Automatically discovering C dependencies

An alternative to gcc -M is fastdep. Fastdep's author reports fastdep to be ten times faster than gcc's -M. If the project takes a while to build, fastdep may be worth a look.

gcc -M file.c does what you need.

What I do in my Makefile is

SRCS=$(wildcard *.c)

depend: $(SRCS)
    gcc -M $(CFLAGS) $(SRCS) >depend

include depend

This means that if any of the source files are updated, the depend rule will run, and use gcc -M to update the file called depend. This is then included in the makefile to provide the dependency rules for all the source files.

Make will check that a file is up to date before including it, so this depend rule will run if necessary whenever you run make without you needing to do a "make depend".

This will run any time any file has changed. I've never found this a problem, but if you had a huge number of files in the directory you might find it took too long, in which case you could try having one dependency file per source file, like this:

SRCS=$(wildcard *.c)
DEPS=$(SRCS:.c=.dep)

%.dep : %.c
    gcc -M $(CFLAGS) $< >$@

include $(DEPS)

Note that you can use -MM instead of -M to not include system headers.