( Automatic Prerequisites

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 4.14 Generating Prerequisites Automatically
 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;  Chains of Implicit Rules Chained 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 ( 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 prerequisites, because it always regenerates any makefile that is out
 of date.   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
    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; rm -f $@; \
               $(CC) -M $(CPPFLAGS) $< > $@.$$$$; \
               sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \
               rm -f $@.$$$$
  Pattern Rules, for information on defining pattern rules.  The
 `-e' flag to the shell causes it to exit immediately if the `$(CC)'
 command (or any other command) fails (exits with a nonzero status).  
    With the GNU C compiler, you may wish to use the `-MM' flag instead
 of `-M'.  This omits prerequisites on system header files.  
 Options Controlling the Preprocessor ( Options,
 for details.
    The purpose of the `sed' command is to translate (for example):
      main.o : main.c defs.h
      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
    Once you've defined the rule to remake the `.d' files, you then use
 the `include' directive to read them all in.   Include.  For
      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'.   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.   Remaking Makefiles.
    Note that the `.d' files contain target definitions; you should be
 sure to place the `include' directive _after_ the first, default goal
 in your makefiles or run the risk of having a random object file become
 the default goal.   How Make Works.
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