2.5 Letting 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.