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(make.info)Automatic Prerequisites

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Generating Prerequisites Automatically
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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; Note: 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 (Note: 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prerequisites, because it always regenerates any makefile that is out
of date.  Note: 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; $(CC) -M $(CPPFLAGS) $< \
                       | sed 's/\($*\)\.o[ :]*/\1.o $@ : /g' > $@; \
                     [ -s $@ ] || rm -f $@

Note: 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.  Note:
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.  Note: 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'.  Note: 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.  Note: Remaking Makefiles.