The following is a brief summary of the improvements made to the
preprocessor's Makefile dependency generation functionality.
Consider the simple example of a file test.c
that includes the file test.h, and the command line
/usr/bin/gcc -MD -c test.c -o subdir/bar.o
With previous versions of CPP, this command line would leave a
file test.d in the current directory, and its contents
would be
test.o: test.c test.h
which is pretty useless. The latest versions of CPP instead do
what you probably want - they make the target
subdir/bar.o, place the dependency file in the same
directory as the object file, and name it after the object file.
Thus in the file subdir/bar.d you will find
subdir/bar.o: test.c test.h
By default CPP uses the main file name, excluding any path, and
appends the object suffix, normally ``.o'', to it to obtain the name
of the target for dependency generation. With -MT or
-MQ you can specify a target yourself, overriding the
default one.
If you want multiple targets, you can specify them as a single
argument to a switch, or use multiple switches. The targets you
specify are output in the order they appear on the command line.
-MQ is identical to -MT, except that the
target name is quoted for Make, but with -MT it isn't.
For example, -MT '$(objpfx)foo.o' gives
$(objpfx)foo.o: /tmp/foo.c
but -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: /tmp/foo.c
The default target is automatically quoted, as if it were given with
-MQ.
When used with -M or -MM,
-MF specifies a file to write the dependencies to.
This allows the preprocessor to write the preprocessed file to
stdout normally. If no -MF switch is given, and one is
not implied by -MD or -MMD, CPP sends the
rules to stdout and suppresses normal preprocessed output.
Finally, -MP instructs CPP to add a phony target
for each dependency other than the main file, causing each to depend
on nothing. These dummy rules work around errors make
gives if you remove header files without updating the
Makefile to match.
This is typical output:-
test.o: /tmp/test.c /tmp/test.h
/tmp/test.h:
Please send FSF & GNU inquiries & questions to
gnu@gnu.org.
There are also other ways
to contact the FSF.
These pages are maintained by
the GCC team.
For questions related to the use of GCC, please consult these web
pages and the GCC manuals. If
that fails, the gcc-help@gcc.gnu.org
mailing list might help.
Please send comments on these web pages and the development of GCC to our
developer mailing list at gcc@gnu.org
or gcc@gcc.gnu.org. All of our lists
have public archives.
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Verbatim copying and distribution of this entire article is
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Last modified 2006-06-21
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