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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 section 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 section Including Other Makefiles).
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that is out of date.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 section How Makefiles Are Remade.
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; $(CC) -M $(CPPFLAGS) $< \
| sed 's/\($*\)\.o[ :]*/\1.o $@ : /g' > $@; \
[ -s $@ ] || rm -f $@
See section Defining and Redefining Pattern Rules, for information on defining pattern rules. The
`-e' flag to the shell makes it exit immediately if the
$(CC) command fails (exits with a nonzero status). Normally the
shell exits with the status of the last command in the pipeline
(sed in this case), so make would not notice a nonzero
status from the compiler.
With the GNU C compiler, you may wish to use the `-MM' flag instead of `-M'. This omits prerequisites on system header files. See section `Options Controlling the Preprocessor' in Using GNU CC, 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 section Including Other Makefiles. 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 section Substitution References, 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 section How Makefiles Are Remade.
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