEFILE_LIST
As make reads various makefiles, including any obtained from the
MAKEFILES variable, the command line, the default files, or
from include directives, their names will be automatically
appended to the MAKEFILE_LIST variable. They are added right
before make begins to parse them.
This means that if the first thing a makefile does is examine the last
word in this variable, it will be the name of the current makefile.
Once the current makefile has used include, however, the last
word will be the just-included makefile.
If a makefile named Makefile has this content:
name1 := $(lastword $(MAKEFILE_LIST))
include inc.mk
name2 := $(lastword $(MAKEFILE_LIST))
all:
@echo name1 = $(name1)
@echo name2 = $(name2)
then you would expect to see this output:
name1 = Makefile
name2 = inc.mk
See Text Functions, for more information on the word and
words functions used above. See The Two Flavors of Variables, for more information on simply-expanded (:=)
variable definitions.
GNU make also supports other special variables. Unless
otherwise documented here, these values lose their special properties
if they are set by a makefile or on the command line.
.DEFAULT_GOAL.DEFAULT_GOAL variable allows you to discover the current
default goal, restart the default goal selection algorithm by clearing
its value, or to explicitly set the default goal. The following
example illustrates these cases:
# Query the default goal.
ifeq ($(.DEFAULT_GOAL),)
$(warning no default goal is set)
endif
.PHONY: foo
foo: ; @echo $@
$(warning default goal is $(.DEFAULT_GOAL))
# Reset the default goal.
.DEFAULT_GOAL :=
.PHONY: bar
bar: ; @echo $@
$(warning default goal is $(.DEFAULT_GOAL))
# Set our own.
.DEFAULT_GOAL := foo
This makefile prints:
no default goal is set
default goal is foo
default goal is bar
foo
Note that assigning more than one target name to .DEFAULT_GOAL is
illegal and will result in an error.
MAKE_RESTARTSmake has
restarted (see How Makefiles Are Remade): it
will contain the number of times this instance has restarted. Note
this is not the same as recursion (counted by the MAKELEVEL
variable). You should not set, modify, or export this variable.
.VARIABLES.FEATURESmake. Possible values include:
ar (archive) files using special filename syntax.
See Using make to Update Archive Files.
-L (--check-symlink-times) flag.
See Summary of Options.
.INCLUDE_DIRSmake searches for
included makefiles (see Including Other Makefiles).
Sometimes makefiles can be remade from other files, such as RCS or SCCS
files. If a makefile can be remade from other files, you probably want
make to get an up-to-date version of the makefile to read in.
To this end, after reading in all makefiles, make will consider
each as a goal target and attempt to update it. If a makefile has a
rule which says how to update it (found either in that very makefile or
in another one) or if an implicit rule applies to it (see Using Implicit Rules), it will be updated if necessary. After
all makefiles have been checked, if any have actually been changed,
make starts with a clean slate and reads all the makefiles over
again. (It will also attempt to update each of them over again, but
normally this will not change them again, since they are already up to
date.)
If you know that one or more of your makefiles cannot be remade and you
want to keep make from performing an implicit rule search on
them, perhaps for efficiency reasons, you can use any normal method of
preventing implicit rule lookup to do so. For example, you can write an
explicit rule with the makefile as the target, and an empty command
string (see Using Empty Commands).
If the makefiles specify a double-colon rule to remake a file with
commands but no prerequisites, that file will always be remade
(see Double-Colon). In the case of makefiles, a makefile that has a
double-colon rule with commands but no prerequisites will be remade every
time make is run, and then again after make starts over
and reads the makefiles in again. This would cause an infinite loop:
make would constantly remake the makefile, and never do anything
else. So, to avoid this, make will not attempt to
remake makefiles which are specified as targets of a double-colon rule
with commands but no prerequisites.
If you do not specify any makefiles to be read with `-f' or
`--file' options, make will try the default makefile names;
see What Name to Give Your Makefile. Unlike
makefiles explicitly requested with `-f' or `--file' options,
make is not certain that these makefiles should exist. However,
if a default makefile does not exist but can be created by running
make rules, you probably want the rules to be run so that the
makefile can be used.
Therefore, if none of the default makefiles exists, make will try
to make each of them in the same order in which they are searched for
(see What Name to Give Your Makefile)
until it succeeds in making one, or it runs out of names to try. Note
that it is not an error if make cannot find or make any makefile;
a makefile is not always necessary.
When you use the `-t' or `--touch' option (see Instead of Executing the Commands), you would not want to use an out-of-date makefile to decide which targets to touch. So the `-t' option has no effect on updating makefiles; they are really updated even if `-t' is specified. Likewise, `-q' (or `--question') and `-n' (or `--just-print') do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, `make -f mfile -n foo' will update mfile, read it in, and then print the commands to update foo and its prerequisites without running them. The commands printed for foo will be those specified in the updated contents of mfile.
However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options `-t' and so on do apply to them.
Thus, `make -f mfile -n mfile foo' would read the makefile mfile, print the commands needed to update it without actually running them, and then print the commands needed to update foo without running them. The commands for foo will be those specified by the existing contents of mfile.
Sometimes it is useful to have a makefile that is mostly just like
another makefile. You can often use the `include' directive to
include one in the other, and add more targets or variable definitions.
However, if the two makefiles give different commands for the same
target, make will not let you just do this. But there is another way.
In the containing makefile (the one that wants to include the other),
you can use a match-anything pattern rule to say that to remake any
target that cannot be made from the information in the containing
makefile, make should look in another makefile.
See Pattern Rules, for more information on pattern rules.
For example, if you have a makefile called Makefile that says how to make the target `foo' (and other targets), you can write a makefile called GNUmakefile that contains:
foo:
frobnicate > foo
%: force
@$(MAKE) -f Makefile $@
force: ;
If you say `make foo', make will find GNUmakefile,
read it, and see that to make foo, it needs to run the command
`frobnicate > foo'. If you say `make bar', make will
find no way to make bar in GNUmakefile, so it will use the
commands from the pattern rule: `make -f Makefile bar'. If
Makefile provides a rule for updating bar, make
will apply the rule. And likewise for any other target that
GNUmakefile does not say how to make.
The way this works is that the pattern rule has a pattern of just
`%', so it matches any target whatever. The rule specifies a
prerequisite force, to guarantee that the commands will be run even
if the target file already exists. We give force target empty
commands to prevent make from searching for an implicit rule to
build it—otherwise it would apply the same match-anything rule to
force itself and create a prerequisite loop!
make Reads a Makefile
GNU make does its work in two distinct phases. During the first
phase it reads all the makefiles, included makefiles, etc. and
internalizes all the variables and their values, implicit and explicit
rules, and constructs a dependency graph of all the targets and their
prerequisites. During the second phase, make uses these internal
structures to determine what targets will need to be rebuilt and to
invoke the rules necessary to do so.
It's important to understand this two-phase approach because it has a
direct impact on how variable and function expansion happens; this is
often a source of some confusion when writing makefiles. Here we will
present a summary of the phases in which expansion happens for different
constructs within the makefile. We say that expansion is
immediate if it happens during the first phase: in this case
make will expand any variables or functions in that section of a
construct as the makefile is parsed. We say that expansion is
deferred if expansion is not performed immediately. Expansion of
deferred construct is not performed until either the construct appears
later in an immediate context, or until the second phase.
You may not be familiar with some of these constructs yet. You can reference this section as you become familiar with them, in later chapters.
Variable definitions are parsed as follows:
immediate = deferred
immediate ?= deferred
immediate := immediate
immediate += deferred or immediate
define immediate
deferred
endef
For the append operator, `+=', the right-hand side is considered immediate if the variable was previously set as a simple variable (`:='), and deferred otherwise.
All instances of conditional syntax are parsed immediately, in their
entirety; this includes the ifdef, ifeq, ifndef,
and ifneq forms. Of course this means that automatic variables
cannot be used in conditional statements, as automatic variables are
not set until the command script for that rule is invoked. If you
need to use automatic variables in a conditional you must use
shell conditional syntax, in your command script proper, for these
tests, not make conditionals.
A rule is always expanded the same way, regardless of the form:
immediate : immediate ; deferred
deferred
That is, the target and prerequisite sections are expanded immediately, and the commands used to construct the target are always deferred. This general rule is true for explicit rules, pattern rules, suffix rules, static pattern rules, and simple prerequisite definitions.
In the previous section we learned that GNU make works in two
distinct phases: a read-in phase and a target-update phase
(see How make Reads a Makefile). GNU
make also has the ability to enable a second expansion of the
prerequisites (only) for some or all targets defined in the makefile.
In order for this second expansion to occur, the special target
.SECONDEXPANSION must be defined before the first prerequisite
list that makes use of this feature.
If that special target is defined then in between the two phases mentioned above, right at the end of the read-in phase, all the prerequisites of the targets defined after the special target are expanded a second time. In most circumstances this secondary expansion will have no effect, since all variable and function references will have been expanded during the initial parsing of the makefiles. In order to take advantage of the secondary expansion phase of the parser, then, it's necessary to escape the variable or function reference in the makefile. In this case the first expansion merely un-escapes the reference but doesn't expand it, and expansion is left to the secondary expansion phase. For example, consider this makefile:
.SECONDEXPANSION:
ONEVAR = onefile
TWOVAR = twofile
myfile: $(ONEVAR) $$(TWOVAR)
After the first expansion phase the prerequisites list of the
myfile target will be onefile and $(TWOVAR); the
first (unescaped) variable reference to ONEVAR is expanded,
while the second (escaped) variable reference is simply unescaped,
without being recognized as a variable reference. Now during the
secondary expansion the first word is expanded again but since it
contains no variable or function references it remains the static
value onefile, while the second word is now a normal reference
to the variable TWOVAR, which is expanded to the value
twofile. The final result is that there are two prerequisites,
onefile and twofile.
Obviously, this is not a very interesting case since the same result could more easily have been achieved simply by having both variables appear, unescaped, in the prerequisites list. One difference becomes apparent if the variables are reset; consider this example:
.SECONDEXPANSION:
AVAR = top
onefile: $(AVAR)
twofile: $$(AVAR)
AVAR = bottom
Here the prerequisite of onefile will be expanded immediately, and resolve to the value top, while the prerequisite of twofile will not be full expanded until the secondary expansion and yield a value of bottom.
This is marginally more exciting, but the true power of this feature
only becomes apparent when you discover that secondary expansions
always take place within the scope of the automatic variables for that
target. This means that you can use variables such as $@,
$*, etc. during the second expansion and they will have their
expected values, just as in the command script. All you have to do is
defer the expansion by escaping the $. Also, secondary
expansion occurs for both explicit and implicit (pattern) rules.
Knowing this, the possible uses for this feature increase
dramatically. For example:
.SECONDEXPANSION:
main_OBJS := main.o try.o test.o
lib_OBJS := lib.o api.o
main lib: $$($$@_OBJS)
Here, after the initial expansion the prerequisites of both the
main and lib targets will be $($@_OBJS). During
the secondary expansion, the $@ variable is set to the name of
the target and so the expansion for the main target will yield
$(main_OBJS), or main.o try.o test.o, while the
secondary expansion for the lib target will yield
$(lib_OBJS), or lib.o api.o.
You can also mix functions here, as long as they are properly escaped:
main_SRCS := main.c try.c test.c
lib_SRCS := lib.c api.c
.SECONDEXPANSION:
main lib: $$(patsubst %.c,%.o,$$($$@_SRCS))
This version allows users to specify source files rather than object files, but gives the same resulting prerequisites list as the previous example.
Evaluation of automatic variables during the secondary expansion
phase, especially of the target name variable $$@, behaves
similarly to evaluation within command scripts. However, there are
some subtle differences and “corner cases” which come into play for
the different types of rule definitions that make understands.
The subtleties of using the different automatic variables are
described below.
During the secondary expansion of explicit rules, $$@ and
$$% evaluate, respectively, to the file name of the target and,
when the target is an archive member, the target member name. The
$$< variable evaluates to the first prerequisite in the first
rule for this target. $$^ and $$+ evaluate to the list
of all prerequisites of rules that have already appeared for
the same target ($$+ with repetitions and $$^
without). The following example will help illustrate these behaviors:
.SECONDEXPANSION:
foo: foo.1 bar.1 $$< $$^ $$+ # line #1
foo: foo.2 bar.2 $$< $$^ $$+ # line #2
foo: foo.3 bar.3 $$< $$^ $$+ # line #3
In the first prerequisite list, all three variables ($$<,
$$^, and $$+) expand to the empty string. In the
second, they will have values foo.1, foo.1 bar.1, and
foo.1 bar.1 respectively. In the third they will have values
foo.1, foo.1 bar.1 foo.2 bar.2, and foo.1 bar.1
foo.2 bar.2 respectively.
Rules undergo secondary expansion in makefile order, except that the rule with the command script is always evaluated last.
The variables $$? and $$* are not available and expand
to the empty string.
Rules for secondary expansion of static pattern rules are identical to
those for explicit rules, above, with one exception: for static
pattern rules the $$* variable is set to the pattern stem. As
with explicit rules, $$? is not available and expands to the
empty string.
As make searches for an implicit rule, it substitutes the stem
and then performs secondary expansion for every rule with a matching
target pattern. The value of the automatic variables is derived in
the same fashion as for static pattern rules. As an example:
.SECONDEXPANSION:
foo: bar
foo foz: fo%: bo%
%oo: $$< $$^ $$+ $$*
When the implicit rule is tried for target foo, $$<
expands to bar, $$^ expands to bar boo,
$$+ also expands to bar boo, and $$* expands to
f.
Note that the directory prefix (D), as described in Implicit Rule Search Algorithm, is appended (after expansion) to all the patterns in the prerequisites list. As an example:
.SECONDEXPANSION:
/tmp/foo.o:
%.o: $$(addsuffix /%.c,foo bar) foo.h
The prerequisite list after the secondary expansion and directory
prefix reconstruction will be /tmp/foo/foo.c /tmp/var/bar/foo.c
foo.h. If you are not interested in this reconstruction, you can use
$$* instead of % in the prerequisites list.
A rule appears in the makefile and says when and how to remake certain files, called the rule's targets (most often only one per rule). It lists the other files that are the prerequisites of the target, and commands to use to create or update the target.
The order of rules is not significant, except for determining the
default goal: the target for make to consider, if you do
not otherwise specify one. The default goal is the target of the first
rule in the first makefile. If the first rule has multiple targets,
only the first target is taken as the default. There are two
exceptions: a target starting with a period is not a default unless it
contains one or more slashes, `/', as well; and, a target that
defines a pattern rule has no effect on the default goal.
(See Defining and Redefining Pattern Rules.)
Therefore, we usually write the makefile so that the first rule is the one for compiling the entire program or all the programs described by the makefile (often with a target called `all'). See Arguments to Specify the Goals.
Here is an example of a rule:
foo.o : foo.c defs.h # module for twiddling the frobs
cc -c -g foo.c
Its target is foo.o and its prerequisites are foo.c and defs.h. It has one command, which is `cc -c -g foo.c'. The command line starts with a tab to identify it as a command.
This rule says two things:
cc as stated.
The command does not explicitly mention defs.h, but we presume
that foo.c includes it, and that that is why defs.h was
added to the prerequisites.
In general, a rule looks like this:
targets : prerequisites
command
...
or like this:
targets : prerequisites ; command
command
...
The targets are file names, separated by spaces. Wildcard characters may be used (see Using Wildcard Characters in File Names) and a name of the form a(m) represents member m in archive file a (see Archive Members as Targets). Usually there is only one target per rule, but occasionally there is a reason to have more (see Multiple Targets in a Rule).
The command lines start with a tab character. The first command may appear on the line after the prerequisites, with a tab character, or may appear on the same line, with a semicolon. Either way, the effect is the same. There are other differences in the syntax of command lines. See Writing the Commands in Rules.
Because dollar signs are used to start make variable
references, if you really want a dollar sign in a target or
prerequisite you must write two of them, `$$' (see How to Use Variables). If you have enabled secondary
expansion (see Secondary Expansion) and you want a literal dollar
sign in the prerequisites lise, you must actually write four
dollar signs (`$$$$').
You may split a long line by inserting a backslash followed by a
newline, but this is not required, as make places no limit on
the length of a line in a makefile.
A rule tells make two things: when the targets are out of date,
and how to update them when necessary.
The criterion for being out of date is specified in terms of the prerequisites, which consist of file names separated by spaces. (Wildcards and archive members (see Archives) are allowed here too.) A target is out of date if it does not exist or if it is older than any of the prerequisites (by comparison of last-modification times). The idea is that the contents of the target file are computed based on information in the prerequisites, so if any of the prerequisites changes, the contents of the existing target file are no longer necessarily valid.
How to update is specified by commands. These are lines to be executed by the shell (normally `sh'), but with some extra features (see Writing the Commands in Rules).
There are actually two different types of prerequisites understood by
GNU make: normal prerequisites such as described in the
previous section, and order-only prerequisites. A normal
prerequisite makes two statements: first, it imposes an order of
execution of build commands: any commands necessary to build any of a
target's prerequisites will be fully executed before any commands
necessary to build the target. Second, it imposes a dependency
relationship: if any prerequisite is newer than the target, then the
target is considered out-of-date and must be rebuilt.
Normally, this is exactly what you want: if a target's prerequisite is updated, then the target should also be updated.
Occasionally, however, you have a situation where you want to impose a
specific ordering on the rules to be invoked without forcing
the target to be updated if one of those rules is executed. In that
case, you want to define order-only prerequisites. Order-only
prerequisites can be specified by placing a pipe symbol (|)
in the prerequisites list: any prerequisites to the left of the pipe
symbol are normal; any prerequisites to the right are order-only:
targets : normal-prerequisites | order-only-prerequisites
The normal prerequisites section may of course be empty. Also, you may still declare multiple lines of prerequisites for the same target: they are appended appropriately. Note that if you declare the same file to be both a normal and an order-only prerequisite, the normal prerequisite takes precedence (since they are a strict superset of the behavior of an order-only prerequisite).
A single file name can specify many files using wildcard characters.
The wildcard characters in make are `*', `?' and
`[...]', the same as in the Bourne shell. For example, *.c
specifies a list of all the files (in the working directory) whose names
end in `.c'.
The character `~' at the beginning of a file name also has special significance. If alone, or followed by a slash, it represents your home directory. For example ~/bin expands to /home/you/bin. If the `~' is followed by a word, the string represents the home directory of the user named by that word. For example ~john/bin expands to /home/john/bin. On systems which don't have a home directory for each user (such as MS-DOS or MS-Windows), this functionality can be simulated by setting the environment variable HOME.
Wildcard expansion is performed by make automatically in
targets and in prerequisites. In commands the shell is responsible
for wildcard expansion. In other contexts, wildcard expansion happens
only if you request it explicitly with the wildcard function.
The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, foo\*bar would refer to a specific file whose name consists of `foo', an asterisk, and `bar'.
Wildcards can be used in the commands of a rule, where they are expanded by the shell. For example, here is a rule to delete all the object files:
clean:
rm -f *.o
Wildcards are also useful in the prerequisites of a rule. With the following rule in the makefile, `make print' will print all the `.c' files that have changed since the last time you printed them:
print: *.c
lpr -p $?
touch print
This rule uses print as an empty target file; see Empty Target Files to Record Events. (The automatic variable `$?' is used to print only those files that have changed; see Automatic Variables.)
Wildcard expansion does not happen when you define a variable. Thus, if you write this:
objects = *.o
then the value of the variable objects is the actual string
`*.o'. However, if you use the value of objects in a target,
prerequisite or command, wildcard expansion will take place at that time.
To set objects to the expansion, instead use:
objects := $(wildcard *.o)
See Wildcard Function.
Now here is an example of a naive way of using wildcard expansion, that does not do what you would intend. Suppose you would like to say that the executable file foo is made from all the object files in the directory, and you write this:
objects = *.o
foo : $(objects)
cc -o foo $(CFLAGS) $(objects)
The value of objects is the actual string `*.o'. Wildcard
expansion happens in the rule for foo, so that each existing
`.o' file becomes a prerequisite of foo and will be recompiled if
necessary.
But what if you delete all the `.o' files? When a wildcard matches
no files, it is left as it is, so then foo will depend on the
oddly-named file *.o. Since no such file is likely to exist,
make will give you an error saying it cannot figure out how to
make *.o. This is not what you want!
Actually it is possible to obtain the desired result with wildcard
expansion, but you need more sophisticated techniques, including the
wildcard function and string substitution.
See The Function wildcard.
Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to separate directories in pathnames, like so:
c:\foo\bar\baz.c
This is equivalent to the Unix-style c:/foo/bar/baz.c (the
c: part is the so-called drive letter). When make runs on
these systems, it supports backslashes as well as the Unix-style forward
slashes in pathnames. However, this support does not include the
wildcard expansion, where backslash is a quote character. Therefore,
you must use Unix-style slashes in these cases.
wildcard
Wildcard expansion happens automatically in rules. But wildcard expansion
does not normally take place when a variable is set, or inside the
arguments of a function. If you want to do wildcard expansion in such
places, you need to use the wildcard function, like this:
$(wildcard pattern...)
This string, used anywhere in a makefile, is replaced by a
space-separated list of names of existing files that match one of the
given file name patterns. If no existing file name matches a pattern,
then that pattern is omitted from the output of the wildcard
function. Note that this is different from how unmatched wildcards
behave in rules, where they are used verbatim rather than ignored
(see Wildcard Pitfall).
One use of the wildcard function is to get a list of all the C source
files in a directory, like this:
$(wildcard *.c)
We can change the list of C source files into a list of object files by replacing the `.c' suffix with `.o' in the result, like this:
$(patsubst %.c,%.o,$(wildcard *.c))
(Here we have used another function, patsubst.
See Functions for String Substitution and Analysis.)
Thus, a makefile to compile all C source files in the directory and then link them together could be written as follows:
objects := $(patsubst %.c,%.o,$(wildcard *.c))
foo : $(objects)
cc -o foo $(objects)
(This takes advantage of the implicit rule for compiling C programs, so there is no need to write explicit rules for compiling the files. See The Two Flavors of Variables, for an explanation of `:=', which is a variant of `='.)
For large systems, it is often desirable to put sources in a separate
directory from the binaries. The directory search features of
make facilitate this by searching several directories
automatically to find a prerequisite. When you redistribute the files
among directories, you do not need to change the individual rules,
just the search paths.
VPATH: Search Path for All Prerequisites
The value of the make variable VPATH specifies a list of
directories that make should search. Most often, the
directories are expected to contain prerequisite files that are not in the
current directory; however, make uses VPATH as a search
list for both prerequisites and targets of rules.
Thus, if a file that is listed as a target or prerequisite does not exist
in the current directory, make searches the directories listed in
VPATH for a file with that name. If a file is found in one of
them, that file may become the prerequisite (see below). Rules may then
specify the names of files in the prerequisite list as if they all
existed in the current directory. See Writing Shell Commands with Directory Search.
In the VPATH variable, directory names are separated by colons or
blanks. The order in which directories are listed is the order followed
by make in its search. (On MS-DOS and MS-Windows, semi-colons
are used as separators of directory names in VPATH, since the
colon can be used in the pathname itself, after the drive letter.)
For example,
VPATH = src:../headers
specifies a path containing two directories, src and
../headers, which make searches in that order.
With this value of VPATH, the following rule,
foo.o : foo.c
is interpreted as if it were written like this:
foo.o : src/foo.c
assuming the file foo.c does not exist in the current directory but is found in the directory src.
vpath Directive
Similar to the VPATH variable, but more selective, is the
vpath directive (note lower case), which allows you to specify a
search path for a particular class of file names: those that match a
particular pattern. Thus you can supply certain search directories for
one class of file names and other directories (or none) for other file
names.
There are three forms of the vpath directive:
vpath pattern directoriesThe search path, directories, is a list of directories to be
searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or
blanks, just like the search path used in the VPATH variable.
vpath patternvpathvpath directives.
A vpath pattern is a string containing a `%' character. The
string must match the file name of a prerequisite that is being searched
for, the `%' character matching any sequence of zero or more
characters (as in pattern rules; see Defining and Redefining Pattern Rules). For example, %.h matches files that
end in .h. (If there is no `%', the pattern must match the
prerequisite exactly, which is not useful very often.)
`%' characters in a vpath directive's pattern can be quoted
with preceding backslashes (`\'). Backslashes that would otherwise
quote `%' characters can be quoted with more backslashes.
Backslashes that quote `%' characters or other backslashes are
removed from the pattern before it is compared to file names. Backslashes
that are not in danger of quoting `%' characters go unmolested.
When a prerequisite fails to exist in the current directory, if the
pattern in a vpath directive matches the name of the
prerequisite file, then the directories in that directive are searched
just like (and before) the directories in the VPATH variable.
For example,
vpath %.h ../headers
tells make to look for any prerequisite whose name ends in .h
in the directory ../headers if the file is not found in the current
directory.
If several vpath patterns match the prerequisite file's name, then
make processes each matching vpath directive one by one,
searching all the directories mentioned in each directive. make
handles multiple vpath directives in the order in which they
appear in the makefile; multiple directives with the same pattern are
independent of each other.
Thus,
vpath %.c foo
vpath % blish
vpath %.c bar
will look for a file ending in `.c' in foo, then blish, then bar, while
vpath %.c foo:bar
vpath % blish
will look for a file ending in `.c' in foo, then bar, then blish.
When a prerequisite is found through directory search, regardless of type
(general or selective), the pathname located may not be the one that
make actually provides you in the prerequisite list. Sometimes
the path discovered through directory search is thrown away.
The algorithm make uses to decide whether to keep or abandon a
path found via directory search is as follows:
make doesn't need to rebuild
the target then you use the path found via directory search.
make must rebuild, then the target is rebuilt locally,
not in the directory found via directory search.
This algorithm may seem complex, but in practice it is quite often exactly what you want.
Other versions of make use a simpler algorithm: if the file does
not exist, and it is found via directory search, then that pathname is
always used whether or not the target needs to be built. Thus, if the
target is rebuilt it is created at the pathname discovered during
directory search.
If, in fact, this is the behavior you want for some or all of your
directories, you can use the GPATH variable to indicate this to
make.
GPATH has the same syntax and format as VPATH (that is, a
space- or colon-delimited list of pathnames). If an out-of-date target
is found by directory search in a directory that also appears in
GPATH, then that pathname is not thrown away. The target is
rebuilt using the expanded path.
When a prerequisite is found in another directory through directory search,
this cannot change the commands of the rule; they will execute as written.
Therefore, you must write the commands with care so that they will look for
the prerequisite in the directory where make finds it.
This is done with the automatic variables such as `$^' (see Automatic Variables). For instance, the value of `$^' is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of `$@' is the target. Thus:
foo.o : foo.c
cc -c $(CFLAGS) $^ -o $@
(The variable CFLAGS exists so you can specify flags for C
compilation by implicit rules; we use it here for consistency so it will
affect all C compilations uniformly;
see Variables Used by Implicit Rules.)
Often the prerequisites include header files as well, which you do not want to mention in the commands. The automatic variable `$<' is just the first prerequisite:
VPATH = src:../headers
foo.o : foo.c defs.h hack.h
cc -c $(CFLAGS) $< -o $@
The search through the directories specified in VPATH or with
vpath also happens during consideration of implicit rules
(see Using Implicit Rules).
For example, when a file foo.o has no explicit rule, make
considers implicit rules, such as the built-in rule to compile
foo.c if that file exists. If such a file is lacking in the
current directory, the appropriate directories are searched for it. If
foo.c exists (or is mentioned in the makefile) in any of the
directories, the implicit rule for C compilation is applied.
The commands of implicit rules normally use automatic variables as a matter of necessity; consequently they will use the file names found by directory search with no extra effort.
Directory search applies in a special way to libraries used with the linker. This special feature comes into play when you write a prerequisite whose name is of the form `-lname'. (You can tell something strange is going on here because the prerequisite is normally the name of a file, and the file name of a library generally looks like libname.a, not like `-lname'.)
When a prerequisite's name has the form `-lname', make
handles it specially by searching for the file libname.so in
the current directory, in directories specified by matching vpath
search paths and the VPATH search path, and then in the
directories /lib, /usr/lib, and prefix/lib
(normally /usr/local/lib, but MS-DOS/MS-Windows versions of
make behave as if prefix is defined to be the root of the
DJGPP installation tree).
If that file is not found, then the file libname.a is searched for, in the same directories as above.
For example, if there is a /usr/lib/libcurses.a library on your system (and no /usr/lib/libcurses.so file), then
foo : foo.c -lcurses
cc $^ -o $@
would cause the command `cc foo.c /usr/lib/libcurses.a -o foo' to be executed when foo is older than foo.c or than /usr/lib/libcurses.a.
Although the default set of files to be searched for is
libname.so and libname.a, this is customizable
via the .LIBPATTERNS variable. Each word in the value of this
variable is a pattern string. When a prerequisite like
`-lname' is seen, make will replace the percent in
each pattern in the list with name and perform the above directory
searches using that library filename. If no library is found, the next
word in the list will be used.
The default value for .LIBPATTERNS is `lib%.so lib%.a',
which provides the default behavior described above.
You can turn off link library expansion completely by setting this variable to an empty value.
A phony target is one that is not really the name of a file. It is just a name for some commands to be executed when you make an explicit request. There are two reasons to use a phony target: to avoid a conflict with a file of the same name, and to improve performance.
If you write a rule whose commands will not create the target file, the commands will be executed every time the target comes up for remaking. Here is an example:
clean:
rm *.o temp
Because the rm command does not create a file named clean,
probably no such file will ever exist. Therefore, the rm command
will be executed every time you say `make clean'.
The phony target will cease to work if anything ever does create a file
named clean in this directory. Since it has no prerequisites, the
file clean would inevitably be considered up to date, and its
commands would not be executed. To avoid this problem, you can explicitly
declare the target to be phony, using the special target .PHONY
(see Special Built-in Target Names) as follows:
.PHONY : clean
Once this is done, `make clean' will run the commands regardless of whether there is a file named clean.
Since it knows that phony targets do not name actual files that could be
remade from other files, make skips the implicit rule search for
phony targets (see Implicit Rules). This is why declaring a target
phony is good for performance, even if you are not worried about the
actual file existing.
Thus, you first write the line that states that clean is a
phony target, then you write the rule, like this:
.PHONY: clean
clean:
rm *.o temp
Another example of the usefulness of phony targets is in conjunction
with recursive invocations of make (for more information, see
Recursive Use of make). In this case the
makefile will often contain a variable which lists a number of
subdirectories to be built. One way to handle this is with one rule
whose command is a shell loop over the subdirectories, like this:
SUBDIRS = foo bar baz
subdirs:
for dir in $(SUBDIRS); do \
$(MAKE) -C $$dir; \
done
There are a few problems with this method, however. First, any error
detected in a submake is not noted by this rule, so it will continue to
build the rest of the directories even when one fails. This can be
overcome by adding shell commands to note the error and exit, but then
it will do so even if make is invoked with the -k option,
which is unfortunate. Second, and perhaps more importantly, you cannot
take advantage of make's ability to build targets in parallel
(see Parallel Execution), since there is only one rule.
By declaring the subdirectories as phony targets (you must do this as the subdirectory obviously always exists; otherwise it won't be built) you can remove these problems:
SUBDIRS = foo bar baz
.PHONY: subdirs $(SUBDIRS)
subdirs: $(SUBDIRS)
$(SUBDIRS):
$(MAKE) -C $@
foo: baz
Here we've also declared that the foo subdirectory cannot be built until after the baz subdirectory is complete; this kind of relationship declaration is particularly important when attempting parallel builds.
A phony target should not be a prerequisite of a real target file; if it
is, its commands are run every time make goes to update that
file. As long as a phony target is never a prerequisite of a real
target, the phony target commands will be executed only when the phony
target is a specified goal (see Arguments to Specify the Goals).
Phony targets can have prerequisites. When one directory contains multiple programs, it is most convenient to describe all of the programs in one makefile ./Makefile. Since the target remade by default will be the first one in the makefile, it is common to make this a phony target named `all' and give it, as prerequisites, all the individual programs. For example:
all : prog1 prog2 prog3
.PHONY : all
prog1 : prog1.o utils.o
cc -o prog1 prog1.o utils.o
prog2 : prog2.o
cc -o prog2 prog2.o
prog3 : prog3.o sort.o utils.o
cc -o prog3 prog3.o sort.o utils.o
Now you can say just `make' to remake all three programs, or specify as arguments the ones to remake (as in `make prog1 prog3'). Phoniness is not inherited: the prerequisites of a phony target are not themselves phony, unless explicitly declared to be so.
When one phony target is a prerequisite of another, it serves as a subroutine of the other. For example, here `make cleanall' will delete the object files, the difference files, and the file program:
.PHONY: cleanall cleanobj cleandiff
cleanall : cleanobj cleandiff
rm program
cleanobj :
rm *.o
cleandiff :
rm *.diff
If a rule has no prerequisites or commands, and the target of the rule
is a nonexistent file, then make imagines this target to have
been updated whenever its rule is run. This implies that all targets
depending on this one will always have their commands run.
An example will illustrate this:
clean: FORCE
rm $(objects)
FORCE:
Here the target `FORCE' satisfies the special conditions, so the target clean that depends on it is forced to run its commands. There is nothing special about the name `FORCE', but that is one name commonly used this way.
As you can see, using `FORCE' this way has the same results as using `.PHONY: clean'.
Using `.PHONY' is more explicit and more efficient. However,
other versions of make do not support `.PHONY'; thus
`FORCE' appears in many makefiles. See Phony Targets.
The empty target is a variant of the phony target; it is used to hold commands for an action that you request explicitly from time to time. Unlike a phony target, this target file can really exist; but the file's contents do not matter, and usually are empty.
The purpose of the empty target file is to record, with its
last-modification time, when the rule's commands were last executed. It
does so because one of the commands is a touch command to update the
target file.
The empty target file should have some prerequisites (otherwise it doesn't make sense). When you ask to remake the empty target, the commands are executed if any prerequisite is more recent than the target; in other words, if a prerequisite has changed since the last time you remade the target. Here is an example:
print: foo.c bar.c
lpr -p $?
touch print
With this rule, `make print' will execute the lpr command if
either source file has changed since the last `make print'. The
automatic variable `$?' is used to print only those files that have
changed (see Automatic Variables).
Certain names have special meanings if they appear as targets.
.PHONY.PHONY are considered to
be phony targets. When it is time to consider such a target,
make will run its commands unconditionally, regardless of
whether a file with that name exists or what its last-modification
time is. See Phony Targets.
.SUFFIXES.SUFFIXES are the list
of suffixes to be used in checking for suffix rules.
See Old-Fashioned Suffix Rules.
.DEFAULT.DEFAULT are used for any target for
which no rules are found (either explicit rules or implicit rules).
See Last Resort. If .DEFAULT commands are specified, every
file mentioned as a prerequisite, but not as a target in a rule, will have
these commands executed on its behalf. See Implicit Rule Search Algorithm.
.PRECIOUS.PRECIOUS depends on are given the following
special treatment: if make is killed or interrupted during the
execution of their commands, the target is not deleted.
See Interrupting or Killing make. Also, if the
target is an intermediate file, it will not be deleted after it is no
longer needed, as is normally done. See Chains of Implicit Rules. In this latter respect it overlaps with the
.SECONDARY special target.
You can also list the target pattern of an implicit rule (such as
`%.o') as a prerequisite file of the special target .PRECIOUS
to preserve intermediate files created by rules whose target patterns
match that file's name.
.INTERMEDIATE.INTERMEDIATE depends on are treated as
intermediate files. See Chains of Implicit Rules.
.INTERMEDIATE with no prerequisites has no effect.
.SECONDARY.SECONDARY depends on are treated as
intermediate files, except that they are never automatically deleted.
See Chains of Implicit Rules.
.SECONDARY with no prerequisites causes all targets to be treated
as secondary (i.e., no target is removed because it is considered
intermediate).
.SECONDEXPANSION.SECONDEXPANSION is mentioned as a target anywhere in the
makefile, then all prerequisite lists defined after it appears
will be expanded a second time after all makefiles have been read in.
See Secondary Expansion.
The prerequisites of the special target .SUFFIXES are the list
of suffixes to be used in checking for suffix rules.
See Old-Fashioned Suffix Rules.
.DELETE_ON_ERROR.DELETE_ON_ERROR is mentioned as a target anywhere in the
makefile, then make will delete the target of a rule if it has
changed and its commands exit with a nonzero exit status, just as it
does when it receives a signal. See Errors in Commands.
.IGNORE.IGNORE, then make will
ignore errors in execution of the commands run for those particular
files. The commands for .IGNORE are not meaningful.
If mentioned as a target with no prerequisites, .IGNORE says to
ignore errors in execution of commands for all files. This usage of
`.IGNORE' is supported only for historical compatibility. Since
this affects every command in the makefile, it is not very useful; we
recommend you use the more selective ways to ignore errors in specific
commands. See Errors in Commands.
.LOW_RESOLUTION_TIME.LOW_RESOLUTION_TIME,
make assumes that these files are created by commands that
generate low resolution time stamps. The commands for
.LOW_RESOLUTION_TIME are not meaningful.
The high resolution file time stamps of many modern hosts lessen the
chance of make incorrectly concluding that a file is up to
date. Unfortunately, these hosts provide no way to set a high
resolution file time stamp, so commands like `cp -p' that
explicitly set a file's time stamp must discard its subsecond part. If
a file is created by such a command, you should list it as a
prerequisite of .LOW_RESOLUTION_TIME so that make does
not mistakenly conclude that the file is out of date. For example:
.LOW_RESOLUTION_TIME: dst
dst: src
cp -p src dst
Since `cp -p' discards the subsecond part of src's time
stamp, dst is typically slightly older than src even when
it is up to date. The .LOW_RESOLUTION_TIME line causes
make to consider dst to be up to date if its time stamp
is at the start of the same second that src's time stamp is in.
Due to a limitation of the archive format, archive member time stamps
are always low resolution. You need not list archive members as
prerequisites of .LOW_RESOLUTION_TIME, as make does this
automatically.
.SILENT.SILENT, then make will
not print the commands to remake those particular files before executing
them. The commands for .SILENT are not meaningful.
If mentioned as a target with no prerequisites, .SILENT says not
to print any commands before executing them. This usage of
`.SILENT' is supported only for historical compatibility. We
recommend you use the more selective ways to silence specific commands.
See Command Echoing. If you want to silence all commands
for a particular run of make, use the `-s' or
`--silent' option (see Options Summary).
.EXPORT_ALL_VARIABLESmake to
export all variables to child processes by default.
See Communicating Variables to a Sub-make.
.NOTPARALLEL.NOTPARALLEL is mentioned as a target, then this invocation of
make will be run serially, even if the `-j' option is
given. Any recursively invoked make command will still be run in
parallel (unless its makefile contains this target). Any prerequisites
on this target are ignored.
Any defined implicit rule suffix also counts as a special target if it appears as a target, and so does the concatenation of two suffixes, such as `.c.o'. These targets are suffix rules, an obsolete way of defining implicit rules (but a way still widely used). In principle, any target name could be special in this way if you break it in two and add both pieces to the suffix list. In practice, suffixes normally begin with `.', so these special target names also begin with `.'. See Old-Fashioned Suffix Rules.
A rule with multiple targets is equivalent to writing many rules, each with one target, and all identical aside from that. The same commands apply to all the targets, but their effects may vary because you can substitute the actual target name into the command using `$@'. The rule contributes the same prerequisites to all the targets also.
This is useful in two cases.
kbd.o command.o files.o: command.h
gives an additional prerequisite to each of the three object files mentioned.
bigoutput littleoutput : text.g
generate text.g -$(subst output,,$@) > $@
bigoutput : text.g
generate text.g -big > bigoutput
littleoutput : text.g
generate text.g -little > littleoutput
Here we assume the hypothetical program generate makes two
types of output, one if given `-big' and one if given
`-little'.
See Functions for String Substitution and Analysis,
for an explanation of the subst function.
Suppose you would like to vary the prerequisites according to the target, much as the variable `$@' allows you to vary the commands. You cannot do this with multiple targets in an ordinary rule, but you can do it with a static pattern rule. See Static Pattern Rules.
One file can be the target of several rules. All the prerequisites mentioned in all the rules are merged into one list of prerequisites for the target. If the target is older than any prerequisite from any rule, the commands are executed.
There can only be one set of commands to be executed for a file. If
more than one rule gives commands for the same file, make uses
the last set given and prints an error message. (As a special case,
if the file's name begins with a dot, no error message is printed.
This odd behavior is only for compatibility with other implementations
of make... you should avoid using it). Occasionally it is
useful to have the same target invoke multiple commands which are
defined in different parts of your makefile; you can use
double-colon rules (see Double-Colon) for this.
An extra rule with just prerequisites can be used to give a few extra
prerequisites to many files at once. For example, makefiles often
have a variable, such as objects, containing a list of all the
compiler output files in the system being made. An easy way to say
that all of them must be recompiled if config.h changes is to
write the following:
objects = foo.o bar.o
foo.o : defs.h
bar.o : defs.h test.h
$(objects) : config.h
This could be inserted or taken out without changing the rules that really specify how to make the object files, making it a convenient form to use if you wish to add the additional prerequisite intermittently.
Another wrinkle is that the additional prerequisites could be specified with
a variable that you set with a command argument to make
(see Overriding Variables). For example,
extradeps=
$(objects) : $(extradeps)
means that the command `make extradeps=foo.h' will consider foo.h as a prerequisite of each object file, but plain `make' will not.
If none of the explicit rules for a target has commands, then make
searches for an applicable implicit rule to find some commands
see Using Implicit Rules).
Static pattern rules are rules which specify multiple targets and construct the prerequisite names for each target based on the target name. They are more general than ordinary rules with multiple targets because the targets do not have to have identical prerequisites. Their prerequisites must be analogous, but not necessarily identical.
Here is the syntax of a static pattern rule:
targets ...: target-pattern: prereq-patterns ...
commands
...
The targets list specifies the targets that the rule applies to. The targets can contain wildcard characters, just like the targets of ordinary rules (see Using Wildcard Characters in File Names).
The target-pattern and prereq-patterns say how to compute the prerequisites of each target. Each target is matched against the target-pattern to extract a part of the target name, called the stem. This stem is substituted into each of the prereq-patterns to make the prerequisite names (one from each prereq-pattern).
Each pattern normally contains the character `%' just once. When the target-pattern matches a target, the `%' can match any part of the target name; this part is called the stem. The rest of the pattern must match exactly. For example, the target foo.o matches the pattern `%.o', with `foo' as the stem. The targets foo.c and foo.out do not match that pattern.
The prerequisite names for each target are made by substituting the stem for the `%' in each prerequisite pattern. For example, if one prerequisite pattern is %.c, then substitution of the stem `foo' gives the prerequisite name foo.c. It is legitimate to write a prerequisite pattern that does not contain `%'; then this prerequisite is the same for all targets.
`%' characters in pattern rules can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared to file names or has a stem substituted into it. Backslashes that are not in danger of quoting `%' characters go unmolested. For example, the pattern the\%weird\\%pattern\\ has `the%weird\' preceding the operative `%' character, and `pattern\\' following it. The final two backslashes are left alone because they cannot affect any `%' character.
Here is an example, which compiles each of foo.o and bar.o from the corresponding .c file:
objects = foo.o bar.o
all: $(objects)
$(objects): %.o: %.c
$(CC) -c $(CFLAGS) $< -o $@
Here `$<' is the automatic variable that holds the name of the prerequisite and `$@' is the automatic variable that holds the name of the target; see Automatic Variables.
Each target specified must match the target pattern; a warning is issued
for each target that does not. If you have a list of files, only some of
which will match the pattern, you can use the filter function to
remove nonmatching file names (see Functions for String Substitution and Analysis):
files = foo.elc bar.o lose.o
$(filter %.o,$(files)): %.o: %.c
$(CC) -c $(CFLAGS) $< -o $@
$(filter %.elc,$(files)): %.elc: %.el
emacs -f batch-byte-compile $<
In this example the result of `$(filter %.o,$(files))' is bar.o lose.o, and the first static pattern rule causes each of these object files to be updated by compiling the corresponding C source file. The result of `$(filter %.elc,$(files))' is foo.elc, so that file is made from foo.el.
Another example shows how to use $* in static pattern rules:
bigoutput littleoutput : %output : text.g
generate text.g -$* > $@
When the generate command is run, $* will expand to the
stem, either `big' or `little'.
A static pattern rule has much in common with an implicit rule defined as a
pattern rule (see Defining and Redefining Pattern Rules).
Both have a pattern for the target and patterns for constructing the
names of prerequisites. The difference is in how make decides
when the rule applies.
An implicit rule can apply to any target that matches its pattern, but it does apply only when the target has no commands otherwise specified, and only when the prerequisites can be found. If more than one implicit rule appears applicable, only one applies; the choice depends on the order of rules.
By contrast, a static pattern rule applies to the precise list of targets that you specify in the rule. It cannot apply to any other target and it invariably does apply to each of the targets specified. If two conflicting rules apply, and both have commands, that's an error.
The static pattern rule can be better than an implicit rule for these reasons:
make to use the wrong implicit rule. The choice
might depend on the order in which the implicit rule search is done.
With static pattern rules, there is no uncertainty: each rule applies
to precisely the targets specified.
Double-colon rules are rules written with `::' instead of `:' after the target names. They are handled differently from ordinary rules when the same target appears in more than one rule.
When a target appears in multiple rules, all the rules must be the same type: all ordinary, or all double-colon. If they are double-colon, each of them is independent of the others. Each double-colon rule's commands are executed if the target is older than any prerequisites of that rule. If there are no prerequisites for that rule, its commands are always executed (even if the target already exists). This can result in executing none, any, or all of the double-colon rules.
Double-colon rules with the same target are in fact completely separate from one another. Each double-colon rule is processed individually, just as rules with different targets are processed.
The double-colon rules for a target are executed in the order they appear in the makefile. However, the cases where double-colon rules really make sense are those where the order of executing the commands would not matter.
Double-colon rules are somewhat obscure and not often very useful; they provide a mechanism for cases in which the method used to update a target differs depending on which prerequisite files caused the update, and such cases are rare.
Each double-colon rule should specify commands; if it does not, an implicit rule will be used if one applies. See Using Implicit Rules.
In the makefile for a program, many of the rules you need to write often
say only that some object file depends on some header
file. For example, if main.c uses defs.h via an
#include, you would write:
main.o: defs.h
You need this rule so that make knows that it must remake
main.o whenever defs.h changes. You can see that for a
large program you would have to write dozens of such rules in your
makefile. And, you must always be very careful to update the makefile
every time you add or remove an #include.
To avoid this hassle, most modern C compilers can write these rules for
you, by looking at the #include lines in the source files.
Usually this is done with the `-M' option to the compiler.
For example, the command:
cc -M main.c
generates the output:
main.o : main.c defs.h
Thus you no longer have to write all those rules yourself. The compiler will do it for you.
Note that such a prerequisite constitutes mentioning main.o in a
makefile, so it can never be considered an intermediate file by implicit
rule search. This means that make won't ever remove the file
after using it; see Chains of Implicit Rules.
With old make programs, it was traditional practice to use this
compiler feature to generate prerequisites on demand with a command like
`make depend'. That command would create a file depend
containing all the automatically-generated prerequisites; then the
makefile could use include to read them in (see Include).
In GNU make, the feature of remaking makefiles makes this
practice obsolete—you need never tell make explicitly to
regenerate the prerequisites, because it always regenerates any makefile
that is out of date. See Remaking Makefiles.
The practice we recommend for automatic prerequisite generation is to have one makefile corresponding to each source file. For each source file name.c there is a makefile name.d which lists what files the object file name.o depends on. That way only the source files that have changed need to be rescanned to produce the new prerequisites.
Here is the pattern rule to generate a file of prerequisites (i.e., a makefile) called name.d from a C source file called name.c:
%.d: %.c
@set -e; rm -f $@; \
$(CC) -M $(CPPFLAGS) $< > $@.$$$$; \
sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \
rm -f $@.$$$$
See Pattern Rules, for information on defining pattern rules. The
`-e' flag to the shell causes it to exit immediately if the
$(CC) command (or any other command) fails (exits with a
nonzero status).
With the GNU C compiler, you may wish to use the `-MM' flag instead
of `-M'. This omits prerequisites on system header files.
See Options Controlling the Preprocessor, for details.
The purpose of the sed command is to translate (for example):
main.o : main.c defs.h
into:
main.o main.d : main.c defs.h
This makes each `.d' file depend on all the source and header files
that the corresponding `.o' file depends on. make then
knows it must regenerate the prerequisites whenever any of the source or
header files changes.
Once you've defined the rule to remake the `.d' files,
you then use the include directive to read them all in.
See Include. For example:
sources = foo.c bar.c
include $(sources:.c=.d)
(This example uses a substitution variable reference to translate the
list of source files `foo.c bar.c' into a list of prerequisite
makefiles, `foo.d bar.d'. See Substitution Refs, for full
information on substitution references.) Since the `.d' files are
makefiles like any others, make will remake them as necessary
with no further work from you. See Remaking Makefiles.
Note that the `.d' files contain target definitions; you should
be sure to place the include directive after the first,
default goal in your makefiles or run the risk of having a random
object file become the default goal.
See How Make Works.
The commands of a rule consist of one or more shell command lines to be executed, one at a time, in the order they appear. Typically, the result of executing these commands is that the target of the rule is brought up to date.
Users use many different shell programs, but commands in makefiles are always interpreted by /bin/sh unless the makefile specifies otherwise. See Command Execution.
Makefiles have the unusual property that there are really two distinct
syntaxes in one file. Most of the makefile uses make syntax
(see Writing Makefiles). However, commands are meant to be
interpreted by the shell and so they are written using shell syntax.
The make program does not try to understand shell syntax: it
performs only a very few specific translations on the content of the
command before handing it to the shell.
Each command line must start with a tab, except that the first command line may be attached to the target-and-prerequisites line with a semicolon in between. Any line in the makefile that begins with a tab and appears in a “rule context” (that is, after a rule has been started until another rule or variable definition) will be considered a command line for that rule. Blank lines and lines of just comments may appear among the command lines; they are ignored.
Some consequences of these rules include:
make comment; it will be
passed to the shell as-is. Whether the shell treats it as a comment
or not depends on your shell.
make variable definition, and passed to the shell.
ifdef, ifeq,
etc. see Syntax of Conditionals) in a “rule
context” which is indented by a tab as the first character on the
line, will be considered a command line and be passed to the shell.
One of the few ways in which make does interpret command lines
is checking for a backslash just before the newline. As in normal
makefile syntax, a single command can be split into multiple lines in
the makefile by placing a backslash before each newline. A sequence
of lines like this is considered a single command, and one instance of
the shell will be invoked to run it.
However, in contrast to how they are treated in other places in a makefile, backslash-newline pairs are not removed from the command. Both the backslash and the newline characters are preserved and passed to the shell. How the backslash-newline is interpreted depends on your shell. If the first character of the next line after the backslash-newline is a tab, then that tab (and only that tab) is removed. Whitespace is never added to the command.
For example, this makefile:
all :
@echo no\
space
@echo no\
space
@echo one \
space
@echo one\
space
consists of four separate shell commands where the output is:
nospace
nospace
one space
one space
As a more complex example, this makefile:
all : ; @echo 'hello \
world' ; echo "hello \
world"
will run one shell with a command script of:
echo 'hello \
world' ; echo "hello \
world"
which, according to shell quoting rules, will yield the following output:
hello \
world
hello world
Notice how the backslash/newline pair was removed inside the string quoted
with double quotes ("..."), but not from the string quoted with single
quotes ('...'). This is the way the default shell (/bin/sh)
handles backslash/newline pairs. If you specify a different shell in your
makefiles it may treat them differently.
Sometimes you want to split a long line inside of single quotes, but
you don't want the backslash-newline to appear in the quoted content.
This is often the case when passing scripts to languages such as Perl,
where extraneous backslashes inside the script can change its meaning
or even be a syntax error. One simple way of handling this is to
place the quoted string, or even the entire command, into a
make variable then use the variable in the command. In this
situation the newline quoting rules for makefiles will be used, and
the backslash-newline will be removed. If we rewrite our example
above using this method:
HELLO = 'hello \
world'
all : ; @echo $(HELLO)
we will get output like this:
hello world
If you like, you can also use target-specific variables (see Target-specific Variable Values) to obtain a tighter correspondence between the variable and the command that uses it.
The other way in which make processes commands is by expanding
any variable references in them (see Basics of Variable References). This occurs after make has finished reading all the
makefiles and the target is determined to be out of date; so, the
commands for targets which are not rebuilt are never expanded.
Variable and function references in commands have identical syntax and
semantics to references elsewhere in the makefile. They also have the
same quoting rules: if you want a dollar sign to appear in your
command, you must double it (`$$'). For shells like the default
shell, that use dollar signs to introduce variables, it's important to
keep clear in your mind whether the variable you want to reference is
a make variable (use a single dollar sign) or a shell variable
(use two dollar signs). For example:
LIST = one two three
all:
for i in $(LIST); do \
echo $$i; \
done
results in the following command being passed to the shell:
for i in one two three; do \
echo $i; \
done
which generates the expected result:
one
two
three
Normally make prints each command line before it is executed.
We call this echoing because it gives the appearance that you
are typing the commands yourself.
When a line starts with `@', the echoing of that line is suppressed.
The `@' is discarded before the command is passed to the shell.
Typically you would use this for a command whose only effect is to print
something, such as an echo command to indicate progress through
the makefile:
@echo About to make distribution files
When make is given the flag `-n' or `--just-print'
it only echoes commands, it won't execute them. See Summary of Options. In this case and only this case, even the
commands starting with `@' are printed. This flag is useful for
finding out which commands make thinks are necessary without
actually doing them.
The `-s' or `--silent'
flag to make prevents all echoing, as if all commands
started with `@'. A rule in the makefile for the special target
.SILENT without prerequisites has the same effect
(see Special Built-in Target Names).
.SILENT is essentially obsolete since `@' is more flexible.
When it is time to execute commands to update a target, they are
executed by invoking a new subshell for each command line. (In
practice, make may take shortcuts that do not affect the
results.)
Please note: this implies that setting shell variables and
invoking shell commands such as cd that set a context local to
each process will not affect the following command lines.2 If you want to use cd to affect the next statement,
put both statements in a single command line. Then make will
invoke one shell to run the entire line, and the shell will execute
the statements in sequence. For example:
foo : bar/lose
cd $(@D) && gobble $(@F) > ../$@
Here we use the shell AND operator (&&) so that if the
cd command fails, the script will fail without trying to invoke
the gobble command in the wrong directory, which could cause
problems (in this case it would certainly cause ../foo to be
truncated, at least).
The program used as the shell is taken from the variable SHELL.
If this variable is not set in your makefile, the program
/bin/sh is used as the shell.
Unlike most variables, the variable SHELL is never set from the
environment. This is because the SHELL environment variable is
used to specify your personal choice of shell program for interactive
use. It would be very bad for personal choices like this to affect the
functioning of makefiles. See Variables from the Environment.
Furthermore, when you do set SHELL in your makefile that value
is not exported in the environment to commands that make
invokes. Instead, the value inherited from the user's environment, if
any, is exported. You can override this behavior by explicitly
exporting SHELL (see Communicating Variables to a Sub-make), forcing it to be passed in the
environment to commands.
However, on MS-DOS and MS-Windows the value of SHELL in the
environment is used, since on those systems most users do not
set this variable, and therefore it is most likely set specifically to
be used by make. On MS-DOS, if the setting of SHELL is
not suitable for make, you can set the variable
MAKESHELL to the shell that make should use; if set it
will be used as the shell instead of the value of SHELL.
Choosing a shell in MS-DOS and MS-Windows is much more complex than on other systems.
On MS-DOS, if SHELL is not set, the value of the variable
COMSPEC (which is always set) is used instead.
The processing of lines that set the variable SHELL in Makefiles
is different on MS-DOS. The stock shell, command.com, is
ridiculously limited in its functionality and many users of make
tend to install a replacement shell. Therefore, on MS-DOS, make
examines the value of SHELL, and changes its behavior based on
whether it points to a Unix-style or DOS-style shell. This allows
reasonable functionality even if SHELL points to
command.com.
If SHELL points to a Unix-style shell, make on MS-DOS
additionally checks whether that shell can indeed be found; if not, it
ignores the line that sets SHELL. In MS-DOS, GNU make
searches for the shell in the following places:
SHELL. For
example, if the makefile specifies `SHELL = /bin/sh', make
will look in the directory /bin on the current drive.
PATH variable, in order.
In every directory it examines, make will first look for the
specific file (sh in the example above). If this is not found,
it will also look in that directory for that file with one of the known
extensions which identify executable files. For example .exe,
.com, .bat, .btm, .sh, and some others.
If any of these attempts is successful, the value of SHELL will
be set to the full pathname of the shell as found. However, if none of
these is found, the value of SHELL will not be changed, and thus
the line that sets it will be effectively ignored. This is so
make will only support features specific to a Unix-style shell if
such a shell is actually installed on the system where make runs.
Note that this extended search for the shell is limited to the cases
where SHELL is set from the Makefile; if it is set in the
environment or command line, you are expected to set it to the full
pathname of the shell, exactly as things are on Unix.
The effect of the above DOS-specific processing is that a Makefile that
contains `SHELL = /bin/sh' (as many Unix makefiles do), will work
on MS-DOS unaltered if you have e.g. sh.exe installed in some
directory along your PATH.
GNU make knows how to execute several commands at once.
Normally, make will execute only one command at a time, waiting
for it to finish before executing the next. However, the `-j' or
`--jobs' option tells make to execute many commands
simultaneously.
On MS-DOS, the `-j' option has no effect, since that system doesn't support multi-processing.
If the `-j' option is followed by an integer, this is the number of commands to execute at once; this is called the number of job slots. If there is nothing looking like an integer after the `-j' option, there is no limit on the number of job slots. The default number of job slots is one, which means serial execution (one thing at a time).
One unpleasant consequence of running several commands simultaneously is that output generated by the commands appears whenever each command sends it, so messages from different commands may be interspersed.
Another problem is that two processes cannot both take input from the
same device; so to make sure that only one command tries to take input
from the terminal at once, make will invalidate the standard
input streams of all but one running command. This means that
attempting to read from standard input will usually be a fatal error (a
`Broken pipe' signal) for most child processes if there are
several.
It is unpredictable which command will have a valid standard input stream
(which will come from the terminal, or wherever you redirect the standard
input of make). The first command run will always get it first, and
the first command started after that one finishes will get it next, and so
on.
We will change how this aspect of make works if we find a better
alternative. In the mean time, you should not rely on any command using
standard input at all if you are using the parallel execution feature; but
if you are not using this feature, then standard input works normally in
all commands.
Finally, handling recursive make invocations raises issues. For
more information on this, see
Communicating Options to a Sub-make.
If a command fails (is killed by a signal or exits with a nonzero
status), and errors are not ignored for that command
(see Errors in Commands),
the remaining command lines to remake the same target will not be run.
If a command fails and the `-k' or `--keep-going'
option was not given
(see Summary of Options),
make aborts execution. If make
terminates for any reason (including a signal) with child processes
running, it waits for them to finish before actually exiting.
When the system is heavily loaded, you will probably want to run fewer jobs
than when it is lightly loaded. You can use the `-l' option to tell
make to limit the number of jobs to run at once, based on the load
average. The `-l' or `--max-load'
option is followed by a floating-point number. For
example,
-l 2.5
will not let make start more than one job if the load average is
above 2.5. The `-l' option with no following number removes the
load limit, if one was given with a previous `-l' option.
More precisely, when make goes to start up a job, and it already has
at least one job running, it checks the current load average; if it is not
lower than the limit given with `-l', make waits until the load
average goes below that limit, or until all the other jobs finish.
By default, there is no load limit.
After each shell command returns, make looks at its exit status.
If the command completed successfully, the next command line is executed
in a new shell; after the last command line is finished, the rule is
finished.
If there is an error (the exit status is nonzero), make gives up on
the current rule, and perhaps on all rules.
Sometimes the failure of a certain command does not indicate a problem.
For example, you may use the mkdir command to ensure that a
directory exists. If the directory already exists, mkdir will
report an error, but you probably want make to continue regardless.
To ignore errors in a command line, write a `-' at the beginning of the line's text (after the initial tab). The `-' is discarded before the command is passed to the shell for execution.
For example,
clean:
-rm -f *.o
This causes rm to continue even if it is unable to remove a file.
When you run make with the `-i' or `--ignore-errors'
flag, errors are ignored in all commands of all rules. A rule in the
makefile for the special target .IGNORE has the same effect, if
there are no prerequisites. These ways of ignoring errors are obsolete
because `-' is more flexible.
When errors are to be ignored, because of either a `-' or the
`-i' flag, make treats an error return just like success,
except that it prints out a message that tells you the status code
the command exited with, and says that the error has been ignored.
When an error happens that make has not been told to ignore,
it implies that the current target cannot be correctly remade, and neither
can any other that depends on it either directly or indirectly. No further
commands will be executed for these targets, since their preconditions
have not been achieved.
Normally make gives up immediately in this circumstance, returning a
nonzero status. However, if the `-k' or `--keep-going'
flag is specified, make
continues to consider the other prerequisites of the pending targets,
remaking them if necessary, before it gives up and returns nonzero status.
For example, after an error in compiling one object file, `make -k'
will continue compiling other object files even though it already knows
that linking them will be impossible. See Summary of Options.
The usual behavior assumes that your purpose is to get the specified
targets up to date; once make learns that this is impossible, it
might as well report the failure immediately. The `-k' option says
that the real purpose is to test as many of the changes made in the
program as possible, perhaps to find several independent problems so
that you can correct them all before the next attempt to compile. This
is why Emacs' compile command passes the `-k' flag by
default.
Usually when a command fails, if it has changed the target file at all,
the file is corrupted and cannot be used—or at least it is not
completely updated. Yet the file's time stamp says that it is now up to
date, so the next time make runs, it will not try to update that
file. The situation is just the same as when the command is killed by a
signal; see Interrupts. So generally the right thing to do is to
delete the target file if the command fails after beginning to change
the file. make will do this if .DELETE_ON_ERROR appears
as a target. This is almost always what you want make to do, but
it is not historical practice; so for compatibility, you must explicitly
request it.
make
If make gets a fatal signal while a command is executing, it may
delete the target file that the command was supposed to update. This is
done if the target file's last-modification time has changed since
make first checked it.
The purpose of deleting the target is to make sure that it is remade from
scratch when make is next run. Why is this? Suppose you type
Ctrl-c while a compiler is running, and it has begun to write an
object file foo.o. The Ctrl-c kills the compiler, resulting
in an incomplete file whose last-modification time is newer than the source
file foo.c. But make also receives the Ctrl-c signal
and deletes this incomplete file. If make did not do this, the next
invocation of make would think that foo.o did not require
updating—resulting in a strange error message from the linker when it
tries to link an object file half of which is missing.
You can prevent the deletion of a target file in this way by making the
special target .PRECIOUS depend on it. Before remaking a target,
make checks to see whether it appears on the prerequisites of
.PRECIOUS, and thereby decides whether the target should be deleted
if a signal happens. Some reasons why you might do this are that the
target is updated in some atomic fashion, or exists only to record a
modification-time (its contents do not matter), or must exist at all
times to prevent other sorts of trouble.
make
Recursive use of make means using make as a command in a
makefile. This technique is useful when you want separate makefiles for
various subsystems that compose a larger system. For example, suppose you
have a subdirectory subdir which has its own makefile, and you would
like the containing directory's makefile to run make on the
subdirectory. You can do it by writing this:
subsystem:
cd subdir && $(MAKE)
or, equivalently, this (see Summary of Options):
subsystem:
$(MAKE) -C subdir
You can write recursive make commands just by copying this example,
but there are many things to know about how they work and why, and about
how the sub-make relates to the top-level make. You may
also find it useful to declare targets that invoke recursive
make commands as `.PHONY' (for more discussion on when
this is useful, see Phony Targets).
For your convenience, when GNU make starts (after it has
processed any -C options) it sets the variable CURDIR to
the pathname of the current working directory. This value is never
touched by make again: in particular note that if you include
files from other directories the value of CURDIR does not
change. The value has the same precedence it would have if it were
set in the makefile (by default, an environment variable CURDIR
will not override this value). Note that setting this variable has no
impact on the operation of make (it does not cause make
to change its working directory, for example).
MAKE Variable Works
Recursive make commands should always use the variable MAKE,
not the explicit command name `make', as shown here:
subsystem:
cd subdir && $(MAKE)
The value of this variable is the file name with which make was
invoked. If this file name was /bin/make, then the command executed
is `cd subdir && /bin/make'. If you use a special version of
make to run the top-level makefile, the same special version will be
executed for recursive invocations.
As a special feature, using the variable MAKE in the commands of
a rule alters the effects of the `-t' (`--touch'), `-n'
(`--just-print'), or `-q' (`--question') option.
Using the MAKE variable has the same effect as using a `+'
character at the beginning of the command line. See Instead of Executing the Commands. This special feature
is only enabled if the MAKE variable appears directly in the
command script: it does not apply if the MAKE variable is
referenced through expansion of another variable. In the latter case
you must use the `+' token to get these special effects.
Consider the command `make -t' in the above example. (The
`-t' option marks targets as up to date without actually running
any commands; see Instead of Execution.) Following the usual
definition of `-t', a `make -t' command in the example would
create a file named subsystem and do nothing else. What you
really want it to do is run `cd subdir && make -t'; but that would
require executing the command, and `-t' says not to execute
commands.
The special feature makes this do what you want: whenever a command
line of a rule contains the variable MAKE, the flags `-t',
`-n' and `-q' do not apply to that line. Command lines
containing MAKE are executed normally despite the presence of a
flag that causes most commands not to be run. The usual
MAKEFLAGS mechanism passes the flags to the sub-make
(see Communicating Options to a Sub-make), so your request to touch the files, or print the
commands, is propagated to the subsystem.
make
Variable values of the top-level make can be passed to the
sub-make through the environment by explicit request. These
variables are defined in the sub-make as defaults, but do not
override what is specified in the makefile used by the sub-make
makefile unless you use the `-e' switch (see Summary of Options).
To pass down, or export, a variable, make adds the variable
and its value to the environment for running each command. The
sub-make, in turn, uses the environment to initialize its table
of variable values. See Variables from the Environment.
Except by explicit request, make exports a variable only if it
is either defined in the environment initially or set on the command
line, and if its name consists only of letters, numbers, and underscores.
Some shells cannot cope with environment variable names consisting of
characters other than letters, numbers, and underscores.
The value of the make variable SHELL is not exported.
Instead, the value of the SHELL variable from the invoking
environment is passed to the sub-make. You can force
make to export its value for SHELL by using the
export directive, described below. See Choosing the Shell.
The special variable MAKEFLAGS is always exported (unless you
unexport it). MAKEFILES is exported if you set it to anything.
make automatically passes down variable values that were defined
on the command line, by putting them in the MAKEFLAGS variable.
See Options/Recursion.
Variables are not normally passed down if they were created by
default by make (see Variables Used by Implicit Rules). The sub-make will define these for
itself.
If you want to export specific variables to a sub-make, use the
export directive, like this:
export variable ...
If you want to prevent a variable from being exported, use the
unexport directive, like this:
unexport variable ...
In both of these forms, the arguments to export and
unexport are expanded, and so could be variables or functions
which expand to a (list of) variable names to be (un)exported.
As a convenience, you can define a variable and export it at the same time by doing:
export variable = value
has the same result as:
variable = value
export variable
and
export variable := value
has the same result as:
variable := value
export variable
Likewise,
export variable += value
is just like:
variable += value
export variable
See Appending More Text to Variables.
You may notice that the export and unexport directives
work in make in the same way they work in the shell, sh.
If you want all variables to be exported by default, you can use
export by itself:
export
This tells make that variables which are not explicitly mentioned
in an export or unexport directive should be exported.
Any variable given in an unexport directive will still not
be exported. If you use export by itself to export variables by
default, variables whose names contain characters other than
alphanumerics and underscores will not be exported unless specifically
mentioned in an export directive.
The behavior elicited by an export directive by itself was the
default in older versions of GNU make. If your makefiles depend
on this behavior and you want to be compatible with old versions of
make, you can write a rule for the special target
.EXPORT_ALL_VARIABLES instead of using the export directive.
This will be ignored by old makes, while the export
directive will cause a syntax error.
Likewise, you can use unexport by itself to tell make
not to export variables by default. Since this is the default
behavior, you would only need to do this if export had been used
by itself earlier (in an included makefile, perhaps). You
cannot use export and unexport by themselves to
have variables exported for some commands and not for others. The last
export or unexport directive that appears by itself
determines the behavior for the entire run of make.
As a special feature, the variable MAKELEVEL is changed when it
is passed down from level to level. This variable's value is a string
which is the depth of the level as a decimal number. The value is
`0' for the top-level make; `1' for a sub-make,
`2' for a sub-sub-make, and so on. The incrementation
happens when make sets up the environment for a command.
The main use of MAKELEVEL is to test it in a conditional
directive (see Conditional Parts of Makefiles); this
way you can write a makefile that behaves one way if run recursively and
another way if run directly by you.
You can use the variable MAKEFILES to cause all sub-make
commands to use additional makefiles. The value of MAKEFILES is
a whitespace-separated list of file names. This variable, if defined in
the outer-level makefile, is passed down through the environment; then
it serves as a list of extra makefiles for the sub-make to read
before the usual or specified ones. See The Variable MAKEFILES.
make
Flags such as `-s' and `-k' are passed automatically to the
sub-make through the variable MAKEFLAGS. This variable is
set up automatically by make to contain the flag letters that
make received. Thus, if you do `make -ks' then
MAKEFLAGS gets the value `ks'.
As a consequence, every sub-make gets a value for MAKEFLAGS
in its environment. In response, it takes the flags from that value and
processes them as if they had been given as arguments.
See Summary of Options.
Likewise variables defined on the command line are passed to the
sub-make through MAKEFLAGS. Words in the value of
MAKEFLAGS that contain `=', make treats as variable
definitions just as if they appeared on the command line.
See Overriding Variables.
The options `-C', `-f', `-o', and `-W' are not put
into MAKEFLAGS; these options are not passed down.
The `-j' option is a special case (see Parallel Execution).
If you set it to some numeric value `N' and your operating system
supports it (most any UNIX system will; others typically won't), the
parent make and all the sub-makes will communicate to
ensure that there are only `N' jobs running at the same time
between them all. Note that any job that is marked recursive
(see Instead of Executing the Commands)
doesn't count against the total jobs (otherwise we could get `N'
sub-makes running and have no slots left over for any real work!)
If your operating system doesn't support the above communication, then
`-j 1' is always put into MAKEFLAGS instead of the value you
specified. This is because if the `-j' option were passed down
to sub-makes, you would get many more jobs running in parallel
than you asked for. If you give `-j' with no numeric argument,
meaning to run as many jobs as possible in parallel, this is passed
down, since multiple infinities are no more than one.
If you do not want to pass the other flags down, you must change the
value of MAKEFLAGS, like this:
subsystem:
cd subdir && $(MAKE) MAKEFLAGS=
The command line variable definitions really appear in the variable
MAKEOVERRIDES, and MAKEFLAGS contains a reference to this
variable. If you do want to pass flags down normally, but don't want to
pass down the command line variable definitions, you can reset
MAKEOVERRIDES to empty, like this:
MAKEOVERRIDES =
This is not usually useful to do. However, some systems have a small
fixed limit on the size of the environment, and putting so much
information into the value of MAKEFLAGS can exceed it. If you
see the error message `Arg list too long', this may be the problem.
(For strict compliance with POSIX.2, changing MAKEOVERRIDES does
not affect MAKEFLAGS if the special target `.POSIX' appears
in the makefile. You probably do not care about this.)
A similar variable MFLAGS exists also, for historical
compatibility. It has the same value as MAKEFLAGS except that it
does not contain the command line variable definitions, and it always
begins with a hyphen unless it is empty (MAKEFLAGS begins with a
hyphen only when it begins with an option that has no single-letter
version, such as `--warn-undefined-variables'). MFLAGS was
traditionally used explicitly in the recursive make command, like
this:
subsystem:
cd subdir && $(MAKE) $(MFLAGS)
but now MAKEFLAGS makes this usage redundant. If you want your
makefiles to be compatible with old make programs, use this
technique; it will work fine with more modern make versions too.
The MAKEFLAGS variable can also be useful if you want to have
certain options, such as `-k' (see Summary of Options), set each time you run make. You simply put a value for
MAKEFLAGS in your environment. You can also set MAKEFLAGS in
a makefile, to specify additional flags that should also be in effect for
that makefile. (Note that you cannot use MFLAGS this way. That
variable is set only for compatibility; make does not interpret a
value you set for it in any way.)
When make interprets the value of MAKEFLAGS (either from the
environment or from a makefile), it first prepends a hyphen if the value
does not already begin with one. Then it chops the value into words
separated by blanks, and parses these words as if they were options given
on the command line (except that `-C', `-f', `-h',
`-o', `-W', and their long-named versions are ignored; and there
is no error for an invalid option).
If you do put MAKEFLAGS in your environment, you should be sure not
to include any options that will drastically affect the actions of
make and undermine the purpose of makefiles and of make
itself. For instance, the `-t', `-n', and `-q' options, if
put in one of these variables, could have disastrous consequences and would
certainly have at least surprising and probably annoying effects.
If you use several levels of recursive make invocations, the
`-w' or `--print-directory' option can make the output a
lot easier to understand by showing each directory as make
starts processing it and as make finishes processing it. For
example, if `make -w' is run in the directory /u/gnu/make,
make will print a line of the form:
make: Entering directory `/u/gnu/make'.
before doing anything else, and a line of the form:
make: Leaving directory `/u/gnu/make'.
when processing is completed.
Normally, you do not need to specify this option because `make'
does it for you: `-w' is turned on automatically when you use the
`-C' option, and in sub-makes. make will not
automatically turn on `-w' if you also use `-s', which says to
be silent, or if you use `--no-print-directory' to explicitly
disable it.
When the same sequence of commands is useful in making various targets, you
can define it as a canned sequence with the define directive, and
refer to the canned sequence from the rules for those targets. The canned
sequence is actually a variable, so the name must not conflict with other
variable names.
Here is an example of defining a canned sequence of commands:
define run-yacc
yacc $(firstword $^)
mv y.tab.c $@
endef
Here run-yacc is the name of the variable being defined;
endef marks the end of the definition; the lines in between are the
commands. The define directive does not expand variable references
and function calls in the canned sequence; the `$' characters,
parentheses, variable names, and so on, all become part of the value of the
variable you are defining.
See Defining Variables Verbatim,
for a complete explanation of define.
The first command in this example runs Yacc on the first prerequisite of whichever rule uses the canned sequence. The output file from Yacc is always named y.tab.c. The second command moves the output to the rule's target file name.
To use the canned sequence, substitute the variable into the commands of a
rule. You can substitute it like any other variable
(see Basics of Variable References).
Because variables defined by define are recursively expanded
variables, all the variable references you wrote inside the define
are expanded now. For example:
foo.c : foo.y
$(run-yacc)
`foo.y' will be substituted for the variable `$^' when it occurs in
run-yacc's value, and `foo.c' for `$@'.
This is a realistic example, but this particular one is not needed in
practice because make has an implicit rule to figure out these
commands based on the file names involved
(see Using Implicit Rules).
In command execution, each line of a canned sequence is treated just as
if the line appeared on its own in the rule, preceded by a tab. In
particular, make invokes a separate subshell for each line. You
can use the special prefix characters that affect command lines
(`@', `-', and `+') on each line of a canned sequence.
See Writing the Commands in Rules.
For example, using this canned sequence:
define frobnicate
@echo "frobnicating target $@"
frob-step-1 $< -o $@-step-1
frob-step-2 $@-step-1 -o $@
endef
make will not echo the first line, the echo command.
But it will echo the following two command lines.
On the other hand, prefix characters on the command line that refers to a canned sequence apply to every line in the sequence. So the rule:
frob.out: frob.in
@$(frobnicate)
does not echo any commands. (See Command Echoing, for a full explanation of `@'.)
It is sometimes useful to define commands which do nothing. This is done simply by giving a command that consists of nothing but whitespace. For example:
target: ;
defines an empty command string for target. You could also use a line beginning with a tab character to define an empty command string, but this would be confusing because such a line looks empty.
You may be wondering why you would want to define a command string that
does nothing. The only reason this is useful is to prevent a target
from getting implicit commands (from implicit rules or the
.DEFAULT special target; see Implicit Rules and
see Defining Last-Resort Default Rules).
You may be inclined to define empty command strings for targets that are not actual files, but only exist so that their prerequisites can be remade. However, this is not the best way to do that, because the prerequisites may not be remade properly if the target file actually does exist. See Phony Targets, for a better way to do this.
A variable is a name defined in a makefile to represent a string
of text, called the variable's value. These values are
substituted by explicit request into targets, prerequisites, commands,
and other parts of the makefile. (In some other versions of make,
variables are called macros.)
Variables and functions in all parts of a makefile are expanded when
read, except for the shell commands in rules, the right-hand sides of
variable definitions using `=', and the bodies of variable
definitions using the define directive.
Variables can represent lists of file names, options to pass to compilers, programs to run, directories to look in for source files, directories to write output in, or anything else you can imagine.
A variable name may be any sequence of characters not containing `:',
`#', `=', or leading or trailing whitespace. However,
variable names containing characters other than letters, numbers, and
underscores should be avoided, as they may be given special meanings in the
future, and with some shells they cannot be passed through the environment to a
sub-make
(see Communicating Variables to a Sub-make).
Variable names are case-sensitive. The names `foo', `FOO', and `Foo' all refer to different variables.
It is traditional to use upper case letters in variable names, but we recommend using lower case letters for variable names that serve internal purposes in the makefile, and reserving upper case for parameters that control implicit rules or for parameters that the user should override with command options (see Overriding Variables).
A few variables have names that are a single punctuation character or just a few characters. These are the automatic variables, and they have particular specialized uses. See Automatic Variables.
To substitute a variable's value, write a dollar sign followed by the name
of the variable in parentheses or braces: either `$(foo)' or
`${foo}' is a valid reference to the variable foo. This
special significance of `$' is why you must write `$$' to have
the effect of a single dollar sign in a file name or command.
Variable references can be used in any context: targets, prerequisites, commands, most directives, and new variable values. Here is an example of a common case, where a variable holds the names of all the object files in a program:
objects = program.o foo.o utils.o
program : $(objects)
cc -o program $(objects)
$(objects) : defs.h
Variable references work by strict textual substitution. Thus, the rule
foo = c
prog.o : prog.$(foo)
$(foo)$(foo) -$(foo) prog.$(foo)
could be used to compile a C program prog.c. Since spaces before
the variable value are ignored in variable assignments, the value of
foo is precisely `c'. (Don't actually write your makefiles
this way!)
A dollar sign followed by a character other than a dollar sign,
open-parenthesis or open-brace treats that single character as the
variable name. Thus, you could reference the variable x with
`$x'. However, this practice is strongly discouraged, except in
the case of the automatic variables (see Automatic Variables).
There are two ways that a variable in GNU make can have a value;
we call them the two flavors of variables. The two flavors are
distinguished in how they are defined and in what they do when expanded.
The first flavor of variable is a recursively expanded variable.
Variables of this sort are defined by lines using `='
(see Setting Variables) or by the define directive
(see Defining Variables Verbatim). The value you specify
is installed verbatim; if it contains references to other variables,
these references are expanded whenever this variable is substituted (in
the course of expanding some other string). When this happens, it is
called recursive expansion.
For example,
foo = $(bar)
bar = $(ugh)
ugh = Huh?
all:;echo $(foo)
will echo `Huh?': `$(foo)' expands to `$(bar)' which expands to `$(ugh)' which finally expands to `Huh?'.
This flavor of variable is the only sort supported by other versions of
make. It has its advantages and its disadvantages. An advantage
(most would say) is that:
CFLAGS = $(include_dirs) -O
include_dirs = -Ifoo -Ibar
will do what was intended: when `CFLAGS' is expanded in a command, it will expand to `-Ifoo -Ibar -O'. A major disadvantage is that you cannot append something on the end of a variable, as in
CFLAGS = $(CFLAGS) -O
because it will cause an infinite loop in the variable expansion.
(Actually make detects the infinite loop and reports an error.)
Another disadvantage is that any functions
(see Functions for Transforming Text)
referenced in the definition will be executed every time the variable is
expanded. This makes make run slower; worse, it causes the
wildcard and shell functions to give unpredictable results
because you cannot easily control when they are called, or even how many
times.
To avoid all the problems and inconveniences of recursively expanded variables, there is another flavor: simply expanded variables.
Simply expanded variables are defined by lines using `:=' (see Setting Variables). The value of a simply expanded variable is scanned once and for all, expanding any references to other variables and functions, when the variable is defined. The actual value of the simply expanded variable is the result of expanding the text that you write. It does not contain any references to other variables; it contains their values as of the time this variable was defined. Therefore,
x := foo
y := $(x) bar
x := later
is equivalent to
y := foo bar
x := later
When a simply expanded variable is referenced, its value is substituted verbatim.
Here is a somewhat more complicated example, illustrating the use of
`:=' in conjunction with the shell function.
(See The shell Function.) This example
also shows use of the variable MAKELEVEL, which is changed
when it is passed down from level to level.
(See Communicating Variables to a Sub-make, for information about MAKELEVEL.)
ifeq (0,${MAKELEVEL})
whoami := $(shell whoami)
host-type := $(shell arch)
MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}
endif
An advantage of this use of `:=' is that a typical `descend into a directory' command then looks like this:
${subdirs}:
${MAKE} -C $@ all
Simply expanded variables generally make complicated makefile programming more predictable because they work like variables in most programming languages. They allow you to redefine a variable using its own value (or its value processed in some way by one of the expansion functions) and to use the expansion functions much more efficiently (see Functions for Transforming Text).
You can also use them to introduce controlled leading whitespace into variable values. Leading whitespace characters are discarded from your input before substitution of variable references and function calls; this means you can include leading spaces in a variable value by protecting them with variable references, like this:
nullstring :=
space := $(nullstring) # end of the line
Here the value of the variable space is precisely one space. The
comment `# end of the line' is included here just for clarity.
Since trailing space characters are not stripped from variable
values, just a space at the end of the line would have the same effect
(but be rather hard to read). If you put whitespace at the end of a
variable value, it is a good idea to put a comment like that at the end
of the line to make your intent clear. Conversely, if you do not
want any whitespace characters at the end of your variable value, you
must remember not to put a random comment on the end of the line after
some whitespace, such as this:
dir := /foo/bar # directory to put the frobs in
Here the value of the variable dir is `/foo/bar '
(with four trailing spaces), which was probably not the intention.
(Imagine something like `$(dir)/file' with this definition!)
There is another assignment operator for variables, `?='. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement:
FOO ?= bar
is exactly equivalent to this
(see The origin Function):
ifeq ($(origin FOO), undefined)
FOO = bar
endif
Note that a variable set to an empty value is still defined, so `?=' will not set that variable.
This section describes some advanced features you can use to reference variables in more flexible ways.
A substitution reference substitutes the value of a variable with alterations that you specify. It has the form `$(var:a=b)' (or `${var:a=b}') and its meaning is to take the value of the variable var, replace every a at the end of a word with b in that value, and substitute the resulting string.
When we say “at the end of a word”, we mean that a must appear either followed by whitespace or at the end of the value in order to be replaced; other occurrences of a in the value are unaltered. For example:
foo := a.o b.o c.o
bar := $(foo:.o=.c)
sets `bar' to `a.c b.c c.c'. See Setting Variables.
A substitution reference is actually an abbreviation for use of the
patsubst expansion function (see Functions for String Substitution and Analysis). We provide
substitution references as well as patsubst for compatibility with
other implementations of make.
Another type of substitution reference lets you use the full power of
the patsubst function. It has the same form
`$(var:a=b)' described above, except that now
a must contain a single `%' character. This case is
equivalent to `$(patsubst a,b,$(var))'.
See Functions for String Substitution and Analysis,
for a description of the patsubst function.
For example:
foo := a.o b.o c.o bar := $(foo:%.o=%.c)
sets `bar' to `a.c b.c c.c'.
Computed variable names are a complicated concept needed only for sophisticated makefile programming. For most purposes you need not consider them, except to know that making a variable with a dollar sign in its name might have strange results. However, if you are the type that wants to understand everything, or you are actually interested in what they do, read on.
Variables may be referenced inside the name of a variable. This is called a computed variable name or a nested variable reference. For example,
x = y
y = z
a := $($(x))
defines a as `z': the `$(x)' inside `$($(x))' expands
to `y', so `$($(x))' expands to `$(y)' which in turn expands
to `z'. Here the name of the variable to reference is not stated
explicitly; it is computed by expansion of `$(x)'. The reference
`$(x)' here is nested within the outer variable reference.
The previous example shows two levels of nesting, but any number of levels is possible. For example, here are three levels:
x = y
y = z
z = u
a := $($($(x)))
Here the innermost `$(x)' expands to `y', so `$($(x))' expands to `$(y)' which in turn expands to `z'; now we have `$(z)', which becomes `u'.
References to recursively-expanded variables within a variable name are reexpanded in the usual fashion. For example:
x = $(y)
y = z
z = Hello
a := $($(x))
defines a as `Hello': `$($(x))' becomes `$($(y))'
which becomes `$(z)' which becomes `Hello'.
Nested variable references can also contain modified references and
function invocations (see Functions for Transforming Text),
just like any other reference.
For example, using the subst function
(see Functions for String Substitution and Analysis):
x = variable1
variable2 := Hello
y = $(subst 1,2,$(x))
z = y
a := $($($(z)))
eventually defines a as `Hello'. It is doubtful that anyone
would ever want to write a nested reference as convoluted as this one, but
it works: `$($($(z)))' expands to `$($(y))' which becomes
`$($(subst 1,2,$(x)))'. This gets the value `variable1' from
x and changes it by substitution to `variable2', so that the
entire string becomes `$(variable2)', a simple variable reference
whose value is `Hello'.
A computed variable name need not consist entirely of a single variable reference. It can contain several variable references, as well as some invariant text. For example,
a_dirs := dira dirb
1_dirs := dir1 dir2
a_files := filea fileb
1_files := file1 file2
ifeq "$(use_a)" "yes"
a1 := a
else
a1 := 1
endif
ifeq "$(use_dirs)" "yes"
df := dirs
else
df := files
endif
dirs := $($(a1)_$(df))
will give dirs the same value as a_dirs, 1_dirs,
a_files or 1_files depending on the settings of use_a
and use_dirs.
Computed variable names can also be used in substitution references:
a_objects := a.o b.o c.o
1_objects := 1.o 2.o 3.o
sources := $($(a1)_objects:.o=.c)
defines sources as either `a.c b.c c.c' or `1.c 2.c 3.c',
depending on the value of a1.
The only restriction on this sort of use of nested variable references is that they cannot specify part of the name of a function to be called. This is because the test for a recognized function name is done before the expansion of nested references. For example,
ifdef do_sort
func := sort
else
func := strip
endif
bar := a d b g q c
foo := $($(func) $(bar))
attempts to give `foo' the value of the variable `sort a d b g
q c' or `strip a d b g q c', rather than giving `a d b g q c'
as the argument to either the sort or the strip function.
This restriction could be removed in the future if that change is shown
to be a good idea.
You can also use computed variable names in the left-hand side of a
variable assignment, or in a define directive, as in:
dir = foo
$(dir)_sources := $(wildcard $(dir)/*.c)
define $(dir)_print
lpr $($(dir)_sources)
endef
This example defines the variables `dir', `foo_sources', and `foo_print'.
Note that nested variable references are quite different from recursively expanded variables (see The Two Flavors of Variables), though both are used together in complex ways when doing makefile programming.
Variables can get values in several different ways:
make.
See Overriding Variables.
make variables.
See Variables from the Environment.
To set a variable from the makefile, write a line starting with the variable name followed by `=' or `:='. Whatever follows the `=' or `:=' on the line becomes the value. For example,
objects = main.o foo.o bar.o utils.o
defines a variable named objects. Whitespace around the variable
name and immediately after the `=' is ignored.
Variables defined with `=' are recursively expanded variables. Variables defined with `:=' are simply expanded variables; these definitions can contain variable references which will be expanded before the definition is made. See The Two Flavors of Variables.
The variable name may contain function and variable references, which are expanded when the line is read to find the actual variable name to use.
There is no limit on the length of the value of a variable except the
amount of swapping space on the computer. When a variable definition is
long, it is a good idea to break it into several lines by inserting
backslash-newline at convenient places in the definition. This will not
affect the functioning of make, but it will make the makefile easier
to read.
Most variable names are considered to have the empty string as a value if you have never set them. Several variables have built-in initial values that are not empty, but you can set them in the usual ways (see Variables Used by Implicit Rules). Several special variables are set automatically to a new value for each rule; these are called the automatic variables (see Automatic Variables).
If you'd like a variable to be set to a value only if it's not already
set, then you can use the shorthand operator `?=' instead of
`='. These two settings of the variable `FOO' are identical
(see The origin Function):
FOO ?= bar
and
ifeq ($(origin FOO), undefined)
FOO = bar
endif
Often it is useful to add more text to the value of a variable already defined. You do this with a line containing `+=', like this:
objects += another.o
This takes the value of the variable objects, and adds the text
`another.o' to it (preceded by a single space). Thus:
objects = main.o foo.o bar.o utils.o
objects += another.o
sets objects to `main.o foo.o bar.o utils.o another.o'.
Using `+=' is similar to:
objects = main.o foo.o bar.o utils.o
objects := $(objects) another.o
but differs in ways that become important when you use more complex values.
When the variable in question has not been defined before, `+=' acts just like normal `=': it defines a recursively-expanded variable. However, when there is a previous definition, exactly what `+=' does depends on what flavor of variable you defined originally. See The Two Flavors of Variables, for an explanation of the two flavors of variables.
When you add to a variable's value with `+=', make acts
essentially as if you had included the extra text in the initial
definition of the variable. If you defined it first with `:=',
making it a simply-expanded variable, `+=' adds to that
simply-expanded definition, and expands the new text before appending it
to the old value just as `:=' does
(see Setting Variables, for a full explanation of `:=').
In fact,
variable := value
variable += more
is exactly equivalent to:
variable := value
variable := $(variable) more
On the other hand, when you use `+=' with a variable that you defined
first to be recursively-expanded using plain `=', make does
something a bit different. Recall that when you define a
recursively-expanded variable, make does not expand the value you set
for variable and function references immediately. Instead it stores the text
verbatim, and saves these variable and function references to be expanded
later, when you refer to the new variable (see The Two Flavors of Variables). When you use `+=' on a recursively-expanded variable,
it is this unexpanded text to which make appends the new text you
specify.
variable = value
variable += more
is roughly equivalent to:
temp = value
variable = $(temp) more
except that of course it never defines a variable called temp.
The importance of this comes when the variable's old value contains
variable references. Take this common example:
CFLAGS = $(includes) -O
...
CFLAGS += -pg # enable profiling
The first line defines the CFLAGS variable with a reference to another
variable, includes. (CFLAGS is used by the rules for C
compilation; see Catalogue of Implicit Rules.)
Using `=' for the definition makes CFLAGS a recursively-expanded
variable, meaning `$(includes) -O' is not expanded when
make processes the definition of CFLAGS. Thus, includes
need not be defined yet for its value to take effect. It only has to be
defined before any reference to CFLAGS. If we tried to append to the
value of CFLAGS without using `+=', we might do it like this:
CFLAGS := $(CFLAGS) -pg # enable profiling
This is pretty close, but not quite what we want. Using `:='
redefines CFLAGS as a simply-expanded variable; this means
make expands the text `$(CFLAGS) -pg' before setting the
variable. If includes is not yet defined, we get ` -O -pg', and a later definition of includes will have no effect.
Conversely, by using `+=' we set CFLAGS to the
unexpanded value `$(includes) -O -pg'. Thus we preserve
the reference to includes, so if that variable gets defined at
any later point, a reference like `$(CFLAGS)' still uses its
value.
override Directive
If a variable has been set with a command argument
(see Overriding Variables),
then ordinary assignments in the makefile are ignored. If you want to set
the variable in the makefile even though it was set with a command
argument, you can use an override directive, which is a line that
looks like this:
override variable = value
or
override variable := value
To append more text to a variable defined on the command line, use:
override variable += more text
See Appending More Text to Variables.
The override directive was not invented for escalation in the war
between makefiles and command arguments. It was invented so you can alter
and add to values that the user specifies with command arguments.
For example, suppose you always want the `-g' switch when you run the
C compiler, but you would like to allow the user to specify the other
switches with a command argument just as usual. You could use this
override directive:
override CFLAGS += -g
You can also use override directives with define directives.
This is done as you might expect:
override define foo
bar
endef
See Defining Variables Verbatim.
Another way to set the value of a variable is to use the define
directive. This directive has an unusual syntax which allows newline
characters to be included in the value, which is convenient for defining
both canned sequences of commands
(see Defining Canned Command Sequences), and also
sections of makefile syntax to use with eval (see Eval Function).
The define directive is followed on the same line by the name of the
variable and nothing more. The value to give the variable appears on the
following lines. The end of the value is marked by a line containing just
the word endef. Aside from this difference in syntax, define
works just like `=': it creates a recursively-expanded variable
(see The Two Flavors of Variables).
The variable name may contain function and variable references, which
are expanded when the directive is read to find the actual variable name
to use.
You may nest define directives: make will keep track of
nested directives and report an error if they are not all properly
closed with endef. Note that lines beginning with tab
characters are considered part of a command script, so any
define or endef strings appearing on such a line will
not be considered make operators.
define two-lines
echo foo
echo $(bar)
endef
The value in an ordinary assignment cannot contain a newline; but the
newlines that separate the lines of the value in a define become
part of the variable's value (except for the final newline which precedes
the endef and is not considered part of the value).
When used in a command script, the previous example is functionally equivalent to this:
two-lines = echo foo; echo $(bar)
since two commands separated by semicolon behave much like two separate
shell commands. However, note that using two separate lines means
make will invoke the shell twice, running an independent subshell
for each line. See Command Execution.
If you want variable definitions made with define to take
precedence over command-line variable definitions, you can use the
override directive together with define:
override define two-lines
foo
$(bar)
endef
Variables in make can come from the environment in which
make is run. Every environment variable that make sees
when it starts up is transformed into a make variable with the
same name and value. However, an explicit assignment in the makefile,
or with a command argument, overrides the environment. (If the
`-e' flag is specified, then values from the environment override
assignments in the makefile. See Summary of Options. But this is not recommended practice.)
Thus, by setting the variable CFLAGS in your environment, you can
cause all C compilations in most makefiles to use the compiler switches you
prefer. This is safe for variables with standard or conventional meanings
because you know that no makefile will use them for other things. (Note
this is not totally reliable; some makefiles set CFLAGS explicitly
and therefore are not affected by the value in the environment.)
When make runs a command script, variables defined in the
makefile are placed into the environment of that command. This allows
you to pass values to sub-make invocations (see Recursive Use of make). By default, only variables that came
from the environment or the command line are passed to recursive
invocations. You can use the export directive to pass other
variables. See Communicating Variables to a Sub-make, for full details.
Other use of variables from the environment is not recommended. It is not wise for makefiles to depend for their functioning on environment variables set up outside their control, since this would cause different users to get different results from the same makefile. This is against the whole purpose of most makefiles.
Such problems would be especially likely with the variable
SHELL, which is normally present in the environment to specify
the user's choice of interactive shell. It would be very undesirable
for this choice to affect make; so, make handles the
SHELL environment variable in a special way; see Choosing the Shell.
Variable values in make are usually global; that is, they are the
same regardless of where they are evaluated (unless they're reset, of
course). One exception to that is automatic variables
(see Automatic Variables).
The other exception is target-specific variable values. This
feature allows you to define different values for the same variable,
based on the target that make is currently building. As with
automatic variables, these values are only available within the context
of a target's command script (and in other target-specific assignments).
Set a target-specific variable value like this:
target ... : variable-assignment
or like this:
target ... : override variable-assignment
or like this:
target ... : export variable-assignment
Multiple target values create a target-specific variable value for each member of the target list individually.
The variable-assignment can be any valid form of assignment; recursive (`='), static (`:='), appending (`+='), or conditional (`?='). All variables that appear within the variable-assignment are evaluated within the context of the target: thus, any previously-defined target-specific variable values will be in effect. Note that this variable is actually distinct from any “global” value: the two variables do not have to have the same flavor (recursive vs. static).
Target-specific variables have the same priority as any other makefile
variable. Variables provided on the command-line (and in the
environment if the `-e' option is in force) will take precedence.
Specifying the override directive will allow the target-specific
variable value to be preferred.
There is one more special feature of target-specific variables: when you define a target-specific variable that variable value is also in effect for all prerequisites of this target, and all their prerequisites, etc. (unless those prerequisites override that variable with their own target-specific variable value). So, for example, a statement like this:
prog : CFLAGS = -g
prog : prog.o foo.o bar.o
will set CFLAGS to `-g' in the command script for
prog, but it will also set CFLAGS to `-g' in the
command scripts that create prog.o, foo.o, and
bar.o, and any command scripts which create their
prerequisites.
Be aware that a given prerequisite will only be built once per invocation of make, at most. If the same file is a prerequisite of multiple targets, and each of those targets has a different value for the same target-specific variable, then the first target to be built will cause that prerequisite to be built and the prerequisite will inherit the target-specific value from the first target. It will ignore the target-specific values from any other targets.
In addition to target-specific variable values
(see Target-specific Variable Values), GNU
make supports pattern-specific variable values. In this form,
the variable is defined for any target that matches the pattern
specified. If a target matches more than one pattern, all the
matching pattern-specific variables are interpreted in the order in
which they were defined in the makefile, and collected together into
one set. Variables defined in this way are searched after any
target-specific variables defined explicitly for that target, and
before target-specific variables defined for the parent target.
Set a pattern-specific variable value like this:
pattern ... : variable-assignment
or like this:
pattern ... : override variable-assignment
where pattern is a %-pattern. As with target-specific variable
values, multiple pattern values create a pattern-specific variable
value for each pattern individually. The variable-assignment can
be any valid form of assignment. Any command-line variable setting will
take precedence, unless override is specified.
For example:
%.o : CFLAGS = -O
will assign CFLAGS the value of `-O' for all targets
matching the pattern %.o.
A conditional causes part of a makefile to be obeyed or ignored
depending on the values of variables. Conditionals can compare the
value of one variable to another, or the value of a variable to
a constant string. Conditionals control what make actually
“sees” in the makefile, so they cannot be used to control shell
commands at the time of execution.
The following example of a conditional tells make to use one set
of libraries if the CC variable is `gcc', and a different
set of libraries otherwise. It works by controlling which of two
command lines will be used as the command for a rule. The result is
that `CC=gcc' as an argument to make changes not only which
compiler is used but also which libraries are linked.
libs_for_gcc = -lgnu
normal_libs =
foo: $(objects)
ifeq ($(CC),gcc)
$(CC) -o foo $(objects) $(libs_for_gcc)
else
$(CC) -o foo $(objects) $(normal_libs)
endif
This conditional uses three directives: one ifeq, one else
and one endif.
The ifeq directive begins the conditional, and specifies the
condition. It contains two arguments, separated by a comma and surrounded
by parentheses. Variable substitution is performed on both arguments and
then they are compared. The lines of the makefile following the
ifeq are obeyed if the two arguments match; otherwise they are
ignored.
The else directive causes the following lines to be obeyed if the
previous conditional failed. In the example above, this means that the
second alternative linking command is used whenever the first alternative
is not used. It is optional to have an else in a conditional.
The endif directive ends the conditional. Every conditional must
end with an endif. Unconditional makefile text follows.
As this example illustrates, conditionals work at the textual level: the lines of the conditional are treated as part of the makefile, or ignored, according to the condition. This is why the larger syntactic units of the makefile, such as rules, may cross the beginning or the end of the conditional.
When the variable CC has the value `gcc', the above example has
this effect:
foo: $(objects)
$(CC) -o foo $(objects) $(libs_for_gcc)
When the variable CC has any other value, the effect is this:
foo: $(objects)
$(CC) -o foo $(objects) $(normal_libs)
Equivalent results can be obtained in another way by conditionalizing a variable assignment and then using the variable unconditionally:
libs_for_gcc = -lgnu
normal_libs =
ifeq ($(CC),gcc)
libs=$(libs_for_gcc)
else
libs=$(normal_libs)
endif
foo: $(objects)
$(CC) -o foo $(objects) $(libs)
The syntax of a simple conditional with no else is as follows:
conditional-directive
text-if-true
endif
The text-if-true may be any lines of text, to be considered as part of the makefile if the condition is true. If the condition is false, no text is used instead.
The syntax of a complex conditional is as follows:
conditional-directive
text-if-true
else
text-if-false
endif
or:
conditional-directive
text-if-one-is-true
else conditional-directive
text-if-true
else
text-if-false
endif
There can be as many “else conditional-directive”
clauses as necessary. Once a given condition is true,
text-if-true is used and no other clause is used; if no
condition is true then text-if-false is used. The
text-if-true and text-if-false can be any number of lines
of text.
The syntax of the conditional-directive is the same whether the
conditional is simple or complex; after an else or not. There
are four different directives that test different conditions. Here is
a table of them:
ifeq (arg1, arg2)ifeq 'arg1' 'arg2'ifeq "arg1" "arg2"ifeq "arg1" 'arg2'ifeq 'arg1' "arg2"Often you want to test if a variable has a non-empty value. When the
value results from complex expansions of variables and functions,
expansions you would consider empty may actually contain whitespace
characters and thus are not seen as empty. However, you can use the
strip function (see Text Functions) to avoid interpreting
whitespace as a non-empty value. For example:
ifeq ($(strip $(foo)),)
text-if-empty
endif
will evaluate text-if-empty even if the expansion of
$(foo) contains whitespace characters.
ifneq (arg1, arg2)ifneq 'arg1' 'arg2'ifneq "arg1" "arg2"ifneq "arg1" 'arg2'ifneq 'arg1' "arg2"ifdef variable-nameifdef form takes the name of a variable as its
argument, not a reference to a variable. The value of that variable
has a non-empty value, the text-if-true is effective; otherwise,
the text-if-false, if any, is effective. Variables that have
never been defined have an empty value. The text variable-name
is expanded, so it could be a variable or function that expands
to the name of a variable. For example:
bar = true
foo = bar
ifdef $(foo)
frobozz = yes
endif
The variable reference $(foo) is expanded, yielding bar,
which is considered to be the name of a variable. The variable
bar is not expanded, but its value is examined to determine if
it is non-empty.
Note that ifdef only tests whether a variable has a value. It
does not expand the variable to see if that value is nonempty.
Consequently, tests using ifdef return true for all definitions
except those like foo =. To test for an empty value, use
ifeq ($(foo),). For example,
bar =
foo = $(bar)
ifdef foo
frobozz = yes
else
frobozz = no
endif
sets `frobozz' to `yes', while:
foo =
ifdef foo
frobozz = yes
else
frobozz = no
endif
sets `frobozz' to `no'.
ifndef variable-nameifdef directive.
Extra spaces are allowed and ignored at the beginning of the conditional directive line, but a tab is not allowed. (If the line begins with a tab, it will be considered a command for a rule.) Aside from this, extra spaces or tabs may be inserted with no effect anywhere except within the directive name or within an argument. A comment starting with `#' may appear at the end of the line.
The other two directives that play a part in a conditional are else
and endif. Each of these directives is written as one word, with no
arguments. Extra spaces are allowed and ignored at the beginning of the
line, and spaces or tabs at the end. A comment starting with `#' may
appear at the end of the line.
Conditionals affect which lines of the makefile make uses. If
the condition is true, make reads the lines of the
text-if-true as part of the makefile; if the condition is false,
make ignores those lines completely. It follows that syntactic
units of the makefile, such as rules, may safely be split across the
beginning or the end of the conditional.
make evaluates conditionals when it reads a makefile.
Consequently, you cannot use automatic variables in the tests of
conditionals because they are not defined until commands are run
(see Automatic Variables).
To prevent intolerable confusion, it is not permitted to start a
conditional in one makefile and end it in another. However, you may
write an include directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.
You can write a conditional that tests make command flags such as
`-t' by using the variable MAKEFLAGS together with the
findstring function
(see Functions for String Substitution and Analysis).
This is useful when touch is not enough to make a file appear up
to date.
The findstring function determines whether one string appears as a
substring of another. If you want to test for the `-t' flag,
use `t' as the first string and the value of MAKEFLAGS as
the other.
For example, here is how to arrange to use `ranlib -t' to finish marking an archive file up to date:
archive.a: ...
ifneq (,$(findstring t,$(MAKEFLAGS)))
+touch archive.a
+ranlib -t archive.a
else
ranlib archive.a
endif
The `+' prefix marks those command lines as “recursive” so
that they will be executed despite use of the `-t' flag.
See Recursive Use of make.
Functions allow you to do text processing in the makefile to compute the files to operate on or the commands to use. You use a function in a function call, where you give the name of the function and some text (the arguments) for the function to operate on. The result of the function's processing is substituted into the makefile at the point of the call, just as a variable might be substituted.
A function call resembles a variable reference. It looks like this:
$(function arguments)
or like this:
${function arguments}
Here function is a function name; one of a short list of names
that are part of make. You can also essentially create your own
functions by using the call builtin function.
The arguments are the arguments of the function. They are separated from the function name by one or more spaces or tabs, and if there is more than one argument, then they are separated by commas. Such whitespace and commas are not part of an argument's value. The delimiters which you use to surround the function call, whether parentheses or braces, can appear in an argument only in matching pairs; the other kind of delimiters may appear singly. If the arguments themselves contain other function calls or variable references, it is wisest to use the same kind of delimiters for all the references; write `$(subst a,b,$(x))', not `$(subst a,b,${x})'. This is because it is clearer, and because only one type of delimiter is matched to find the end of the reference.
The text written for each argument is processed by substitution of variables and function calls to produce the argument value, which is the text on which the function acts. The substitution is done in the order in which the arguments appear.
Commas and unmatched parentheses or braces cannot appear in the text of an
argument as written; leading spaces cannot appear in the text of the first
argument as written. These characters can be put into the argument value
by variable substitution. First define variables comma and
space whose values are isolated comma and space characters, then
substitute these variables where such characters are wanted, like this:
comma:= ,
empty:=
space:= $(empty) $(empty)
foo:= a b c
bar:= $(subst $(space),$(comma),$(foo))
# bar is now `a,b,c'.
Here the subst function replaces each space with a comma, through
the value of foo, and substitutes the result.
Here are some functions that operate on strings:
$(subst from,to,text) $(subst ee,EE,feet on the street)
substitutes the string `fEEt on the strEEt'.
$(patsubst pattern,replacement,text)`%' characters in patsubst function invocations can be
quoted with preceding backslashes (`\'). Backslashes that would
otherwise quote `%' characters can be quoted with more backslashes.
Backslashes that quote `%' characters or other backslashes are
removed from the pattern before it is compared file names or has a stem
substituted into it. Backslashes that are not in danger of quoting
`%' characters go unmolested. For example, the pattern
the\%weird\\%pattern\\ has `the%weird\' preceding the
operative `%' character, and `pattern\\' following it. The
final two backslashes are left alone because they cannot affect any
`%' character.
Whitespace between words is folded into single space characters; leading and trailing whitespace is discarded.
For example,
$(patsubst %.c,%.o,x.c.c bar.c)
produces the value `x.c.o bar.o'.
Substitution references (see Substitution References) are a simpler way to get the effect of the patsubst
function:
$(var:pattern=replacement)
is equivalent to
$(patsubst pattern,replacement,$(var))
The second shorthand simplifies one of the most common uses of
patsubst: replacing the suffix at the end of file names.
$(var:suffix=replacement)
is equivalent to
$(patsubst %suffix,%replacement,$(var))
For example, you might have a list of object files:
objects = foo.o bar.o baz.o
To get the list of corresponding source files, you could simply write:
$(objects:.o=.c)
instead of using the general form:
$(patsubst %.o,%.c,$(objects))
$(strip string)The function strip can be very useful when used in conjunction
with conditionals. When comparing something with the empty string
`' using ifeq or ifneq, you usually want a string of
just whitespace to match the empty string (see Conditionals).
Thus, the following may fail to have the desired results:
.PHONY: all
ifneq "$(needs_made)" ""
all: $(needs_made)
else
all:;@echo 'Nothing to make!'
endif
Replacing the variable reference `$(needs_made)' with the
function call `$(strip $(needs_made))' in the ifneq
directive would make it more robust.
$(findstring find,in) $(findstring a,a b c)
$(findstring a,b c)
produce the values `a' and `' (the empty string),
respectively. See Testing Flags, for a practical application of
findstring.
$(filter pattern...,text)patsubst function above.
The filter function can be used to separate out different types
of strings (such as file names) in a variable. For example:
sources := foo.c bar.c baz.s ugh.h
foo: $(sources)
cc $(filter %.c %.s,$(sources)) -o foo
says that foo depends of foo.c, bar.c,
baz.s and ugh.h but only foo.c, bar.c and
baz.s should be specified in the command to the
compiler.
$(filter-out pattern...,text)filter
function.
For example, given:
objects=main1.o foo.o main2.o bar.o
mains=main1.o main2.o
the following generates a list which contains all the object files not in `mains':
$(filter-out $(mains),$(objects))
$(sort list) $(sort foo bar lose)
returns the value `bar foo lose'.
Incidentally, since sort removes duplicate words, you can use
it for this purpose even if you don't care about the sort order.
$(word n,text) $(word 2, foo bar baz)
returns `bar'.
$(wordlist s,e,text) $(wordlist 2, 3, foo bar baz)
returns `bar baz'.
$(words text)$(word $(words text),text).
$(firstword names...)For example,
$(firstword foo bar)
produces the result `foo'. Although $(firstword
text) is the same as $(word 1,text), the
firstword function is retained for its simplicity.
$(lastword names...)For example,
$(lastword foo bar)
produces the result `bar'. Although $(lastword
text) is the same as $(word $(words text),text),
the lastword function was added for its simplicity and better
performance.
Here is a realistic example of the use of subst and
patsubst. Suppose that a makefile uses the VPATH variable
to specify a list of directories that make should search for
prerequisite files
(see VPATH Search Path for All Prerequisites).
This example shows how to
tell the C compiler to search for header files in the same list of
directories.
The value of VPATH is a list of directories separated by colons,
such as `src:../headers'. First, the subst function is used to
change the colons to spaces:
$(subst :, ,$(VPATH))
This produces `src ../headers'. Then patsubst is used to turn
each directory name into a `-I' flag. These can be added to the
value of the variable CFLAGS, which is passed automatically to the C
compiler, like this:
override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH)))
The effect is to append the text `-Isrc -I../headers' to the
previously given value of CFLAGS. The override directive is
used so that the new value is assigned even if the previous value of
CFLAGS was specified with a command argument (see The override Directive).
Several of the built-in expansion functions relate specifically to taking apart file names or lists of file names.
Each of the following functions performs a specific transformation on a file name. The argument of the function is regarded as a series of file names, separated by whitespace. (Leading and trailing whitespace is ignored.) Each file name in the series is transformed in the same way and the results are concatenated with single spaces between them.
$(dir names...) $(dir src/foo.c hacks)
produces the result `src/ ./'.
$(notdir names...)A file name that ends with a slash becomes an empty string. This is unfortunate, because it means that the result does not always have the same number of whitespace-separated file names as the argument had; but we do not see any other valid alternative.
For example,
$(notdir src/foo.c hacks)
produces the result `foo.c hacks'.
$(suffix names...)For example,
$(suffix src/foo.c src-1.0/bar.c hacks)
produces the result `.c .c'.
$(basename names...) $(basename src/foo.c src-1.0/bar hacks)
produces the result `src/foo src-1.0/bar hacks'.
$(addsuffix suffix,names...) $(addsuffix .c,foo bar)
produces the result `foo.c bar.c'.
$(addprefix prefix,names...) $(addprefix src/,foo bar)
produces the result `src/foo src/bar'.
$(join list1,list2)For example, `$(join a b,.c .o)' produces `a.c b.o'.
Whitespace between the words in the lists is not preserved; it is replaced with a single space.
This function can merge the results of the dir and
notdir functions, to produce the original list of files which
was given to those two functions.
$(wildcard pattern)wildcard is a space-separated list of the names of existing files
that match the pattern.
See Using Wildcard Characters in File Names.
$(realpath names...). or .. components,
nor any repeated path separators (/) or symlinks. In case of a
failure the empty string is returned. Consult the realpath(3)
documentation for a list of possible failure causes.
$(abspath names...). or .. components, nor any repeated path
separators (/). Note that, in contrast to realpath
function, abspath does not resolve symlinks and does not require
the file names to refer to an existing file or directory. Use the
wildcard function to test for existence.
There are three functions that provide conditional expansion. A key aspect of these functions is that not all of the arguments are expanded initially. Only those arguments which need to be expanded, will be expanded.
$(if condition,then-part[,else-part])if function provides support for conditional expansion in a
functional context (as opposed to the GNU make makefile
conditionals such as ifeq (see Syntax of Conditionals).
The first argument, condition, first has all preceding and trailing whitespace stripped, then is expanded. If it expands to any non-empty string, then the condition is considered to be true. If it expands to an empty string, the condition is considered to be false.
If the condition is true then the second argument, then-part, is
evaluated and this is used as the result of the evaluation of the entire
if function.
If the condition is false then the third argument, else-part, is
evaluated and this is the result of the if function. If there is
no third argument, the if function evaluates to nothing (the
empty string).
Note that only one of the then-part or the else-part will be
evaluated, never both. Thus, either can contain side-effects (such as
shell function calls, etc.)
$(or condition1[,condition2[,condition3...]])or function provides a “short-circuiting” OR operation.
Each argument is expanded, in order. If an argument expands to a
non-empty string the processing stops and the result of the expansion
is that string. If, after all arguments are expanded, all of them are
false (empty), then the result of the expansion is the empty string.
$(and condition1[,condition2[,condition3...]])and function provides a “short-circuiting” AND operation.
Each argument is expanded, in order. If an argument expands to an
empty string the processing stops and the result of the expansion is
the empty string. If all arguments expand to a non-empty string then
the result of the expansion is the expansion of the last argument.
foreach Function
The foreach function is very different from other functions. It
causes one piece of text to be used repeatedly, each time with a different
substitution performed on it. It resembles the for command in the
shell sh and the foreach command in the C-shell csh.
The syntax of the foreach function is:
$(foreach var,list,text)
The first two arguments, var and list, are expanded before anything else is done; note that the last argument, text, is not expanded at the same time. Then for each word of the expanded value of list, the variable named by the expanded value of var is set to that word, and text is expanded. Presumably text contains references to that variable, so its expansion will be different each time.
The result is that text is expanded as many times as there are
whitespace-separated words in list. The multiple expansions of
text are concatenated, with spaces between them, to make the result
of foreach.
This simple example sets the variable `files' to the list of all files in the directories in the list `dirs':
dirs := a b c d
files := $(foreach dir,$(dirs),$(wildcard $(dir)/*))
Here text is `$(wildcard $(dir)/*)'. The first repetition
finds the value `a' for dir, so it produces the same result
as `$(wildcard a/*)'; the second repetition produces the result
of `$(wildcard b/*)'; and the third, that of `$(wildcard c/*)'.
This example has the same result (except for setting `dirs') as the following example:
files := $(wildcard a/* b/* c/* d/*)
When text is complicated, you can improve readability by giving it a name, with an additional variable:
find_files = $(wildcard $(dir)/*)
dirs := a b c d
files := $(foreach dir,$(dirs),$(find_files))
Here we use the variable find_files this way. We use plain `='
to define a recursively-expanding variable, so that its value contains an
actual function call to be reexpanded under the control of foreach;
a simply-expanded variable would not do, since wildcard would be
called only once at the time of defining find_files.
The foreach function has no permanent effect on the variable
var; its value and flavor after the foreach function call are
the same as they were beforehand. The other values which are taken from
list are in effect only temporarily, during the execution of
foreach. The variable var is a simply-expanded variable
during the execution of foreach. If var was undefined
before the foreach function call, it is undefined after the call.
See The Two Flavors of Variables.
You must take care when using complex variable expressions that result in variable names because many strange things are valid variable names, but are probably not what you intended. For example,
files := $(foreach Esta escrito en espanol!,b c ch,$(find_files))
might be useful if the value of find_files references the variable
whose name is `Esta escrito en espanol!' (es un nombre bastante largo,
no?), but it is more likely to be a mistake.
call Function
The call function is unique in that it can be used to create new
parameterized functions. You can write a complex expression as the
value of a variable, then use call to expand it with different
values.
The syntax of the call function is:
$(call variable,param,param,...)
When make expands this function, it assigns each param to
temporary variables $(1), $(2), etc. The variable
$(0) will contain variable. There is no maximum number of
parameter arguments. There is no minimum, either, but it doesn't make
sense to use call with no parameters.
Then variable is expanded as a make variable in the context
of these temporary assignments. Thus, any reference to $(1) in
the value of variable will resolve to the first param in the
invocation of call.
Note that variable is the name of a variable, not a reference to that variable. Therefore you would not normally use a `$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.)
If variable is the name of a builtin function, the builtin function
is always invoked (even if a make variable by that name also
exists).
The call function expands the param arguments before
assigning them to temporary variables. This means that variable
values containing references to builtin functions that have special
expansion rules, like foreach or if, may not work as you
expect.
Some examples may make this clearer.
This macro simply reverses its arguments:
reverse = $(2) $(1)
foo = $(call reverse,a,b)
Here foo will contain `b a'.
This one is slightly more interesting: it defines a macro to search for
the first instance of a program in PATH:
pathsearch = $(firstword $(wildcard $(addsuffix /$(1),$(subst :, ,$(PATH)))))
LS := $(call pathsearch,ls)
Now the variable LS contains /bin/ls or similar.
The call function can be nested. Each recursive invocation gets
its own local values for $(1), etc. that mask the values of
higher-level call. For example, here is an implementation of a
map function:
map = $(foreach a,$(2),$(call $(1),$(a)))
Now you can map a function that normally takes only one argument,
such as origin, to multiple values in one step:
o = $(call map,origin,o map MAKE)
and end up with o containing something like `file file default'.
A final caution: be careful when adding whitespace to the arguments to
call. As with other functions, any whitespace contained in the
second and subsequent arguments is kept; this can cause strange
effects. It's generally safest to remove all extraneous whitespace when
providing parameters to call.
value Function
The value function provides a way for you to use the value of a
variable without having it expanded. Please note that this
does not undo expansions which have already occurred; for example if
you create a simply expanded variable its value is expanded during the
definition; in that case the value function will return the
same result as using the variable directly.
The syntax of the value function is:
$(value variable)
Note that variable is the name of a variable; not a reference to that variable. Therefore you would not normally use a `$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.)
The result of this function is a string containing the value of variable, without any expansion occurring. For example, in this makefile:
FOO = $PATH
all:
@echo $(FOO)
@echo $(value FOO)
The first output line would be ATH, since the “$P” would be
expanded as a make variable, while the second output line would
be the current value of your $PATH environment variable, since
the value function avoided the expansion.
The value function is most often used in conjunction with the
eval function (see Eval Function).
eval Function
The eval function is very special: it allows you to define new
makefile constructs that are not constant; which are the result of
evaluating other variables and functions. The argument to the
eval function is expanded, then the results of that expansion
are parsed as makefile syntax. The expanded results can define new
make variables, targets, implicit or explicit rules, etc.
The result of the eval function is always the empty string;
thus, it can be placed virtually anywhere in a makefile without
causing syntax errors.
It's important to realize that the eval argument is expanded
twice; first by the eval function, then the results of
that expansion are expanded again when they are parsed as makefile
syntax. This means you may need to provide extra levels of escaping
for “$” characters when using eval. The value
function (see Value Function) can sometimes be useful in these
situations, to circumvent unwanted expansions.
Here is an example of how eval can be used; this example
combines a number of concepts and other functions. Although it might
seem overly complex to use eval in this example, rather than
just writing out the rules, consider two things: first, the template
definition (in PROGRAM_template) could need to be much more
complex than it is here; and second, you might put the complex,
“generic” part of this example into another makefile, then include
it in all the individual makefiles. Now your individual makefiles are
quite straightforward.
PROGRAMS = server client
server_OBJS = server.o server_priv.o server_access.o
server_LIBS = priv protocol
client_OBJS = client.o client_api.o client_mem.o
client_LIBS = protocol
# Everything after this is generic
.PHONY: all
all: $(PROGRAMS)
define PROGRAM_template
$(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%)
ALL_OBJS += $$($(1)_OBJS)
endef
$(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog))))
$(PROGRAMS):
$(LINK.o) $^ $(LDLIBS) -o $@
clean:
rm -f $(ALL_OBJS) $(PROGRAMS)
origin Function
The origin function is unlike most other functions in that it does
not operate on the values of variables; it tells you something about
a variable. Specifically, it tells you where it came from.
The syntax of the origin function is:
$(origin variable)
Note that variable is the name of a variable to inquire about; not a reference to that variable. Therefore you would not normally use a `$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.)
The result of this function is a string telling you how the variable variable was defined:
CC
and so on. See Variables Used by Implicit Rules.
Note that if you have redefined a default variable, the origin
function will return the origin of the later definition.
override directive in a
makefile (see The override Directive).
This information is primarily useful (other than for your curiosity) to
determine if you want to believe the value of a variable. For example,
suppose you have a makefile foo that includes another makefile
bar. You want a variable bletch to be defined in bar
if you run the command `make -f bar', even if the environment contains
a definition of bletch. However, if foo defined
bletch before including bar, you do not want to override that
definition. This could be done by using an override directive in
foo, giving that definition precedence over the later definition in
bar; unfortunately, the override directive would also
override any command line definitions. So, bar could
include:
ifdef bletch
ifeq "$(origin bletch)" "environment"
bletch = barf, gag, etc.
endif
endif
If bletch has been defined from the environment, this will redefine
it.
If you want to override a previous definition of bletch if it came
from the environment, even under `-e', you could instead write:
ifneq "$(findstring environment,$(origin bletch))" ""
bletch = barf, gag, etc.
endif
Here the redefinition takes place if `$(origin bletch)' returns either `environment' or `environment override'. See Functions for String Substitution and Analysis.
flavor Function
The flavor function is unlike most other functions (and like
origin function) in that it does not operate on the values of
variables; it tells you something about a variable.
Specifically, it tells you the flavor of a variable (see The Two Flavors of Variables).
The syntax of the flavor function is:
$(flavor variable)
Note that variable is the name of a variable to inquire about; not a reference to that variable. Therefore you would not normally use a `$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.)
The result of this function is a string that identifies the flavor of the variable variable:
shell Function
The shell function is unlike any other function other than the
wildcard function
(see The Function wildcard) in that it
communicates with the world outside of make.
The shell function performs the same function that backquotes
(``') perform in most shells: it does command expansion.
This means that it takes as an argument a shell command and evaluates
to the output of the command. The only processing make does on
the result is to convert each newline (or carriage-return / newline
pair) to a single space. If there is a trailing (carriage-return
and) newline it will simply be removed.
The commands run by calls to the shell function are run when the
function calls are expanded (see How make Reads a Makefile). Because this function involves
spawning a new shell, you should carefully consider the performance
implications of using the shell function within recursively
expanded variables vs. simply expanded variables (see The Two Flavors of Variables).
Here are some examples of the use of the shell function:
contents := $(shell cat foo)
sets contents to the contents of the file foo, with a space
(rather than a newline) separating each line.
files := $(shell echo *.c)
sets files to the expansion of `*.c'. Unless make is
using a very strange shell, this has the same result as
`$(wildcard *.c)' (as long as at least one `.c' file
exists).
These functions control the way make runs. Generally, they are used to provide information to the user of the makefile or to cause make to stop if some sort of environmental error is detected.
$(error text...)For example,
ifdef ERROR1
$(error error is $(ERROR1))
endif
will generate a fatal error during the read of the makefile if the
make variable ERROR1 is defined. Or,
ERR = $(error found an error!)
.PHONY: err
err: ; $(ERR)
will generate a fatal error while make is running, if the
err target is invoked.
$(warning text...)error function, above,
except that make doesn't exit. Instead, text is expanded
and the resulting message is displayed, but processing of the makefile
continues.
The result of the expansion of this function is the empty string.
$(info text...)makeA makefile that says how to recompile a program can be used in more
than one way. The simplest use is to recompile every file that is out
of date. Usually, makefiles are written so that if you run
make with no arguments, it does just that.
But you might want to update only some of the files; you might want to use a different compiler or different compiler options; you might want just to find out which files are out of date without changing them.
By giving arguments when you run make, you can do any of these
things and many others.
The exit status of make is always one of three values:
0make is successful.
2make encounters any errors.
It will print messages describing the particular errors.
1make
determines that some target is not already up to date.
See Instead of Executing the Commands.
The way to specify the name of the makefile is with the `-f' or `--file' option (`--makefile' also works). For example, `-f altmake' says to use the file altmake as the makefile.
If you use the `-f' flag several times and follow each `-f' with an argument, all the specified files are used jointly as makefiles.
If you do not use the `-f' or `--file' flag, the default is to try GNUmakefile, makefile, and Makefile, in that order, and use the first of these three which exists or can be made (see Writing Makefiles).
The goals are the targets that make should strive ultimately
to update. Other targets are updated as well if they appear as
prerequisites of goals, or prerequisites of prerequisites of goals, etc.
By default, the goal is the first target in the makefile (not counting
targets that start with a period). Therefore, makefiles are usually
written so that the first target is for compiling the entire program or
programs they describe. If the first rule in the makefile has several
targets, only the first target in the rule becomes the default goal, not
the whole list. You can manage the selection of the default goal from
within your makefile using the .DEFAULT_GOAL variable
(see Other Special Variables).
You can also specify a different goal or goals with command-line
arguments to make. Use the name of the goal as an argument.
If you specify several goals, make processes each of them in
turn, in the order you name them.
Any target in the makefile may be specified as a goal (unless it
starts with `-' or contains an `=', in which case it will be
parsed as a switch or variable definition, respectively). Even
targets not in the makefile may be specified, if make can find
implicit rules that say how to make them.
Make will set the special variable MAKECMDGOALS to the
list of goals you specified on the command line. If no goals were given
on the command line, this variable is empty. Note that this variable
should be used only in special circumstances.
An example of appropriate use is to avoid including .d files
during clean rules (see Automatic Prerequisites), so
make won't create them only to immediately remove them
again:
sources = foo.c bar.c
ifneq ($(MAKECMDGOALS),clean)
include $(sources:.c=.d)
endif
One use of specifying a goal is if you want to compile only a part of the program, or only one of several programs. Specify as a goal each file that you wish to remake. For example, consider a directory containing several programs, with a makefile that starts like this:
.PHONY: all
all: size nm ld ar as
If you are working on the program size, you might want to say
`make size' so that only the files of that program are recompiled.
Another use of specifying a goal is to make files that are not normally made. For example, there may be a file of debugging output, or a version of the program that is compiled specially for testing, which has a rule in the makefile but is not a prerequisite of the default goal.
Another use of specifying a goal is to run the commands associated with a phony target (see Phony Targets) or empty target (see Empty Target Files to Record Events). Many makefiles contain a phony target named clean which deletes everything except source files. Naturally, this is done only if you request it explicitly with `make clean'. Following is a list of typical phony and empty target names. See Standard Targets, for a detailed list of all the standard target names which GNU software packages use.
make.
The makefile tells make how to tell whether a target is up to date,
and how to update each target. But updating the targets is not always
what you want. Certain options specify other activities for make.
make pretends to compile
the targets but does not really change their contents.
make as being the present
time, although the actual modification times remain the same.
You can use the `-W' flag in conjunction with the `-n' flag
to see what would happen if you were to modify specific files.
With the `-n' flag, make prints the commands that it would
normally execute but does not execute them.
With the `-t' flag, make ignores the commands in the rules
and uses (in effect) the command touch for each target that needs to
be remade. The touch command is also printed, unless `-s' or
.SILENT is used. For speed, make does not actually invoke
the program touch. It does the work directly.
With the `-q' flag, make prints nothing and executes no
commands, but the exit status code it returns is zero if and only if the
targets to be considered are already up to date. If the exit status is
one, then some updating needs to be done. If make encounters an
error, the exit status is two, so you can distinguish an error from a
target that is not up to date.
It is an error to use more than one of these three flags in the same
invocation of make.
The `-n', `-t', and `-q' options do not affect command
lines that begin with `+' characters or contain the strings
`$(MAKE)' or `${MAKE}'. Note that only the line containing
the `+' character or the strings `$(MAKE)' or `${MAKE}'
is run regardless of these options. Other lines in the same rule are
not run unless they too begin with `+' or contain `$(MAKE)' or
`${MAKE}' (See How the MAKE Variable Works.)
The `-W' flag provides two features:
make would do if you were to modify some files.
make is actually
executing commands, the `-W' flag can direct make to act
as if some files had been modified, without actually modifying the
files.
Note that the options `-p' and `-v' allow you to obtain other
information about make or about the makefiles in use
(see Summary of Options).
Sometimes you may have changed a source file but you do not want to
recompile all the files that depend on it. For example, suppose you add
a macro or a declaration to a header file that many other files depend
on. Being conservative, make assumes that any change in the
header file requires recompilation of all dependent files, but you know
that they do not need to be recompiled and you would rather not waste
the time waiting for them to compile.
If you anticipate the problem before changing the header file, you can
use the `-t' flag. This flag tells make not to run the
commands in the rules, but rather to mark the target up to date by
changing its last-modification date. You would follow this procedure:
make, the changes in the
header files will not cause any recompilation.
If you have already changed the header file at a time when some files do need recompilation, it is too late to do this. Instead, you can use the `-o file' flag, which marks a specified file as “old” (see Summary of Options). This means that the file itself will not be remade, and nothing else will be remade on its account. Follow this procedure:
An argument that contains `=' specifies the value of a variable: `v=x' sets the value of the variable v to x. If you specify a value in this way, all ordinary assignments of the same variable in the makefile are ignored; we say they have been overridden by the command line argument.
The most common way to use this facility is to pass extra flags to
compilers. For example, in a properly written makefile, the variable
CFLAGS is included in each command that runs the C compiler, so a
file foo.c would be compiled something like this:
cc -c $(CFLAGS) foo.c
Thus, whatever value you set for CFLAGS affects each compilation
that occurs. The makefile probably specifies the usual value for
CFLAGS, like this:
CFLAGS=-g
Each time you run make, you can override this value if you
wish. For example, if you say `make CFLAGS='-g -O'', each C
compilation will be done with `cc -c -g -O'. (This also
illustrates how you can use quoting in the shell to enclose spaces and
other special characters in the value of a variable when you override
it.)
The variable CFLAGS is only one of many standard variables that
exist just so that you can change them this way. See Variables Used by Implicit Rules, for a complete list.
You can also program the makefile to look at additional variables of your own, giving the user the ability to control other aspects of how the makefile works by changing the variables.
When you override a variable with a command argument, you can define either a recursively-expanded variable or a simply-expanded variable. The examples shown above make a recursively-expanded variable; to make a simply-expanded variable, write `:=' instead of `='. But, unless you want to include a variable reference or function call in the value that you specify, it makes no difference which kind of variable you create.
There is one way that the makefile can change a variable that you have
overridden. This is to use the override directive, which is a line
that looks like this: `override variable = value'
(see The override Directive).
Normally, when an error happens in executing a shell command, make
gives up immediately, returning a nonzero status. No further commands are
executed for any target. The error implies that the goal cannot be
correctly remade, and make reports this as soon as it knows.
When you are compiling a program that you have just changed, this is not
what you want. Instead, you would rather that make try compiling
every file that can be tried, to show you as many compilation errors
as possible.
On these occasions, you should use the `-k' or
`--keep-going' flag. This tells make to continue to
consider the other prerequisites of the pending targets, remaking them
if necessary, before it gives up and returns nonzero status. For
example, after an error in compiling one object file, `make -k'
will continue compiling other object files even though it already
knows that linking them will be impossible. In addition to continuing
after failed shell commands, `make -k' will continue as much as
possible after discovering that it does not know how to make a target
or prerequisite file. This will always cause an error message, but
without `-k', it is a fatal error (see Summary of Options).
The usual behavior of make assumes that your purpose is to get the
goals up to date; once make learns that this is impossible, it might
as well report the failure immediately. The `-k' flag says that the
real purpose is to test as much as possible of the changes made in the
program, perhaps to find several independent problems so that you can
correct them all before the next attempt to compile. This is why Emacs'
M-x compile command passes the `-k' flag by default.
Here is a table of all the options make understands:
make.
make proceeds to
consider targets and their prerequisites using the normal algorithms;
however, all targets so considered are always remade regardless of the
status of their prerequisites. To avoid infinite recursion, if
MAKE_RESTARTS (see Other Special Variables) is set to a number greater than 0 this option is disabled
when considering whether to remake makefiles (see How Makefiles Are Remade).
make
(see Recursive Use of make).
Print debugging information in addition to normal processing. The
debugging information says which files are being considered for
remaking, which file-times are being compared and with what results,
which files actually need to be remade, which implicit rules are
considered and which are applied—everything interesting about how
make decides what to do. The -d option is equivalent to
`--debug=a' (see below).
Print debugging information in addition to normal processing. Various levels and types of output can be chosen. With no arguments, print the “basic” level of debugging. Possible arguments are below; only the first character is considered, and values must be comma- or space-separated.
a (all)b (basic)v (verbose)i (implicit)j (jobs)m (makefile)Remind you of the options that make understands and then exit.
make runs as many jobs simultaneously as possible. If
there is more than one `-j' option, the last one is effective.
See Parallel Execution,
for more information on how commands are run.
Note that this option is ignored on MS-DOS.
make
to consider the timestamps on any symbolic links in addition to the
timestamp on the file referenced by those links. When this option is
provided, the most recent timestamp among the file and the symbolic
links is taken as the modification time for this target file.
Print the commands that would be executed, but do not execute them.
See Instead of Executing the Commands.
.SUFFIXES, and then define your own suffix rules. Note that only
rules are affected by the -r option; default variables
remain in effect (see Variables Used by Implicit Rules); see the `-R' option below.
Silent operation; do not print the commands as they are executed.
See Command Echoing.
Cancel the effect of the `-k' option. This is never necessary
except in a recursive make where `-k' might be inherited
from the top-level make via MAKEFLAGS
(see Recursive Use of make)
or if you set `-k' in MAKEFLAGS in your environment.
Touch files (mark them up to date without really changing them)
instead of running their commands. This is used to pretend that the
commands were done, in order to fool future invocations of
make. See Instead of Executing the Commands.
make program plus a copyright, a list
of authors, and a notice that there is no warranty; then exit.
make commands.
See Recursive Use of make. (In practice, you
rarely need to specify this option since `make' does it for you;
see The `--print-directory' Option.)
-w.
This option is useful when -w is turned on automatically,
but you do not want to see the extra messages.
See The `--print-directory' Option.
touch command on the given file before running
make, except that the modification time is changed only in the
imagination of make.
See Instead of Executing the Commands.
make sees a reference to an
undefined variable. This can be helpful when you are trying to debug
makefiles which use variables in complex ways.
Certain standard ways of remaking target files are used very often. For
example, one customary way to make an object file is from a C source file
using the C compiler, cc.
Implicit rules tell make how to use customary techniques so
that you do not have to specify them in detail when you want to use
them. For example, there is an implicit rule for C compilation. File
names determine which implicit rules are run. For example, C
compilation typically takes a .c file and makes a .o file.
So make applies the implicit rule for C compilation when it sees
this combination of file name endings.
A chain of implicit rules can apply in sequence; for example, make
will remake a .o file from a .y file by way of a .c file.
The built-in implicit rules use several variables in their commands so
that, by changing the values of the variables, you can change the way the
implicit rule works. For example, the variable CFLAGS controls the
flags given to the C compiler by the implicit rule for C compilation.
You can define your own implicit rules by writing pattern rules.
Suffix rules are a more limited way to define implicit rules. Pattern rules are more general and clearer, but suffix rules are retained for compatibility.
To allow make to find a customary method for updating a target file,
all you have to do is refrain from specifying commands yourself. Either
write a rule with no command lines, or don't write a rule at all. Then
make will figure out which implicit rule to use based on which
kind of source file exists or can be made.
For example, suppose the makefile looks like this:
foo : foo.o bar.o
cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
Because you mention foo.o but do not give a rule for it, make
will automatically look for an implicit rule that tells how to update it.
This happens whether or not the file foo.o currently exists.
If an implicit rule is found, it can supply both commands and one or more prerequisites (the source files). You would want to write a rule for foo.o with no command lines if you need to specify additional prerequisites, such as header files, that the implicit rule cannot supply.
Each implicit rule has a target pattern and prerequisite patterns. There may
be many implicit rules with the same target pattern. For example, numerous
rules make `.o' files: one, from a `.c' file with the C compiler;
another, from a `.p' file with the Pascal compiler; and so on. The rule
that actually applies is the one whose prerequisites exist or can be made.
So, if you have a file foo.c, make will run the C compiler;
otherwise, if you have a file foo.p, make will run the Pascal
compiler; and so on.
Of course, when you write the makefile, you know which implicit rule you
want make to use, and you know it will choose that one because you
know which possible prerequisite files are supposed to exist.
See Catalogue of Implicit Rules,
for a catalogue of all the predefined implicit rules.
Above, we said an implicit rule applies if the required prerequisites “exist or can be made”. A file “can be made” if it is mentioned explicitly in the makefile as a target or a prerequisite, or if an implicit rule can be recursively found for how to make it. When an implicit prerequisite is the result of another implicit rule, we say that chaining is occurring. See Chains of Implicit Rules.
In general, make searches for an implicit rule for each target, and
for each double-colon rule, that has no commands. A file that is mentioned
only as a prerequisite is considered a target whose rule specifies nothing,
so implicit rule search happens for it. See Implicit Rule Search Algorithm, for the
details of how the search is done.
Note that explicit prerequisites do not influence implicit rule search. For example, consider this explicit rule:
foo.o: foo.p
The prerequisite on foo.p does not necessarily mean that
make will remake foo.o according to the implicit rule to
make an object file, a .o file, from a Pascal source file, a
.p file. For example, if foo.c also exists, the implicit
rule to make an object file from a C source file is used instead,
because it appears before the Pascal rule in the list of predefined
implicit rules (see Catalogue of Implicit Rules).
If you do not want an implicit rule to be used for a target that has no commands, you can give that target empty commands by writing a semicolon (see Defining Empty Commands).
Here is a catalogue of predefined implicit rules which are always available unless the makefile explicitly overrides or cancels them. See Canceling Implicit Rules, for information on canceling or overriding an implicit rule. The `-r' or `--no-builtin-rules' option cancels all predefined rules.
This manual only documents the default rules available on POSIX-based
operating systems. Other operating systems, such as VMS, Windows,
OS/2, etc. may have different sets of default rules. To see the full
list of default rules and variables available in your version of GNU
make, run `make -p' in a directory with no makefile.
Not all of these rules will always be defined, even when the `-r'
option is not given. Many of the predefined implicit rules are
implemented in make as suffix rules, so which ones will be
defined depends on the suffix list (the list of prerequisites of
the special target .SUFFIXES). The default suffix list is:
.out, .a, .ln, .o, .c, .cc,
.C, .cpp, .p, .f, .F, .r, .y,
.l, .s, .S, .mod, .sym, .def,
.h, .info, .dvi, .tex, .texinfo,
.texi, .txinfo, .w, .ch .web,
.sh, .elc, .el. All of the implicit rules
described below whose prerequisites have one of these suffixes are
actually suffix rules. If you modify the suffix list, the only
predefined suffix rules in effect will be those named by one or two of
the suffixes that are on the list you specify; rules whose suffixes fail
to be on the list are disabled. See Old-Fashioned Suffix Rules, for full details on suffix rules.
as. The precise command is
`$(AS) $(ASFLAGS)'.
n.s is made automatically from n.S by
running the C preprocessor, cpp. The precise command is
`$(CPP) $(CPPFLAGS)'.
ld) via the C compiler. The precise
command used is `$(CC) $(LDFLAGS) n.o $(LOADLIBES) $(LDLIBS)'.
This rule does the right thing for a simple program with only one source file. It will also do the right thing if there are multiple object files (presumably coming from various other source files), one of which has a name matching that of the executable file. Thus,
x: y.o z.o
when x.c, y.c and z.c all exist will execute:
cc -c x.c -o x.o
cc -c y.c -o y.o
cc -c z.c -o z.o
cc x.o y.o z.o -o x
rm -f x.o
rm -f y.o
rm -f z.o
In more complicated cases, such as when there is no object file whose name derives from the executable file name, you must write an explicit command for linking.
Each kind of file automatically made into `.o' object files will
be automatically linked by using the compiler (`$(CC)',
`$(FC)' or `$(PC)'; the C compiler `$(CC)' is used to
assemble `.s' files) without the `-c' option. This could be
done by using the `.o' object files as intermediates, but it is
faster to do the compiling and linking in one step, so that's how it's
done.
The convention of using the same suffix `.l' for all Lex files
regardless of whether they produce C code or Ratfor code makes it
impossible for make to determine automatically which of the two
languages you are using in any particular case. If make is
called upon to remake an object file from a `.l' file, it must
guess which compiler to use. It will guess the C compiler, because
that is more common. If you are using Ratfor, make sure make
knows this by mentioning n.r in the makefile. Or, if you
are using Ratfor exclusively, with no C files, remove `.c' from
the list of implicit rule suffixes with:
.SUFFIXES:
.SUFFIXES: .o .r .f .l ...
lint.
The precise command is `$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i'.
The same command is used on the C code produced from
n.y or n.l.
For the benefit of SCCS, a file n is copied from n.sh and made executable (by everyone). This is for shell scripts that are checked into SCCS. Since RCS preserves the execution permission of a file, you do not need to use this feature with RCS.
We recommend that you avoid using of SCCS. RCS is widely held to be superior, and is also free. By choosing free software in place of comparable (or inferior) proprietary software, you support the free software movement.
Usually, you want to change only the variables listed in the table above, which are documented in the following section.
However, the commands in built-in implicit rules actually use
variables such as COMPILE.c, LINK.p, and
PREPROCESS.S, whose values contain the commands listed above.
make follows the convention that the rule to compile a
.x source file uses the variable COMPILE.x.
Similarly, the rule to produce an executable from a .x
file uses LINK.x; and the rule to preprocess a
.x file uses PREPROCESS.x.
Every rule that produces an object file uses the variable
OUTPUT_OPTION. make defines this variable either to
contain `-o $@', or to be empty, depending on a compile-time
option. You need the `-o' option to ensure that the output goes
into the right file when the source file is in a different directory,
as when using VPATH (see Directory Search). However,
compilers on some systems do not accept a `-o' switch for object
files. If you use such a system, and use VPATH, some
compilations will put their output in the wrong place.
A possible workaround for this problem is to give OUTPUT_OPTION
the value `; mv $*.o $@'.
The commands in built-in implicit rules make liberal use of certain
predefined variables. You can alter the values of these variables in
the makefile, with arguments to make, or in the environment to
alter how the implicit rules work without redefining the rules
themselves. You can cancel all variables used by implicit rules with
the `-R' or `--no-builtin-variables' option.
For example, the command used to compile a C source file actually says `$(CC) -c $(CFLAGS) $(CPPFLAGS)'. The default values of the variables used are `cc' and nothing, resulting in the command `cc -c'. By redefining `CC' to `ncc', you could cause `ncc' to be used for all C compilations performed by the implicit rule. By redefining `CFLAGS' to be `-g', you could pass the `-g' option to each compilation. All implicit rules that do C compilation use `$(CC)' to get the program name for the compiler and all include `$(CFLAGS)' among the arguments given to the compiler.
The variables used in implicit rules fall into two classes: those that are
names of programs (like CC) and those that contain arguments for the
programs (like CFLAGS). (The “name of a program” may also contain
some command arguments, but it must start with an actual executable program
name.) If a variable value contains more than one argument, separate them
with spaces.
The following tables describe of some of the more commonly-used predefined
variables. This list is not exhaustive, and the default values shown here may
not be what are selected by make for your environment. To see the
complete list of predefined variables for your instance of GNU make you
can run `make -p' in a directory with no makefiles.
Here is a table of some of the more common variables used as names of programs in built-in rules: makefiles.
ARASCCCOCXXCOCPPFCGETLEXYACCLINTM2CPCMAKEINFOTEXTEXI2DVIWEAVECWEAVETANGLECTANGLERMHere is a table of variables whose values are additional arguments for the programs above. The default values for all of these is the empty string, unless otherwise noted.
ARFLAGSASFLAGSCFLAGSCXXFLAGSCOFLAGSco program.
CPPFLAGSFFLAGSGFLAGSget program.
LDFLAGSLFLAGSYFLAGSPFLAGSRFLAGSLINTFLAGSSometimes a file can be made by a sequence of implicit rules. For example,
a file n.o could be made from n.y by running
first Yacc and then cc. Such a sequence is called a chain.
If the file n.c exists, or is mentioned in the makefile, no
special searching is required: make finds that the object file can
be made by C compilation from n.c; later on, when considering
how to make n.c, the rule for running Yacc is
used. Ultimately both n.c and n.o are
updated.
However, even if n.c does not exist and is not mentioned,
make knows how to envision it as the missing link between
n.o and n.y! In this case, n.c is
called an intermediate file. Once make has decided to use the
intermediate file, it is entered in the data base as if it had been
mentioned in the makefile, along with the implicit rule that says how to
create it.
Intermediate files are remade using their rules just like all other files. But intermediate files are treated differently in two ways.
The first difference is what happens if the intermediate file does not
exist. If an ordinary file b does not exist, and make
considers a target that depends on b, it invariably creates
b and then updates the target from b. But if b is an
intermediate file, then make can leave well enough alone. It
won't bother updating b, or the ultimate target, unless some
prerequisite of b is newer than that target or there is some other
reason to update that target.
The second difference is that if make does create b
in order to update something else, it deletes b later on after it
is no longer needed. Therefore, an intermediate file which did not
exist before make also does not exist after make.
make reports the deletion to you by printing a `rm -f'
command showing which file it is deleting.
Ordinarily, a file cannot be intermediate if it is mentioned in the
makefile as a target or prerequisite. However, you can explicitly mark a
file as intermediate by listing it as a prerequisite of the special target
.INTERMEDIATE. This takes effect even if the file is mentioned
explicitly in some other way.
You can prevent automatic deletion of an intermediate file by marking it
as a secondary file. To do this, list it as a prerequisite of the
special target .SECONDARY. When a file is secondary, make
will not create the file merely because it does not already exist, but
make does not automatically delete the file. Marking a file as
secondary also marks it as intermediate.
You can list the target pattern of an implicit rule (such as `%.o')
as a prerequisite of the special target .PRECIOUS to preserve
intermediate files made by implicit rules whose target patterns match
that file's name; see Interrupts.
A chain can involve more than two implicit rules. For example, it is
possible to make a file foo from RCS/foo.y,v by running RCS,
Yacc and cc. Then both foo.y and foo.c are
intermediate files that are deleted at the end.
No single implicit rule can appear more than once in a chain. This means
that make will not even consider such a ridiculous thing as making
foo from foo.o.o by running the linker twice. This
constraint has the added benefit of preventing any infinite loop in the
search for an implicit rule chain.
There are some special implicit rules to optimize certain cases that would
otherwise be handled by rule chains. For example, making foo from
foo.c could be handled by compiling and linking with separate
chained rules, using foo.o as an intermediate file. But what
actually happens is that a special rule for this case does the compilation
and linking with a single cc command. The optimized rule is used in
preference to the step-by-step chain because it comes earlier in the
ordering of rules.
You define an implicit rule by writing a pattern rule. A pattern rule looks like an ordinary rule, except that its target contains the character `%' (exactly one of them). The target is considered a pattern for matching file names; the `%' can match any nonempty substring, while other characters match only themselves. The prerequisites likewise use `%' to show how their names relate to the target name.
Thus, a pattern rule `%.o : %.c' says how to make any file stem.o from another file stem.c.
Note that expansion using `%' in pattern rules occurs after any variable or function expansions, which take place when the makefile is read. See How to Use Variables, and Functions for Transforming Text.
A pattern rule contains the character `%' (exactly one of them) in the target; otherwise, it looks exactly like an ordinary rule. The target is a pattern for matching file names; the `%' matches any nonempty substring, while other characters match only themselves. For example, `%.c' as a pattern matches any file name that ends in `.c'. `s.%.c' as a pattern matches any file name that starts with `s.', ends in `.c' and is at least five characters long. (There must be at least one character to match the `%'.) The substring that the `%' matches is called the stem.
`%' in a prerequisite of a pattern rule stands for the same stem that was matched by the `%' in the target. In order for the pattern rule to apply, its target pattern must match the file name under consideration and all of its prerequisites (after pattern substitution) must name files that exist or can be made. These files become prerequisites of the target. Thus, a rule of the form
%.o : %.c ; command...
specifies how to make a file n.o, with another file n.c as its prerequisite, provided that n.c exists or can be made.
There may also be prerequisites that do not use `%'; such a prerequisite attaches to every file made by this pattern rule. These unvarying prerequisites are useful occasionally.
A pattern rule need not have any prerequisites that contain `%', or in fact any prerequisites at all. Such a rule is effectively a general wildcard. It provides a way to make any file that matches the target pattern. See Last Resort.
Pattern rules may have more than one target. Unlike normal rules, this
does not act as many different rules with the same prerequisites and
commands. If a pattern rule has multiple targets, make knows that
the rule's commands are responsible for making all of the targets. The
commands are executed only once to make all the targets. When searching
for a pattern rule to match a target, the target patterns of a rule other
than the one that matches the target in need of a rule are incidental:
make worries only about giving commands and prerequisites to the file
presently in question. However, when this file's commands are run, the
other targets are marked as having been updated themselves.
The order in which pattern rules appear in the makefile is important
since this is the order in which they are considered.
Of equally applicable
rules, only the first one found is used. The rules you write take precedence
over those that are built in. Note however, that a rule whose
prerequisites actually exist or are mentioned always takes priority over a
rule with prerequisites that must be made by chaining other implicit rules.
Here are some examples of pattern rules actually predefined in
make. First, the rule that compiles `.c' files into `.o'
files:
%.o : %.c
$(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@
defines a rule that can make any file x.o from x.c. The command uses the automatic variables `$@' and `$<' to substitute the names of the target file and the source file in each case where the rule applies (see Automatic Variables).
Here is a second built-in rule:
% :: RCS/%,v
$(CO) $(COFLAGS) $<
defines a rule that can make any file x whatsoever from a corresponding file x,v in the subdirectory RCS. Since the target is `%', this rule will apply to any file whatever, provided the appropriate prerequisite file exists. The double colon makes the rule terminal, which means that its prerequisite may not be an intermediate file (see Match-Anything Pattern Rules).
This pattern rule has two targets:
%.tab.c %.tab.h: %.y
bison -d $<
This tells make that the command `bison -d x.y' will
make both x.tab.c and x.tab.h. If the file
foo depends on the files parse.tab.o and scan.o
and the file scan.o depends on the file parse.tab.h,
when parse.y is changed, the command `bison -d parse.y'
will be executed only once, and the prerequisites of both
parse.tab.o and scan.o will be satisfied. (Presumably
the file parse.tab.o will be recompiled from parse.tab.c
and the file scan.o from scan.c, while foo is
linked from parse.tab.o, scan.o, and its other
prerequisites, and it will execute happily ever after.)
Suppose you are writing a pattern rule to compile a `.c' file into a `.o' file: how do you write the `cc' command so that it operates on the right source file name? You cannot write the name in the command, because the name is different each time the implicit rule is applied.
What you do is use a special feature of make, the automatic
variables. These variables have values computed afresh for each rule that
is executed, based on the target and prerequisites of the rule. In this
example, you would use `$@' for the object file name and `$<'
for the source file name.
It's very important that you recognize the limited scope in which
automatic variable values are available: they only have values within
the command script. In particular, you cannot use them anywhere
within the target list of a rule; they have no value there and will
expand to the empty string. Also, they cannot be accessed directly
within the prerequisite list of a rule. A common mistake is
attempting to use $@ within the prerequisites list; this will
not work. However, there is a special feature of GNU make,
secondary expansion (see Secondary Expansion), which will allow
automatic variable values to be used in prerequisite lists.
Here is a table of automatic variables:
$@$%$<$?$^$^ contains just one copy of the name. This list
does not contain any of the order-only prerequisites; for those
see the `$|' variable, below.
$+$|$*In an explicit rule, there is no stem; so `$*' cannot be determined
in that way. Instead, if the target name ends with a recognized suffix
(see Old-Fashioned Suffix Rules), `$*' is set to
the target name minus the suffix. For example, if the target name is
`foo.c', then `$*' is set to `foo', since `.c' is a
suffix. GNU make does this bizarre thing only for compatibility
with other implementations of make. You should generally avoid
using `$*' except in implicit rules or static pattern rules.
If the target name in an explicit rule does not end with a recognized suffix, `$*' is set to the empty string for that rule.
`$?' is useful even in explicit rules when you wish to operate on only the prerequisites that have changed. For example, suppose that an archive named lib is supposed to contain copies of several object files. This rule copies just the changed object files into the archive:
lib: foo.o bar.o lose.o win.o
ar r lib $?
Of the variables listed above, four have values that are single file
names, and three have values that are lists of file names. These seven
have variants that get just the file's directory name or just the file
name within the directory. The variant variables' names are formed by
appending `D' or `F', respectively. These variants are
semi-obsolete in GNU make since the functions dir and
notdir can be used to get a similar effect (see Functions for File Names). Note, however, that the
`D' variants all omit the trailing slash which always appears in
the output of the dir function. Here is a table of the variants:
Note that we use a special stylistic convention when we talk about these
automatic variables; we write “the value of `$<'”, rather than
“the variable <” as we would write for ordinary variables
such as objects and CFLAGS. We think this convention
looks more natural in this special case. Please do not assume it has a
deep significance; `$<' refers to the variable named < just
as `$(CFLAGS)' refers to the variable named CFLAGS.
You could just as well use `$(<)' in place of `$<'.
A target pattern is composed of a `%' between a prefix and a suffix, either or both of which may be empty. The pattern matches a file name only if the file name starts with the prefix and ends with the suffix, without overlap. The text between the prefix and the suffix is called the stem. Thus, when the pattern `%.o' matches the file name test.o, the stem is `test'. The pattern rule prerequisites are turned into actual file names by substituting the stem for the character `%'. Thus, if in the same example one of the prerequisites is written as `%.c', it expands to `test.c'.
When the target pattern does not contain a slash (and it usually does not), directory names in the file names are removed from the file name before it is compared with the target prefix and suffix. After the comparison of the file name to the target pattern, the directory names, along with the slash that ends them, are added on to the prerequisite file names generated from the pattern rule's prerequisite patterns and the file name. The directories are ignored only for the purpose of finding an implicit rule to use, not in the application of that rule. Thus, `e%t' matches the file name src/eat, with `src/a' as the stem. When prerequisites are turned into file names, the directories from the stem are added at the front, while the rest of the stem is substituted for the `%'. The stem `src/a' with a prerequisite pattern `c%r' gives the file name src/car.
When a pattern rule's target is just `%', it matches any file name
whatever. We call these rules match-anything rules. They are very
useful, but it can take a lot of time for make to think about them,
because it must consider every such rule for each file name listed either
as a target or as a prerequisite.
Suppose the makefile mentions foo.c. For this target, make
would have to consider making it by linking an object file foo.c.o,
or by C compilation-and-linking in one step from foo.c.c, or by
Pascal compilation-and-linking from foo.c.p, and many other
possibilities.
We know these possibilities are ridiculous since foo.c is a C source
file, not an executable. If make did consider these possibilities,
it would ultimately reject them, because files such as foo.c.o and
foo.c.p would not exist. But these possibilities are so
numerous that make would run very slowly if it had to consider
them.
To gain speed, we have put various constraints on the way make
considers match-anything rules. There are two different constraints that
can be applied, and each time you define a match-anything rule you must
choose one or the other for that rule.
One choice is to mark the match-anything rule as terminal by defining it with a double colon. When a rule is terminal, it does not apply unless its prerequisites actually exist. Prerequisites that could be made with other implicit rules are not good enough. In other words, no further chaining is allowed beyond a terminal rule.
For example, the built-in implicit rules for extracting sources from RCS
and SCCS files are terminal; as a result, if the file foo.c,v does
not exist, make will not even consider trying to make it as an
intermediate file from foo.c,v.o or from RCS/SCCS/s.foo.c,v.
RCS and SCCS files are generally ultimate source files, which should not be
remade from any other files; therefore, make can save time by not
looking for ways to remake them.
If you do not mark the match-anything rule as terminal, then it is nonterminal. A nonterminal match-anything rule cannot apply to a file name that indicates a specific type of data. A file name indicates a specific type of data if some non-match-anything implicit rule target matches it.
For example, the file name foo.c matches the target for the pattern
rule `%.c : %.y' (the rule to run Yacc). Regardless of whether this
rule is actually applicable (which happens only if there is a file
foo.y), the fact that its target matches is enough to prevent
consideration of any nonterminal match-anything rules for the file
foo.c. Thus, make will not even consider trying to make
foo.c as an executable file from foo.c.o, foo.c.c,
foo.c.p, etc.
The motivation for this constraint is that nonterminal match-anything rules are used for making files containing specific types of data (such as executable files) and a file name with a recognized suffix indicates some other specific type of data (such as a C source file).
Special built-in dummy pattern rules are provided solely to recognize certain file names so that nonterminal match-anything rules will not be considered. These dummy rules have no prerequisites and no commands, and they are ignored for all other purposes. For example, the built-in implicit rule
%.p :
exists to make sure that Pascal source files such as foo.p match a specific target pattern and thereby prevent time from being wasted looking for foo.p.o or foo.p.c.
Dummy pattern rules such as the one for `%.p' are made for every suffix listed as valid for use in suffix rules (see Old-Fashioned Suffix Rules).
You can override a built-in implicit rule (or one you have defined yourself) by defining a new pattern rule with the same target and prerequisites, but different commands. When the new rule is defined, the built-in one is replaced. The new rule's position in the sequence of implicit rules is determined by where you write the new rule.
You can cancel a built-in implicit rule by defining a pattern rule with the same target and prerequisites, but no commands. For example, the following would cancel the rule that runs the assembler:
%.o : %.s
You can define a last-resort implicit rule by writing a terminal match-anything pattern rule with no prerequisites (see Match-Anything Rules). This is just like any other pattern rule; the only thing special about it is that it will match any target. So such a rule's commands are used for all targets and prerequisites that have no commands of their own and for which no other implicit rule applies.
For example, when testing a makefile, you might not care if the source files contain real data, only that they exist. Then you might do this:
%::
touch $@
to cause all the source files needed (as prerequisites) to be created automatically.
You can instead define commands to be used for targets for which there
are no rules at all, even ones which don't specify commands. You do
this by writing a rule for the target .DEFAULT. Such a rule's
commands are used for all prerequisites which do not appear as targets in
any explicit rule, and for which no implicit rule applies. Naturally,
there is no .DEFAULT rule unless you write one.
If you use .DEFAULT with no commands or prerequisites:
.DEFAULT:
the commands previously stored for .DEFAULT are cleared.
Then make acts as if you had never defined .DEFAULT at all.
If you do not want a target to get the commands from a match-anything
pattern rule or .DEFAULT, but you also do not want any commands
to be run for the target, you can give it empty commands (see Defining Empty Commands).
You can use a last-resort rule to override part of another makefile. See Overriding Part of Another Makefile.
Suffix rules are the old-fashioned way of defining implicit rules for
make. Suffix rules are obsolete because pattern rules are more
general and clearer. They are supported in GNU make for
compatibility with old makefiles. They come in two kinds:
double-suffix and single-suffix.
A double-suffix rule is defined by a pair of suffixes: the target suffix and the source suffix. It matches any file whose name ends with the target suffix. The corresponding implicit prerequisite is made by replacing the target suffix with the source suffix in the file name. A two-suffix rule whose target and source suffixes are `.o' and `.c' is equivalent to the pattern rule `%.o : %.c'.
A single-suffix rule is defined by a single suffix, which is the source suffix. It matches any file name, and the corresponding implicit prerequisite name is made by appending the source suffix. A single-suffix rule whose source suffix is `.c' is equivalent to the pattern rule `% : %.c'.
Suffix rule definitions are recognized by comparing each rule's target
against a defined list of known suffixes. When make sees a rule
whose target is a known suffix, this rule is considered a single-suffix
rule. When make sees a rule whose target is two known suffixes
concatenated, this rule is taken as a double-suffix rule.
For example, `.c' and `.o' are both on the default list of
known suffixes. Therefore, if you define a rule whose target is
`.c.o', make takes it to be a double-suffix rule with source
suffix `.c' and target suffix `.o'. Here is the old-fashioned
way to define the rule for compiling a C source file:
.c.o:
$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
Suffix rules cannot have any prerequisites of their own. If they have any, they are treated as normal files with funny names, not as suffix rules. Thus, the rule:
.c.o: foo.h
$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
tells how to make the file .c.o from the prerequisite file foo.h, and is not at all like the pattern rule:
%.o: %.c foo.h
$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
which tells how to make `.o' files from `.c' files, and makes all `.o' files using this pattern rule also depend on foo.h.
Suffix rules with no commands are also meaningless. They do not remove previous rules as do pattern rules with no commands (see Canceling Implicit Rules). They simply enter the suffix or pair of suffixes concatenated as a target in the data base.
The known suffixes are simply the names of the prerequisites of the special
target .SUFFIXES. You can add your own suffixes by writing a rule
for .SUFFIXES that adds more prerequisites, as in:
.SUFFIXES: .hack .win
which adds `.hack' and `.win' to the end of the list of suffixes.
If you wish to eliminate the default known suffixes instead of just adding
to them, write a rule for .SUFFIXES with no prerequisites. By
special dispensation, this eliminates all existing prerequisites of
.SUFFIXES. You can then write another rule to add the suffixes you
want. For example,
.SUFFIXES: # Delete the default suffixes .SUFFIXES: .c .o .h # Define our suffix list
The `-r' or `--no-builtin-rules' flag causes the default list of suffixes to be empty.
The variable SUFFIXES is defined to the default list of suffixes
before make reads any makefiles. You can change the list of suffixes
with a rule for the special target .SUFFIXES, but that does not alter
this variable.
Here is the procedure make uses for searching for an implicit rule
for a target t. This procedure is followed for each double-colon
rule with no commands, for each target of ordinary rules none of which have
commands, and for each prerequisite that is not the target of any rule. It
is also followed recursively for prerequisites that come from implicit
rules, in the search for a chain of rules.
Suffix rules are not mentioned in this algorithm because suffix rules are converted to equivalent pattern rules once the makefiles have been read in.
For an archive member target of the form `archive(member)', the following algorithm is run twice, first using the entire target name t, and second using `(member)' as the target t if the first run found no rule.
If all prerequisites exist or ought to exist, or there are no prerequisites, then this rule applies.
.DEFAULT, if any,
applies. In that case, give t the same commands that
.DEFAULT has. Otherwise, there are no commands for t.
Once a rule that applies has been found, for each target pattern of the rule other than the one that matched t or n, the `%' in the pattern is replaced with s and the resultant file name is stored until the commands to remake the target file t are executed. After these commands are executed, each of these stored file names are entered into the data base and marked as having been updated and having the same update status as the file t.
When the commands of a pattern rule are executed for t, the automatic variables are set corresponding to the target and prerequisites. See Automatic Variables.
make to Update Archive Files
Archive files are files containing named subfiles called
members; they are maintained with the program ar and their
main use is as subroutine libraries for linking.
An individual member of an archive file can be used as a target or
prerequisite in make. You specify the member named member in
archive file archive as follows:
archive(member)
This construct is available only in targets and prerequisites, not in
commands! Most programs that you might use in commands do not support this
syntax and cannot act directly on archive members. Only ar and
other programs specifically designed to operate on archives can do so.
Therefore, valid commands to update an archive member target probably must
use ar. For example, this rule says to create a member
hack.o in archive foolib by copying the file hack.o:
foolib(hack.o) : hack.o
ar cr foolib hack.o
In fact, nearly all archive member targets are updated in just this way
and there is an implicit rule to do it for you. Please note: The
`c' flag to ar is required if the archive file does not
already exist.
To specify several members in the same archive, you can write all the member names together between the parentheses. For example:
foolib(hack.o kludge.o)
is equivalent to:
foolib(hack.o) foolib(kludge.o)
You can also use shell-style wildcards in an archive member reference. See Using Wildcard Characters in File Names. For example, `foolib(*.o)' expands to all existing members of the foolib archive whose names end in `.o'; perhaps `foolib(hack.o) foolib(kludge.o)'.
Recall that a target that looks like a(m) stands for the member named m in the archive file a.
When make looks for an implicit rule for such a target, as a special
feature it considers implicit rules that match (m), as well as
those that match the actual target a(m).
This causes one special rule whose target is (%) to match. This rule updates the target a(m) by copying the file m into the archive. For example, it will update the archive member target foo.a(bar.o) by copying the file bar.o into the archive foo.a as a member named bar.o.
When this rule is chained with others, the result is very powerful. Thus, `make "foo.a(bar.o)"' (the quotes are needed to protect the `(' and `)' from being interpreted specially by the shell) in the presence of a file bar.c is enough to cause the following commands to be run, even without a makefile:
cc -c bar.c -o bar.o
ar r foo.a bar.o
rm -f bar.o
Here make has envisioned the file bar.o as an intermediate
file. See Chains of Implicit Rules.
Implicit rules such as this one are written using the automatic variable `$%'. See Automatic Variables.
An archive member name in an archive cannot contain a directory name, but
it may be useful in a makefile to pretend that it does. If you write an
archive member target foo.a(dir/file.o), make will perform
automatic updating with this command:
ar r foo.a dir/file.o
which has the effect of copying the file dir/file.o into a member
named file.o. In connection with such usage, the automatic variables
%D and %F may be useful.
An archive file that is used as a library usually contains a special member
named __.SYMDEF that contains a directory of the external symbol
names defined by all the other members. After you update any other
members, you need to update __.SYMDEF so that it will summarize the
other members properly. This is done by running the ranlib program:
ranlib archivefile
Normally you would put this command in the rule for the archive file, and make all the members of the archive file prerequisites of that rule. For example,
libfoo.a: libfoo.a(x.o) libfoo.a(y.o) ...
ranlib libfoo.a
The effect of this is to update archive members x.o, y.o,
etc., and then update the symbol directory member __.SYMDEF by
running ranlib. The rules for updating the members are not shown
here; most likely you can omit them and use the implicit rule which copies
files into the archive, as described in the preceding section.
This is not necessary when using the GNU ar program, which
updates the __.SYMDEF member automatically.
It is important to be careful when using parallel execution (the
-j switch; see Parallel Execution) and archives.
If multiple ar commands run at the same time on the same archive
file, they will not know about each other and can corrupt the file.
Possibly a future version of make will provide a mechanism to
circumvent this problem by serializing all commands that operate on the
same archive file. But for the time being, you must either write your
makefiles to avoid this problem in some other way, or not use -j.
You can write a special kind of suffix rule for dealing with archive
files. See Suffix Rules, for a full explanation of suffix rules.
Archive suffix rules are obsolete in GNU make, because pattern
rules for archives are a more general mechanism (see Archive Update). But they are retained for compatibility with other
makes.
To write a suffix rule for archives, you simply write a suffix rule using the target suffix `.a' (the usual suffix for archive files). For example, here is the old-fashioned suffix rule to update a library archive from C source files:
.c.a:
$(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
$(AR) r $@ $*.o
$(RM) $*.o
This works just as if you had written the pattern rule:
(%.o): %.c
$(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
$(AR) r $@ $*.o
$(RM) $*.o
In fact, this is just what make does when it sees a suffix rule
with `.a' as the target suffix. Any double-suffix rule
`.x.a' is converted to a pattern rule with the target
pattern `(%.o)' and a prerequisite pattern of `%.x'.
Since you might want to use `.a' as the suffix for some other kind
of file, make also converts archive suffix rules to pattern rules
in the normal way (see Suffix Rules). Thus a double-suffix rule
`.x.a' produces two pattern rules: `(%.o):
%.x' and `%.a: %.x'.
make
Here is a summary of the features of GNU make, for comparison
with and credit to other versions of make. We consider the
features of make in 4.2 BSD systems as a baseline. If you are
concerned with writing portable makefiles, you should not use the
features of make listed here, nor the ones in Missing.
Many features come from the version of make in System V.
VPATH variable and its special meaning.
See Searching Directories for Prerequisites.
This feature exists in System V make, but is undocumented.
It is documented in 4.3 BSD make (which says it mimics System V's
VPATH feature).
MAKEFLAGS to recursive
invocations of make.
See Communicating Options to a Sub-make.
$% is set to the member name
in an archive reference. See Automatic Variables.
$@, $*, $<, $%,
and $? have corresponding forms like $(@F) and
$(@D). We have generalized this to $^ as an obvious
extension. See Automatic Variables.
make, these options actually do something.
make via the variable
MAKE even if `-n', `-q' or `-t' is specified.
See Recursive Use of make.
make, because the
general feature of rule chaining (see Chains of Implicit Rules) allows one pattern rule for installing members in an
archive (see Archive Update) to be sufficient.
The following features were inspired by various other versions of
make. In some cases it is unclear exactly which versions inspired
which others.
make.
We're not sure who invented it first, but it's been spread around a bit.
See Defining and Redefining Pattern Rules.
make
for AT&T Eighth Edition Research Unix, and later by Andrew Hume of
AT&T Bell Labs in his mk program (where he terms it
“transitive closure”). We do not really know if
we got this from either of them or thought it up ourselves at the
same time. See Chains of Implicit Rules.
$^ containing a list of all prerequisites
of the current target. We did not invent this, but we have no idea who
did. See Automatic Variables. The automatic variable
$+ is a simple extension of $^.
make) was (as far as we know)
invented by Andrew Hume in mk.
See Instead of Executing the Commands.
make and similar programs, though not in the
System V or BSD implementations. See Command Execution.
make by the
patsubst function before the alternate syntax was implemented
for compatibility with SunOS 4. It is not altogether clear who
inspired whom, since GNU make had patsubst before SunOS
4 was released.
make. See Appending More Text to Variables.
make.
See Archive Members.
-include directive to include makefiles with no error for a
nonexistent file comes from SunOS 4 make. (But note that SunOS 4
make does not allow multiple makefiles to be specified in one
-include directive.) The same feature appears with the name
sinclude in SGI make and perhaps others.
The remaining features are inventions new in GNU make:
make.
MAKE to recursive make invocations.
See Recursive Use of make.
define.
See Defining Variables Verbatim.
.PHONY.
Andrew Hume of AT&T Bell Labs implemented a similar feature with a
different syntax in his mk program. This seems to be a case of
parallel discovery. See Phony Targets.
This feature has been implemented numerous times in various versions
of make; it seems a natural extension derived from the features
of the C preprocessor and similar macro languages and is not a
revolutionary concept. See Conditional Parts of Makefiles.
MAKEFILES.
make, they must begin with
`.' and not contain any `/' characters.
make recursion using the
variable MAKELEVEL. See Recursive Use of make.
MAKECMDGOALS. See Arguments to Specify the Goals.
vpath search.
See Searching Directories for Prerequisites.
make has a very, very limited form of this
functionality in that it will check out SCCS files for makefiles.
make.
The make programs in various other systems support a few features
that are not implemented in GNU make. The POSIX.2 standard
(IEEE Standard 1003.2-1992) which specifies make does not
require any of these features.
This feature was not put into GNU make because of the
nonmodularity of putting knowledge into make of the internal
format of archive file symbol tables.
See Updating Archive Symbol Directories.
make;
they refer to the SCCS file that corresponds
to the file one would get without the `~'. For example, the
suffix rule `.c~.o' would make the file n.o from
the SCCS file s.n.c. For complete coverage, a whole
series of such suffix rules is required.
See Old-Fashioned Suffix Rules.
In GNU make, this entire series of cases is handled by two
pattern rules for extraction from SCCS, in combination with the
general feature of rule chaining.
See Chains of Implicit Rules.
make, files found by VPATH search
(see Searching Directories for Prerequisites) have their names changed inside command
strings. We feel it is much cleaner to always use automatic variables
and thus make this feature obsolete.
makes, the automatic variable $* appearing in
the prerequisites of a rule has the amazingly strange “feature” of
expanding to the full name of the target of that rule. We cannot
imagine what went on in the minds of Unix make developers to do
this; it is utterly inconsistent with the normal definition of $*.
makes, implicit rule search
(see Using Implicit Rules) is apparently done for
all targets, not just those without commands. This means you can
do:
foo.o:
cc -c foo.c
and Unix make will intuit that foo.o depends on
foo.c.
We feel that such usage is broken. The prerequisite properties of
make are well-defined (for GNU make, at least),
and doing such a thing simply does not fit the model.
make does not include any built-in implicit rules for
compiling or preprocessing EFL programs. If we hear of anyone who is
using EFL, we will gladly add them.
make, a suffix rule can be specified with
no commands, and it is treated as if it had empty commands
(see Empty Commands). For example:
.c.a:
will override the built-in .c.a suffix rule.
We feel that it is cleaner for a rule without commands to always simply
add to the prerequisite list for the target. The above example can be
easily rewritten to get the desired behavior in GNU make:
.c.a: ;
make invoke the shell with the `-e' flag,
except under `-k' (see Testing the Compilation of a Program). The `-e' flag tells the shell to exit as soon as any
program it runs returns a nonzero status. We feel it is cleaner to
write each shell command line to stand on its own and not require this
special treatment.
This describes conventions for writing the Makefiles for GNU programs. Using Automake will help you write a Makefile that follows these conventions.
Every Makefile should contain this line:
SHELL = /bin/sh
to avoid trouble on systems where the SHELL variable might be
inherited from the environment. (This is never a problem with GNU
make.)
Different make programs have incompatible suffix lists and
implicit rules, and this sometimes creates confusion or misbehavior. So
it is a good idea to set the suffix list explicitly using only the
suffixes you need in the particular Makefile, like this:
.SUFFIXES:
.SUFFIXES: .c .o
The first line clears out the suffix list, the second introduces all suffixes which may be subject to implicit rules in this Makefile.
Don't assume that . is in the path for command execution. When you need to run programs that are a part of your package during the make, please make sure that it uses ./ if the program is built as part of the make or $(srcdir)/ if the file is an unchanging part of the source code. Without one of these prefixes, the current search path is used.
The distinction between ./ (the build directory) and $(srcdir)/ (the source directory) is important because users can build in a separate directory using the `--srcdir' option to configure. A rule of the form:
foo.1 : foo.man sedscript
sed -e sedscript foo.man > foo.1
will fail when the build directory is not the source directory, because foo.man and sedscript are in the source directory.
When using GNU make, relying on `VPATH' to find the source
file will work in the case where there is a single dependency file,
since the make automatic variable `$<' will represent the
source file wherever it is. (Many versions of make set `$<'
only in implicit rules.) A Makefile target like
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c bar.c -o foo.o
should instead be written as
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c $< -o $@
in order to allow `VPATH' to work correctly. When the target has multiple dependencies, using an explicit `$(srcdir)' is the easiest way to make the rule work well. For example, the target above for foo.1 is best written as:
foo.1 : foo.man sedscript
sed -e $(srcdir)/sedscript $(srcdir)/foo.man > $@
GNU distributions usually contain some files which are not source files—for example, Info files, and the output from Autoconf, Automake, Bison or Flex. Since these files normally appear in the source directory, they should always appear in the source directory, not in the build directory. So Makefile rules to update them should put the updated files in the source directory.
However, if a file does not appear in the distribution, then the Makefile should not put it in the source directory, because building a program in ordinary circumstances should not modify the source directory in any way.
Try to make the build and installation targets, at least (and all their
subtargets) work correctly with a parallel make.
Write the Makefile commands (and any shell scripts, such as
configure) to run in sh, not in csh. Don't use any
special features of ksh or bash.
The configure script and the Makefile rules for building and
installation should not use any utilities directly except these:
cat cmp cp diff echo egrep expr false grep install-info
ln ls mkdir mv pwd rm rmdir sed sleep sort tar test touch true
The compression program gzip can be used in the dist rule.
Stick to the generally supported options for these programs. For example, don't use `mkdir -p', convenient as it may be, because most systems don't support it.
It is a good idea to avoid creating symbolic links in makefiles, since a few systems don't support them.
The Makefile rules for building and installation can also use compilers
and related programs, but should do so via make variables so that the
user can substitute alternatives. Here are some of the programs we
mean:
ar bison cc flex install ld ldconfig lex
make makeinfo ranlib texi2dvi yacc
Use the following make variables to run those programs:
$(AR) $(BISON) $(CC) $(FLEX) $(INSTALL) $(LD) $(LDCONFIG) $(LEX)
$(MAKE) $(MAKEINFO) $(RANLIB) $(TEXI2DVI) $(YACC)
When you use ranlib or ldconfig, you should make sure
nothing bad happens if the system does not have the program in question.
Arrange to ignore an error from that command, and print a message before
the command to tell the user that failure of this command does not mean
a problem. (The Autoconf `AC_PROG_RANLIB' macro can help with
this.)
If you use symbolic links, you should implement a fallback for systems that don't have symbolic links.
Additional utilities that can be used via Make variables are:
chgrp chmod chown mknod
It is ok to use other utilities in Makefile portions (or scripts) intended only for particular systems where you know those utilities exist.
Makefiles should provide variables for overriding certain commands, options, and so on.
In particular, you should run most utility programs via variables.
Thus, if you use Bison, have a variable named BISON whose default
value is set with `BISON = bison', and refer to it with
$(BISON) whenever you need to use Bison.
File management utilities such as ln, rm, mv, and
so on, need not be referred to through variables in this way, since users
don't need to replace them with other programs.
Each program-name variable should come with an options variable that is
used to supply options to the program. Append `FLAGS' to the
program-name variable name to get the options variable name—for
example, BISONFLAGS. (The names CFLAGS for the C
compiler, YFLAGS for yacc, and LFLAGS for lex, are
exceptions to this rule, but we keep them because they are standard.)
Use CPPFLAGS in any compilation command that runs the
preprocessor, and use LDFLAGS in any compilation command that
does linking as well as in any direct use of ld.
If there are C compiler options that must be used for proper
compilation of certain files, do not include them in CFLAGS.
Users expect to be able to specify CFLAGS freely themselves.
Instead, arrange to pass the necessary options to the C compiler
independently of CFLAGS, by writing them explicitly in the
compilation commands or by defining an implicit rule, like this:
CFLAGS = -g
ALL_CFLAGS = -I. $(CFLAGS)
.c.o:
$(CC) -c $(CPPFLAGS) $(ALL_CFLAGS) $<
Do include the `-g' option in CFLAGS, because that is not
required for proper compilation. You can consider it a default
that is only recommended. If the package is set up so that it is
compiled with GCC by default, then you might as well include `-O'
in the default value of CFLAGS as well.
Put CFLAGS last in the compilation command, after other variables
containing compiler options, so the user can use CFLAGS to
override the others.
CFLAGS should be used in every invocation of the C compiler,
both those which do compilation and those which do linking.
Every Makefile should define the variable INSTALL, which is the
basic command for installing a file into the system.
Every Makefile should also define the variables INSTALL_PROGRAM
and INSTALL_DATA. (The default for INSTALL_PROGRAM should
be $(INSTALL); the default for INSTALL_DATA should be
${INSTALL} -m 644.) Then it should use those variables as the
commands for actual installation, for executables and nonexecutables
respectively. Use these variables as follows:
$(INSTALL_PROGRAM) foo $(bindir)/foo
$(INSTALL_DATA) libfoo.a $(libdir)/libfoo.a
Optionally, you may prepend the value of DESTDIR to the target
filename. Doing this allows the installer to create a snapshot of the
installation to be copied onto the real target filesystem later. Do not
set the value of DESTDIR in your Makefile, and do not include it
in any installed files. With support for DESTDIR, the above
examples become:
$(INSTALL_PROGRAM) foo $(DESTDIR)$(bindir)/foo
$(INSTALL_DATA) libfoo.a $(DESTDIR)$(libdir)/libfoo.a
Always use a file name, not a directory name, as the second argument of the installation commands. Use a separate command for each file to be installed.
Installation directories should always be named by variables, so it is easy to install in a nonstandard place. The standard names for these variables and the values they should have in GNU packages are described below. They are based on a standard filesystem layout; variants of it are used in GNU/Linux and other modern operating systems.
Installers are expected to override these values when calling make (e.g., make prefix=/usr install or configure (e.g., configure --prefix=/usr). GNU packages should not try to guess which value should be appropriate for these variables on the system they are being installed onto: use the default settings specified here so that all GNU packages behave identically, allowing the installer to achieve any desired layout.
These two variables set the root for the installation. All the other installation directories should be subdirectories of one of these two, and nothing should be directly installed into these two directories.
prefixprefix should be /usr/local.
When building the complete GNU system, the prefix will be empty and
/usr will be a symbolic link to /.
(If you are using Autoconf, write it as `@prefix@'.)
Running `make install' with a different value of prefix from
the one used to build the program should not recompile the
program.
exec_prefixexec_prefix should
be $(prefix).
(If you are using Autoconf, write it as `@exec_prefix@'.)
Generally, $(exec_prefix) is used for directories that contain
machine-specific files (such as executables and subroutine libraries),
while $(prefix) is used directly for other directories.
Running `make install' with a different value of exec_prefix
from the one used to build the program should not recompile the
program.
Executable programs are installed in one of the following directories.
bindirsbindirlibexecdirThe definition of `libexecdir' is the same for all packages, so you should install your data in a subdirectory thereof. Most packages install their data under $(libexecdir)/package-name/, possibly within additional subdirectories thereof, such as $(libexecdir)/package-name/machine/version.
Data files used by the program during its execution are divided into categories in two ways.
This makes for six different possibilities. However, we want to discourage the use of architecture-dependent files, aside from object files and libraries. It is much cleaner to make other data files architecture-independent, and it is generally not hard.
Here are the variables Makefiles should use to specify directories to put these various kinds of files in:
This should normally be /usr/local/share, but write it as $(datarootdir). (If you are using Autoconf, write it as `@datadir@'.)
The definition of `datadir' is the same for all packages, so you
should install your data in a subdirectory thereof. Most packages
install their data under $(datadir)/package-name/.
Do not install executables here in this directory (they probably belong
in $(libexecdir) or $(sbindir)). Also do not install
files that are modified in the normal course of their use (programs
whose purpose is to change the configuration of the system excluded).
Those probably belong in $(localstatedir).
These variables specify the directory for installing certain specific types of files, if your program has them. Every GNU package should have Info files, so every program needs `infodir', but not all need `libdir' or `lispdir'.
Most compilers other than GCC do not look for header files in directory
/usr/local/include. So installing the header files this way is
only useful with GCC. Sometimes this is not a problem because some
libraries are only really intended to work with GCC. But some libraries
are intended to work with other compilers. They should install their
header files in two places, one specified by includedir and one
specified by oldincludedir.
The Makefile commands should check whether the value of
oldincludedir is empty. If it is, they should not try to use
it; they should cancel the second installation of the header files.
A package should not replace an existing header in this directory unless
the header came from the same package. Thus, if your Foo package
provides a header file foo.h, then it should install the header
file in the oldincludedir directory if either (1) there is no
foo.h there or (2) the foo.h that exists came from the Foo
package.
To tell whether foo.h came from the Foo package, put a magic
string in the file—part of a comment—and grep for that string.
infodir is separate from
docdir for compatibility with existing practice.
$(docdir) by default. (If
you are using Autoconf, write them as `@htmldir@',
`@dvidir@', etc.) Packages which supply several translations
of their documentation should install them in
`$(htmldir)/'ll, `$(pdfdir)/'ll, etc. where
ll is a locale abbreviation such as `en' or `pt_BR'.
libdir should normally be
/usr/local/lib, but write it as $(exec_prefix)/lib.
(If you are using Autoconf, write it as `@libdir@'.)
If you are using Autoconf, write the default as `@lispdir@'. In order to make `@lispdir@' work, you need the following lines in your configure.in file:
lispdir='${datarootdir}/emacs/site-lisp'
AC_SUBST(lispdir)
Unix-style man pages are installed in one of the following:
And finally, you should set the following variable:
configure shell script.
(If you are using Autconf, use `srcdir = @srcdir@'.)
For example:
# Common prefix for installation directories.
# NOTE: This directory must exist when you start the install.
prefix = /usr/local
datarootdir = $(prefix)/share
datadir = $(datarootdir)
exec_prefix = $(prefix)
# Where to put the executable for the command `gcc'.
bindir = $(exec_prefix)/bin
# Where to put the directories used by the compiler.
libexecdir = $(exec_prefix)/libexec
# Where to put the Info files.
infodir = $(datarootdir)/info
If your program installs a large number of files into one of the
standard user-specified directories, it might be useful to group them
into a subdirectory particular to that program. If you do this, you
should write the install rule to create these subdirectories.
Do not expect the user to include the subdirectory name in the value of any of the variables listed above. The idea of having a uniform set of variable names for installation directories is to enable the user to specify the exact same values for several different GNU packages. In order for this to be useful, all the packages must be designed so that they will work sensibly when the user does so.
All GNU programs should have the following targets in their Makefiles:
By default, the Make rules should compile and link with `-g', so
that executable programs have debugging symbols. Users who don't mind
being helpless can strip the executables later if they wish.
Do not strip executables when installing them. Devil-may-care users can
use the install-strip target to do that.
If possible, write the install target rule so that it does not
modify anything in the directory where the program was built, provided
`make all' has just been done. This is convenient for building the
program under one user name and installing it under another.
The commands should create all the directories in which files are to be
installed, if they don't already exist. This includes the directories
specified as the values of the variables prefix and
exec_prefix, as well as all subdirectories that are needed.
One way to do this is by means of an installdirs target
as described below.
Use `-' before any command for installing a man page, so that
make will ignore any errors. This is in case there are systems
that don't have the Unix man page documentation system installed.
The way to install Info files is to copy them into $(infodir)
with $(INSTALL_DATA) (see Command Variables), and then run
the install-info program if it is present. install-info
is a program that edits the Info dir file to add or update the
menu entry for the given Info file; it is part of the Texinfo package.
Here is a sample rule to install an Info file:
$(DESTDIR)$(infodir)/foo.info: foo.info
$(POST_INSTALL)
# There may be a newer info file in . than in srcdir.
-if test -f foo.info; then d=.; \
else d=$(srcdir); fi; \
$(INSTALL_DATA) $$d/foo.info $(DESTDIR)$@; \
# Run install-info only if it exists.
# Use `if' instead of just prepending `-' to the
# line so we notice real errors from install-info.
# We use `$(SHELL) -c' because some shells do not
# fail gracefully when there is an unknown command.
if $(SHELL) -c 'install-info --version' \
>/dev/null 2>&1; then \
install-info --dir-file=$(DESTDIR)$(infodir)/dir \
$(DESTDIR)$(infodir)/foo.info; \
else true; fi
When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands. See Install Command Categories.
install target.
When you have many documentation files to install, we recommend that
you avoid collisions and clutter by arranging for these targets to
install in subdirectories of the appropriate installation directory,
such as htmldir. As one example, if your package has multiple
manuals, and you wish to install HTML documentation with many files
(such as the “split” mode output by makeinfo --html), you'll
certainly want to use subdirectories, or two nodes with the same name
in different manuals will overwrite each other.
This rule should not modify the directories where compilation is done, only the directories where files are installed.
The uninstallation commands are divided into three categories, just like
the installation commands. See Install Command Categories.
install, but strip the executable files while installing
them. In simple cases, this target can use the install target in
a simple way:
install-strip:
$(MAKE) INSTALL_PROGRAM='$(INSTALL_PROGRAM) -s' \
install
But if the package installs scripts as well as real executables, the
install-strip target can't just refer to the install
target; it has to strip the executables but not the scripts.
install-strip should not strip the executables in the build
directory which are being copied for installation. It should only strip
the copies that are installed.
Normally we do not recommend stripping an executable unless you are sure
the program has no bugs. However, it can be reasonable to install a
stripped executable for actual execution while saving the unstripped
executable elsewhere in case there is a bug.
Delete .dvi files here if they are not part of the distribution.
distclean, plus
more: C source files produced by Bison, tags tables, Info files, and
so on.
The reason we say “almost everything” is that running the command
`make maintainer-clean' should not delete configure even
if configure can be remade using a rule in the Makefile. More
generally, `make maintainer-clean' should not delete anything
that needs to exist in order to run configure and then begin to
build the program. Also, there is no need to delete parent
directories that were created with `mkdir -p', since they could
have existed anyway. These are the only exceptions;
maintainer-clean should delete everything else that can be
rebuilt.
The `maintainer-clean' target is intended to be used by a maintainer of the package, not by ordinary users. You may need special tools to reconstruct some of the files that `make maintainer-clean' deletes. Since these files are normally included in the distribution, we don't take care to make them easy to reconstruct. If you find you need to unpack the full distribution again, don't blame us.
To help make users aware of this, the commands for the special
maintainer-clean target should start with these two:
@echo 'This command is intended for maintainers to use; it'
@echo 'deletes files that may need special tools to rebuild.'
info: foo.info
foo.info: foo.texi chap1.texi chap2.texi
$(MAKEINFO) $(srcdir)/foo.texi
You must define the variable MAKEINFO in the Makefile. It should
run the makeinfo program, which is part of the Texinfo
distribution.
Normally a GNU distribution comes with Info files, and that means the
Info files are present in the source directory. Therefore, the Make
rule for an info file should update it in the source directory. When
users build the package, ordinarily Make will not update the Info files
because they will already be up to date.
dvi: foo.dvi
foo.dvi: foo.texi chap1.texi chap2.texi
$(TEXI2DVI) $(srcdir)/foo.texi
You must define the variable TEXI2DVI in the Makefile. It should
run the program texi2dvi, which is part of the Texinfo
distribution.3 Alternatively,
write just the dependencies, and allow GNU make to provide the command.
Here's another example, this one for generating HTML from Texinfo:
html: foo.html
foo.html: foo.texi chap1.texi chap2.texi
$(TEXI2HTML) $(srcdir)/foo.texi
Again, you would define the variable TEXI2HTML in the Makefile;
for example, it might run makeinfo --no-split --html
(makeinfo is part of the Texinfo distribution).
For example, the distribution tar file of GCC version 1.40 unpacks into a subdirectory named gcc-1.40.
The easiest way to do this is to create a subdirectory appropriately
named, use ln or cp to install the proper files in it, and
then tar that subdirectory.
Compress the tar file with gzip. For example, the actual
distribution file for GCC version 1.40 is called gcc-1.40.tar.gz.
The dist target should explicitly depend on all non-source files
that are in the distribution, to make sure they are up to date in the
distribution.
See Making Releases.
The following targets are suggested as conventional names, for programs in which they are useful.
installcheckinstalldirs # Make sure all installation directories (e.g. $(bindir))
# actually exist by making them if necessary.
installdirs: mkinstalldirs
$(srcdir)/mkinstalldirs $(bindir) $(datadir) \
$(libdir) $(infodir) \
$(mandir)
or, if you wish to support DESTDIR,
# Make sure all installation directories (e.g. $(bindir))
# actually exist by making them if necessary.
installdirs: mkinstalldirs
$(srcdir)/mkinstalldirs \
$(DESTDIR)$(bindir) $(DESTDIR)$(datadir) \
$(DESTDIR)$(libdir) $(DESTDIR)$(infodir) \
$(DESTDIR)$(mandir)
This rule should not modify the directories where compilation is done. It should do nothing but create installation directories.
When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands.
Normal commands move files into their proper places, and set their modes. They may not alter any files except the ones that come entirely from the package they belong to.
Pre-installation and post-installation commands may alter other files; in particular, they can edit global configuration files or data bases.
Pre-installation commands are typically executed before the normal commands, and post-installation commands are typically run after the normal commands.
The most common use for a post-installation command is to run
install-info. This cannot be done with a normal command, since
it alters a file (the Info directory) which does not come entirely and
solely from the package being installed. It is a post-installation
command because it needs to be done after the normal command which
installs the package's Info files.
Most programs don't need any pre-installation commands, but we have the feature just in case it is needed.
To classify the commands in the install rule into these three
categories, insert category lines among them. A category line
specifies the category for the commands that follow.
A category line consists of a tab and a reference to a special Make variable, plus an optional comment at the end. There are three variables you can use, one for each category; the variable name specifies the category. Category lines are no-ops in ordinary execution because these three Make variables are normally undefined (and you should not define them in the makefile).
Here are the three possible category lines, each with a comment that explains what it means:
$(PRE_INSTALL) # Pre-install commands follow. $(POST_INSTALL) # Post-install commands follow. $(NORMAL_INSTALL) # Normal commands follow.
If you don't use a category line at the beginning of the install
rule, all the commands are classified as normal until the first category
line. If you don't use any category lines, all the commands are
classified as normal.
These are the category lines for uninstall:
$(PRE_UNINSTALL) # Pre-uninstall commands follow. $(POST_UNINSTALL) # Post-uninstall commands follow. $(NORMAL_UNINSTALL) # Normal commands follow.
Typically, a pre-uninstall command would be used for deleting entries from the Info directory.
If the install or uninstall target has any dependencies
which act as subroutines of installation, then you should start
each dependency's commands with a category line, and start the
main target's commands with a category line also. This way, you can
ensure that each command is placed in the right category regardless of
which of the dependencies actually run.
Pre-installation and post-installation commands should not run any programs except for these:
[ basename bash cat chgrp chmod chown cmp cp dd diff echo
egrep expand expr false fgrep find getopt grep gunzip gzip
hostname install install-info kill ldconfig ln ls md5sum
mkdir mkfifo mknod mv printenv pwd rm rmdir sed sort tee
test touch true uname xargs yes
The reason for distinguishing the commands in this way is for the sake of making binary packages. Typically a binary package contains all the executables and other files that need to be installed, and has its own method of installing them—so it does not need to run the normal installation commands. But installing the binary package does need to execute the pre-installation and post-installation commands.
Programs to build binary packages work by extracting the pre-installation and post-installation commands. Here is one way of extracting the pre-installation commands (the -s option to make is needed to silence messages about entering subdirectories):
make -s -n install -o all \
PRE_INSTALL=pre-install \
POST_INSTALL=post-install \
NORMAL_INSTALL=normal-install \
| gawk -f pre-install.awk
where the file pre-install.awk could contain this:
$0 ~ /^(normal-install|post-install)[ \t]*$/ {on = 0}
on {print $0}
$0 ~ /^pre-install[ \t]*$/ {on = 1}
This appendix summarizes the directives, text manipulation functions,
and special variables which GNU make understands.
See Special Targets, Catalogue of Implicit Rules,
and Summary of Options,
for other summaries.
Here is a summary of the directives GNU make recognizes:
define variableendefifdef variableifndef variableifeq (a,b)ifeq "a" "b"ifeq 'a' 'b'ifneq (a,b)ifneq "a" "b"ifneq 'a' 'b'elseendifinclude file-include filesinclude fileoverride variable = valueoverride variable := valueoverride variable += valueoverride variable ?= valueoverride define variableendefoverride Directive.
exportmake to export all variables to child processes by default.make.
export variableexport variable = valueexport variable := valueexport variable += valueexport variable ?= valueunexport variablemake whether or not to export a particular variable to child
processes.make.
vpath pattern pathvpath Directive.
vpath patternvpathvpath
directive.
Here is a summary of the built-in functions (see Functions):
$(subst from,to,text)$(patsubst pattern,replacement,text)$(strip string)$(findstring find,text)$(filter pattern...,text)$(filter-out pattern...,text)$(sort list)$(word n,text)$(words text)$(wordlist s,e,text)$(firstword names...)$(lastword names...)$(dir names...)$(notdir names...)$(suffix names...)$(basename names...)$(addsuffix suffix,names...)$(addprefix prefix,names...)$(join list1,list2)$(wildcard pattern...)wildcard.
$(realpath names...)., .., nor symlinks.$(abspath names...). or .. components, but preserves
symlinks.$(error text...)make generates a fatal error
with the message text.$(warning text...)make generates a warning with
the message text.$(shell command)shell Function.
$(origin variable)make variable variable was
defined.origin Function.
$(flavor variable)make variable
variable.flavor Function.
$(foreach var,words,text)foreach Function.
$(call var,param,...)$(1),
$(2) with the first, second, etc. param values.call Function.
$(eval text)eval Function.
$(value var)value Function.
Here is a summary of the automatic variables. See Automatic Variables, for full information.
$@$%$<$?$^$+$^ omits duplicate
prerequisites, while $+ retains them and preserves their order.
$*$(@D)$(@F)$@.
$(*D)$(*F)$*.
$(%D)$(%F)$%.
$(<D)$(<F)$<.
$(^D)$(^F)$^.
$(+D)$(+F)$+.
$(?D)$(?F)$?.
These variables are used specially by GNU make:
MAKEFILESmake.MAKEFILES.
VPATHVPATH Search Path for All Prerequisites.
SHELLSHELL in the makefile to change the shell used to run
commands. See Command Execution. The SHELL
variable is handled specially when importing from and exporting to the
environment. See Choosing the Shell.
MAKESHELLmake. This value takes precedence over the value of
SHELL. See MAKESHELL variable.
MAKEmake was invoked.
Using this variable in commands has special meaning.
See How the MAKE Variable Works.
MAKELEVELmakes).MAKEFLAGSmake. You can set this in the environment or
a makefile to set flags.make.
It is never appropriate to use MAKEFLAGS directly on a
command line: its contents may not be quoted correctly for use in the
shell. Always allow recursive make's to obtain these values
through the environment from its parent.
MAKECMDGOALSmake on the command line. Setting this
variable has no effect on the operation of make.CURDIR-C options are processed, if any). Setting this variable has no
effect on the operation of make.make.
SUFFIXESmake reads any makefiles.
.LIBPATTERNSmake searches for, and their
order.Here is a list of the more common errors you might see generated by
make, and some information about what they mean and how to fix
them.
Sometimes make errors are not fatal, especially in the presence
of a - prefix on a command script line, or the -k command
line option. Errors that are fatal are prefixed with the string
***.
Error messages are all either prefixed with the name of the program (usually `make'), or, if the error is found in a makefile, the name of the file and linenumber containing the problem.
In the table below, these common prefixes are left off.
make errors at all. They mean that a
program that make invoked as part of a command script returned a
non-0 error code (`Error NN'), which make interprets
as failure, or it exited in some other abnormal fashion (with a
signal of some type). See Errors in Commands.
If no *** is attached to the message, then the subprocess failed
but the rule in the makefile was prefixed with the - special
character, so make ignored the error.
make could not understand much of anything about
the command line it just read. GNU make looks for various kinds
of separators (:, =, TAB characters, etc.) to help it
decide what kind of commandline it's seeing. This means it couldn't
find a valid one.
One of the most common reasons for this message is that you (or perhaps
your oh-so-helpful editor, as is the case with many MS-Windows editors)
have attempted to indent your command scripts with spaces instead of a
TAB character. In this case, make will use the second form of
the error above. Remember that every line in the command script must
begin with a TAB character. Eight spaces do not count. See Rule Syntax.
make command (such as a variable assignment). Command scripts
must always be associated with a target.
The second form is generated if the line has a semicolon as the first
non-whitespace character; make interprets this to mean you left
out the "target: prerequisite" section of a rule. See Rule Syntax.
make decided it needed to build a target, but
then couldn't find any instructions in the makefile on how to do that,
either explicit or implicit (including in the default rules database).
If you want that file to be built, you will need to add a rule to your
makefile describing how that target can be built. Other possible
sources of this problem are typos in the makefile (if that filename is
wrong) or a corrupted source tree (if that file is not supposed to be
built, but rather only a prerequisite).
make couldn't find any makefiles to read in.
The latter means that some makefile was found, but it didn't contain any
default goal and none was given on the command line. GNU make
has nothing to do in these situations.
See Arguments to Specify the Makefile.
make allows commands to be specified only once per target
(except for double-colon rules). If you give commands for a target
which already has been defined to have commands, this warning is issued
and the second set of commands will overwrite the first set.
See Multiple Rules for One Target.
make detected a loop in the dependency graph:
after tracing the prerequisite yyy of target xxx, and its
prerequisites, etc., one of them depended on xxx again.
make variable
xxx that, when it's expanded, will refer to itself (xxx).
This is not allowed; either use simply-expanded variables (:=) or
use the append operator (+=). See How to Use Variables.
%); and the
fourth means that all three parts of the static pattern rule contain
pattern characters (%)–only the first two parts should.
See Syntax of Static Pattern Rules.
make detects error
conditions related to parallel processing on systems where
sub-makes can communicate (see Communicating Options to a Sub-make). This warning is
generated if a recursive invocation of a make process is forced
to have `-jN' in its argument list (where N is greater
than one). This could happen, for example, if you set the MAKE
environment variable to `make -j2'. In this case, the
sub-make doesn't communicate with other make processes and
will simply pretend it has two jobs of its own.
make processes to communicate, the parent will pass
information to the child. Since this could result in problems if the
child process isn't actually a make, the parent will only do this
if it thinks the child is a make. The parent uses the normal
algorithms to determine this (see How the MAKE Variable Works). If the makefile is constructed such that the parent
doesn't know the child is a make process, then the child will
receive only part of the information necessary. In this case, the child
will generate this warning message and proceed with its build in a
sequential manner.
Here is the makefile for the GNU tar program. This is a
moderately complex makefile.
Because it is the first target, the default goal is `all'. An
interesting feature of this makefile is that testpad.h is a
source file automatically created by the testpad program,
itself compiled from testpad.c.
If you type `make' or `make all', then make creates
the tar executable, the rmt daemon that provides
remote tape access, and the tar.info Info file.
If you type `make install', then make not only creates
tar, rmt, and tar.info, but also installs
them.
If you type `make clean', then make removes the `.o'
files, and the tar, rmt, testpad,
testpad.h, and core files.
If you type `make distclean', then make not only removes
the same files as does `make clean' but also the
TAGS, Makefile, and config.status files.
(Although it is not evident, this makefile (and
config.status) is generated by the user with the
configure program, which is provided in the tar
distribution, but is not shown here.)
If you type `make realclean', then make removes the same
files as does `make distclean' and also removes the Info files
generated from tar.texinfo.
In addition, there are targets shar and dist that create
distribution kits.
# Generated automatically from Makefile.in by configure.
# Un*x Makefile for GNU tar program.
# Copyright (C) 1991 Free Software Foundation, Inc.
# This program is free software; you can redistribute
# it and/or modify it under the terms of the GNU
# General Public License ...
...
...
SHELL = /bin/sh
#### Start of system configuration section. ####
srcdir = .
# If you use gcc, you should either run the
# fixincludes script that comes with it or else use
# gcc with the -traditional option. Otherwise ioctl
# calls will be compiled incorrectly on some systems.
CC = gcc -O
YACC = bison -y
INSTALL = /usr/local/bin/install -c
INSTALLDATA = /usr/local/bin/install -c -m 644
# Things you might add to DEFS:
# -DSTDC_HEADERS If you have ANSI C headers and
# libraries.
# -DPOSIX If you have POSIX.1 headers and
# libraries.
# -DBSD42 If you have sys/dir.h (unless
# you use -DPOSIX), sys/file.h,
# and st_blocks in `struct stat'.
# -DUSG If you have System V/ANSI C
# string and memory functions
# and headers, sys/sysmacros.h,
# fcntl.h, getcwd, no valloc,
# and ndir.h (unless
# you use -DDIRENT).
# -DNO_MEMORY_H If USG or STDC_HEADERS but do not
# include memory.h.
# -DDIRENT If USG and you have dirent.h
# instead of ndir.h.
# -DSIGTYPE=int If your signal handlers
# return int, not void.
# -DNO_MTIO If you lack sys/mtio.h
# (magtape ioctls).
# -DNO_REMOTE If you do not have a remote shell
# or rexec.
# -DUSE_REXEC To use rexec for remote tape
# operations instead of
# forking rsh or remsh.
# -DVPRINTF_MISSING If you lack vprintf function
# (but have _doprnt).
# -DDOPRNT_MISSING If you lack _doprnt function.
# Also need to define
# -DVPRINTF_MISSING.
# -DFTIME_MISSING If you lack ftime system call.
# -DSTRSTR_MISSING If you lack strstr function.
# -DVALLOC_MISSING If you lack valloc function.
# -DMKDIR_MISSING If you lack mkdir and
# rmdir system calls.
# -DRENAME_MISSING If you lack rename system call.
# -DFTRUNCATE_MISSING If you lack ftruncate
# system call.
# -DV7 On Version 7 Unix (not
# tested in a long time).
# -DEMUL_OPEN3 If you lack a 3-argument version
# of open, and want to emulate it
# with system calls you do have.
# -DNO_OPEN3 If you lack the 3-argument open
# and want to disable the tar -k
# option instead of emulating open.
# -DXENIX If you have sys/inode.h
# and need it 94 to be included.
DEFS = -DSIGTYPE=int -DDIRENT -DSTRSTR_MISSING \
-DVPRINTF_MISSING -DBSD42
# Set this to rtapelib.o unless you defined NO_REMOTE,
# in which case make it empty.
RTAPELIB = rtapelib.o
LIBS =
DEF_AR_FILE = /dev/rmt8
DEFBLOCKING = 20
CDEBUG = -g
CFLAGS = $(CDEBUG) -I. -I$(srcdir) $(DEFS) \
-DDEF_AR_FILE=\"$(DEF_AR_FILE)\" \
-DDEFBLOCKING=$(DEFBLOCKING)
LDFLAGS = -g
prefix = /usr/local
# Prefix for each installed program,
# normally empty or `g'.
binprefix =
# The directory to install tar in.
bindir = $(prefix)/bin
# The directory to install the info files in.
infodir = $(prefix)/info
#### End of system configuration section. ####
SRC1 = tar.c create.c extract.c buffer.c \
getoldopt.c update.c gnu.c mangle.c
SRC2 = version.c list.c names.c diffarch.c \
port.c wildmat.c getopt.c
SRC3 = getopt1.c regex.c getdate.y
SRCS = $(SRC1) $(SRC2) $(SRC3)
OBJ1 = tar.o create.o extract.o buffer.o \
getoldopt.o update.o gnu.o mangle.o
OBJ2 = version.o list.o names.o diffarch.o \
port.o wildmat.o getopt.o
OBJ3 = getopt1.o regex.o getdate.o $(RTAPELIB)
OBJS = $(OBJ1) $(OBJ2) $(OBJ3)
AUX = README COPYING ChangeLog Makefile.in \
makefile.pc configure configure.in \
tar.texinfo tar.info* texinfo.tex \
tar.h port.h open3.h getopt.h regex.h \
rmt.h rmt.c rtapelib.c alloca.c \
msd_dir.h msd_dir.c tcexparg.c \
level-0 level-1 backup-specs testpad.c
.PHONY: all
all: tar rmt tar.info
.PHONY: tar
tar: $(OBJS)
$(CC) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
rmt: rmt.c
$(CC) $(CFLAGS) $(LDFLAGS) -o $@ rmt.c
tar.info: tar.texinfo
makeinfo tar.texinfo
.PHONY: install
install: all
$(INSTALL) tar $(bindir)/$(binprefix)tar
-test ! -f rmt || $(INSTALL) rmt /etc/rmt
$(INSTALLDATA) $(srcdir)/tar.info* $(infodir)
$(OBJS): tar.h port.h testpad.h
regex.o buffer.o tar.o: regex.h
# getdate.y has 8 shift/reduce conflicts.
testpad.h: testpad
./testpad
testpad: testpad.o
$(CC) -o $@ testpad.o
TAGS: $(SRCS)
etags $(SRCS)
.PHONY: clean
clean:
rm -f *.o tar rmt testpad testpad.h core
.PHONY: distclean
distclean: clean
rm -f TAGS Makefile config.status
.PHONY: realclean
realclean: distclean
rm -f tar.info*
.PHONY: shar
shar: $(SRCS) $(AUX)
shar $(SRCS) $(AUX) | compress \
> tar-`sed -e '/version_string/!d' \
-e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
-e q
version.c`.shar.Z
.PHONY: dist
dist: $(SRCS) $(AUX)
echo tar-`sed \
-e '/version_string/!d' \
-e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
-e q
version.c` > .fname
-rm -rf `cat .fname`
mkdir `cat .fname`
ln $(SRCS) $(AUX) `cat .fname`
tar chZf `cat .fname`.tar.Z `cat .fname`
-rm -rf `cat .fname` .fname
tar.zoo: $(SRCS) $(AUX)
-rm -rf tmp.dir
-mkdir tmp.dir
-rm tar.zoo
for X in $(SRCS) $(AUX) ; do \
echo $$X ; \
sed 's/$$/^M/' $$X \
> tmp.dir/$$X ; done
cd tmp.dir ; zoo aM ../tar.zoo *
-rm -rf tmp.dir
Copyright © 2000,2001,2002 Free Software Foundation, Inc.
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# (comments), in commands: Command Syntax# (comments), in makefile: Makefile Contents#include: Automatic Prerequisites$, in function call: Syntax of Functions$, in rules: Rule Syntax$, in variable name: Computed Names$, in variable reference: Reference%, in pattern rules: Pattern Intro%, quoting in patsubst: Text Functions%, quoting in static pattern: Static Usage%, quoting in vpath: Selective Search%, quoting with \ (backslash): Text Functions%, quoting with \ (backslash): Static Usage%, quoting with \ (backslash): Selective Search* (wildcard character): Wildcardsdefine: Sequences,v (RCS file extension): Catalogue of Rules- (in commands): Errorsdefine: Sequences--always-make: Options Summary--assume-new: Options Summary--assume-new: Instead of Execution--assume-new, and recursion: Options/Recursion--assume-old: Options Summary--assume-old: Avoiding Compilation--assume-old, and recursion: Options/Recursion--check-symlink-times: Options Summary--debug: Options Summary--directory: Options Summary--directory: Recursion--directory, and --print-directory: -w Option--directory, and recursion: Options/Recursion--dry-run: Options Summary--dry-run: Instead of Execution--dry-run: Echoing--environment-overrides: Options Summary--file: Options Summary--file: Makefile Arguments--file: Makefile Names--file, and recursion: Options/Recursion--help: Options Summary--ignore-errors: Options Summary--ignore-errors: Errors--include-dir: Options Summary--include-dir: Include--jobs: Options Summary--jobs: Parallel--jobs, and recursion: Options/Recursion--just-print: Options Summary--just-print: Instead of Execution--just-print: Echoing--keep-going: Options Summary--keep-going: Testing--keep-going: Errors--load-average: Options Summary--load-average: Parallel--makefile: Options Summary--makefile: Makefile Arguments--makefile: Makefile Names--max-load: Options Summary--max-load: Parallel--new-file: Options Summary--new-file: Instead of Execution--new-file, and recursion: Options/Recursion--no-builtin-rules: Options Summary--no-builtin-variables: Options Summary--no-keep-going: Options Summary--no-print-directory: Options Summary--no-print-directory: -w Option--old-file: Options Summary--old-file: Avoiding Compilation--old-file, and recursion: Options/Recursion--print-data-base: Options Summary--print-directory: Options Summary--print-directory, and --directory: -w Option--print-directory, and recursion: -w Option--print-directory, disabling: -w Option--question: Options Summary--question: Instead of Execution--quiet: Options Summary--quiet: Echoing--recon: Options Summary--recon: Instead of Execution--recon: Echoing--silent: Options Summary--silent: Echoing--stop: Options Summary--touch: Options Summary--touch: Instead of Execution--touch, and recursion: MAKE Variable--version: Options Summary--warn-undefined-variables: Options Summary--what-if: Options Summary--what-if: Instead of Execution-B: Options Summary-b: Options Summary-C: Options Summary-C: Recursion-C, and -w: -w Option-C, and recursion: Options/Recursion-d: Options Summary-e: Options Summary-e (shell flag): Automatic Prerequisites-f: Options Summary-f: Makefile Arguments-f: Makefile Names-f, and recursion: Options/Recursion-h: Options Summary-I: Options Summary-i: Options Summary-i: Errors-I: Include-j: Options Summary-j: Parallel-j, and archive update: Archive Pitfalls-j, and recursion: Options/Recursion-k: Options Summary-k: Testing-k: Errors-L: Options Summary-l: Options Summary-l (library search): Libraries/Search-l (load average): Parallel-m: Options Summary-M (to compiler): Automatic Prerequisites-MM (to GNU compiler): Automatic Prerequisites-n: Options Summary-n: Instead of Execution-n: Echoing-o: Options Summary-o: Avoiding Compilation-o, and recursion: Options/Recursion-p: Options Summary-q: Options Summary-q: Instead of Execution-R: Options Summary-r: Options Summary-S: Options Summary-s: Options Summary-s: Echoing-t: Options Summary-t: Instead of Execution-t, and recursion: MAKE Variable-v: Options Summary-W: Options Summary-w: Options Summary-W: Instead of Execution-w, and -C: -w Option-w, and recursion: -w Option-W, and recursion: Options/Recursion-w, disabling: -w Option.a (archives): Archive Suffix Rules.C: Catalogue of Rules.c: Catalogue of Rules.cc: Catalogue of Rules.ch: Catalogue of Rules.cpp: Catalogue of Rules.d: Automatic Prerequisites.def: Catalogue of Rules.dvi: Catalogue of Rules.F: Catalogue of Rules.f: Catalogue of Rules.info: Catalogue of Rules.l: Catalogue of Rules.LIBPATTERNS, and link libraries: Libraries/Search.ln: Catalogue of Rules.mod: Catalogue of Rules.o: Catalogue of Rules.p: Catalogue of Rules.PRECIOUS intermediate files: Chained Rules.r: Catalogue of Rules.S: Catalogue of Rules.s: Catalogue of Rules.sh: Catalogue of Rules.sym: Catalogue of Rules.tex: Catalogue of Rules.texi: Catalogue of Rules.texinfo: Catalogue of Rules.txinfo: Catalogue of Rules.w: Catalogue of Rules.web: Catalogue of Rules.y: Catalogue of Rules:: rules (double-colon): Double-Colon? (wildcard character): Wildcards@ (in commands): Echoingdefine: Sequences[...] (wildcard characters): Wildcards\ (backslash), for continuation lines: Simple Makefile\ (backslash), in commands: Splitting Lines\ (backslash), to quote %: Text Functions\ (backslash), to quote %: Static Usage\ (backslash), to quote %: Selective Search__.SYMDEF: Archive Symbolsall (standard target): Goalsar: Implicit Variables-j: Archive Pitfallsas: Implicit Variablesas: Catalogue of Rules\), for continuation lines: Simple Makefile\), in commands: Splitting Lines\), to quote %: Text Functions\), to quote %: Static Usage\), to quote %: Selective Searchcc: Implicit Variablescc: Catalogue of Rulescd (shell command): MAKE Variablecd (shell command): Executioncheck (standard target): Goalsclean (standard target): Goalsclean target: Cleanupclean target: Simple Makefileclobber (standard target): Goalsco: Implicit Variablesco: Catalogue of Rules\) in: Splitting Linesctangle: Implicit Variablesctangle: Catalogue of Rulescweave: Implicit Variablescweave: Catalogue of Rulesmake rules: Options SummaryVPATH): Directory SearchVPATH), and implicit rules: Implicit/SearchVPATH), and link libraries: Libraries/SearchVPATH), and shell commands: Commands/Searchdist (standard target): Goalsdistclean (standard target): Goals$), in function call: Syntax of Functions$), in rules: Rule Syntax$), in variable name: Computed Names$), in variable reference: ReferenceM-x compile): ErrorsSHELL in: Choosing the Shellf77: Implicit Variablesf77: Catalogue of Rulesmake: FeaturesFORCE: Force Targetsg++: Implicit Variablesg++: Catalogue of Rulesgcc: Catalogue of Rulesget: Implicit Variablesget: Catalogue of RulesVPATH: Implicit/SearchMAKEFILE_LIST variable): MAKEFILE_LIST VariableMAKEFILES variable): MAKEFILES Variableinstall (standard target): Goalsld: Catalogue of Ruleslex: Implicit Variableslex: Catalogue of Ruleslint: Implicit Variableslint: Catalogue of Ruleslint, rule to run: Catalogue of Ruleslpr (shell command): Empty Targetslpr (shell command): Wildcard Examplesm2c: Implicit Variablesm2c: Catalogue of Rulesmake depend: Automatic PrerequisitesMAKEFILES variable: MAKEFILES Variablemake processes: How Make WorksMAKEFILE_LIST variable: MAKEFILE_LIST Variablemakeinfo: Implicit Variablesmakeinfo: Catalogue of Rulesmostlyclean (standard target): Goals::): Double-ColonOBJ: Variables Simplifyobj: Variables SimplifyOBJECTS: Variables Simplifyobjects: Variables SimplifyOBJS: Variables Simplifyobjs: Variables Simplifyoverride: Override Directivepc: Implicit Variablespc: Catalogue of Rules.PRECIOUS: Chained Rules.PRECIOUS: Special Targets.SECONDARY: Special Targetsprint (standard target): Goalsprint target: Empty Targetsprint target: Wildcard Examples%, in patsubst: Text Functions%, in static pattern: Static Usage%, in vpath: Selective SearchREADME: Makefile Namesrealclean (standard target): Goals-C: Options/Recursion-f: Options/Recursion-j: Options/Recursion-o: Options/Recursion-t: MAKE Variable-w: -w Option-W: Options/RecursionMAKE variable: MAKE VariableMAKEFILES variable: MAKEFILES Variablerm: Implicit Variablesrm (shell command): Errorsrm (shell command): Phony Targetsrm (shell command): Wildcard Examplesrm (shell command): Simple Makefile::): Double-ColonVPATH: Implicit/Search$: Rule Syntaxs. (SCCS file prefix): Catalogue of RulesVPATH): Directory SearchVPATH), and implicit rules: Implicit/SearchVPATH), and link libraries: Libraries/Searchsed (shell command): Automatic Prerequisitesshar (standard target): Goalsinclude): Includeinclude): IncludeSHELL, MS-DOS specifics: Choosing the ShellSHELL, value of: Choosing the Shellmake: Variables/RecursionTAGS (standard target): Goalstangle: Implicit Variablestangle: Catalogue of Rulestar (standard target): Goalstest (standard target): Goalstex: Implicit Variablestex: Catalogue of Rulestexi2dvi: Implicit Variablestexi2dvi: Catalogue of Rules~): Wildcardstouch (shell command): Empty Targetstouch (shell command): Wildcard ExamplesVPATH, and implicit rules: Implicit/SearchVPATH, and link libraries: Libraries/Searchweave: Implicit Variablesweave: Catalogue of Rulesinclude: Includeyacc: Implicit Variablesyacc: Catalogue of Rulesyacc: Sequences~ (tilde): Wildcards$%: Automatic Variables$(%D): Automatic Variables$(%F): Automatic Variables$(*D): Automatic Variables$(*F): Automatic Variables$(+D): Automatic Variables$(+F): Automatic Variables$(<D): Automatic Variables$(<F): Automatic Variables$(?D): Automatic Variables$(?F): Automatic Variables$(@D): Automatic Variables$(@F): Automatic Variables$(^D): Automatic Variables$(^F): Automatic Variables$*: Automatic Variables$*, and static pattern: Static Usage$+: Automatic Variables$<: Automatic Variables$?: Automatic Variables$@: Automatic Variables$^: Automatic Variables$|: Automatic Variables% (automatic variable): Automatic Variables%D (automatic variable): Automatic Variables%F (automatic variable): Automatic Variables* (automatic variable): Automatic Variables* (automatic variable), unsupported bizarre usage: Missing*D (automatic variable): Automatic Variables*F (automatic variable): Automatic Variables+ (automatic variable): Automatic Variables+D (automatic variable): Automatic Variables+F (automatic variable): Automatic Variables.DEFAULT: Last Resort.DEFAULT: Special Targets.DEFAULT, and empty commands: Empty Commands.DEFAULT_GOAL (define default goal): Special Variables.DELETE_ON_ERROR: Errors.DELETE_ON_ERROR: Special Targets.EXPORT_ALL_VARIABLES: Variables/Recursion.EXPORT_ALL_VARIABLES: Special Targets.FEATURES (list of supported features): Special Variables.IGNORE: Errors.IGNORE: Special Targets.INCLUDE_DIRS (list of include directories): Special Variables.INTERMEDIATE: Special Targets.LIBPATTERNS: Libraries/Search.LOW_RESOLUTION_TIME: Special Targets.NOTPARALLEL: Special Targets.PHONY: Special Targets.PHONY: Phony Targets.POSIX: Options/Recursion.PRECIOUS: Interrupts.PRECIOUS: Special Targets.SECONDARY: Special Targets.SECONDEXPANSION: Special Targets.SECONDEXPANSION: Secondary Expansion.SILENT: Echoing.SILENT: Special Targets.SUFFIXES: Suffix Rules.SUFFIXES: Special Targets.VARIABLES (list of variables): Special Variables/usr/gnu/include: Include/usr/include: Include/usr/local/include: Include< (automatic variable): Automatic Variables<D (automatic variable): Automatic Variables<F (automatic variable): Automatic Variables? (automatic variable): Automatic Variables?D (automatic variable): Automatic Variables?F (automatic variable): Automatic Variables@ (automatic variable): Automatic Variables@D (automatic variable): Automatic Variables@F (automatic variable): Automatic Variables^ (automatic variable): Automatic Variables^D (automatic variable): Automatic Variables^F (automatic variable): Automatic Variablesabspath: File Name Functionsaddprefix: File Name Functionsaddsuffix: File Name Functionsand: Conditional FunctionsAR: Implicit VariablesARFLAGS: Implicit VariablesAS: Implicit VariablesASFLAGS: Implicit Variablesbasename: File Name Functionsbindir: Directory Variablescall: Call FunctionCC: Implicit VariablesCFLAGS: Implicit VariablesCO: Implicit VariablesCOFLAGS: Implicit VariablesCOMSPEC: Choosing the ShellCPP: Implicit VariablesCPPFLAGS: Implicit VariablesCTANGLE: Implicit VariablesCURDIR: RecursionCWEAVE: Implicit VariablesCXX: Implicit VariablesCXXFLAGS: Implicit Variablesdefine: Definingdir: File Name Functionselse: Conditional Syntaxendef: Definingendif: Conditional Syntaxerror: Make Control Functionseval: Eval Functionexec_prefix: Directory Variablesexport: Variables/RecursionFC: Implicit VariablesFFLAGS: Implicit Variablesfilter: Text Functionsfilter-out: Text Functionsfindstring: Text Functionsfirstword: Text Functionsflavor: Flavor Functionforeach: Foreach FunctionGET: Implicit VariablesGFLAGS: Implicit VariablesGNUmakefile: Makefile NamesGPATH: Search Algorithmif: Conditional Functionsifdef: Conditional Syntaxifeq: Conditional Syntaxifndef: Conditional Syntaxifneq: Conditional Syntaxinclude: Includeinfo: Make Control Functionsjoin: File Name Functionslastword: Text FunctionsLDFLAGS: Implicit VariablesLEX: Implicit VariablesLFLAGS: Implicit Variableslibexecdir: Directory VariablesLINT: Implicit VariablesLINTFLAGS: Implicit VariablesM2C: Implicit VariablesMAKE: FlavorsMAKE: MAKE VariableMAKE_RESTARTS (number of times make has restarted): Special VariablesMAKE_VERSION: FeaturesMAKECMDGOALS: Goalsmakefile: Makefile NamesMakefile: Makefile NamesMAKEFILE_LIST: MAKEFILE_LIST VariableMAKEFILES: Variables/RecursionMAKEFILES: MAKEFILES VariableMAKEFLAGS: Options/RecursionMAKEINFO: Implicit VariablesMAKELEVEL: FlavorsMAKELEVEL: Variables/RecursionMAKEOVERRIDES: Options/RecursionMAKESHELL (MS-DOS alternative to SHELL): Choosing the ShellMFLAGS: Options/Recursionnotdir: File Name Functionsor: Conditional Functionsorigin: Origin FunctionOUTPUT_OPTION: Catalogue of Rulesoverride: Override Directivepatsubst: Text Functionspatsubst: Substitution RefsPC: Implicit VariablesPFLAGS: Implicit Variablesprefix: Directory Variablesrealpath: File Name FunctionsRFLAGS: Implicit VariablesRM: Implicit Variablessbindir: Directory Variablesshell: Shell FunctionSHELL: Choosing the ShellSHELL (command execution): Executionsort: Text Functionsstrip: Text Functionssubst: Text Functionssubst: Multiple Targetssuffix: File Name FunctionsSUFFIXES: Suffix RulesTANGLE: Implicit VariablesTEX: Implicit VariablesTEXI2DVI: Implicit Variablesunexport: Variables/Recursionvalue: Value Functionvpath: Selective SearchVPATH: General Searchvpath: Directory SearchVPATH: Directory Searchwarning: Make Control FunctionsWEAVE: Implicit Variableswildcard: File Name Functionswildcard: Wildcard Functionword: Text Functionswordlist: Text Functionswords: Text FunctionsYACC: Implicit VariablesYFLAGS: Implicit Variables| (automatic variable): Automatic Variables[1] GNU Make compiled for MS-DOS and MS-Windows behaves as if prefix has been defined to be the root of the DJGPP tree hierarchy.
[2] On MS-DOS, the value of current working directory is global, so changing it will affect the following command lines on those systems.
[3] texi2dvi uses TeX to do the real work
of formatting. TeX is not distributed with Texinfo.