File: make.info,  Node: Multiple Rules,  Next: Static Pattern,  Prev: Multiple Targets,  Up: Rules

4.11 Multiple Rules for One Target
==================================

One file can be the target of several rules.  All the prerequisites
mentioned in all the rules are merged into one list of prerequisites for
the target.  If the target is older than any prerequisite from any rule,
the recipe is executed.

   There can only be one recipe to be executed for a file.  If more than
one rule gives a recipe for the same file, 'make' uses the last one
given and prints an error message.  (As a special case, if the file's
name begins with a dot, no error message is printed.  This odd behavior
is only for compatibility with other implementations of 'make'... you
should avoid using it).  Occasionally it is useful to have the same
target invoke multiple recipes which are defined in different parts of
your makefile; you can use "double-colon rules" (*note Double-Colon::)
for this.

   An extra rule with just prerequisites can be used to give a few extra
prerequisites to many files at once.  For example, makefiles often have
a variable, such as 'objects', containing a list of all the compiler
output files in the system being made.  An easy way to say that all of
them must be recompiled if 'config.h' changes is to write the following:

     objects = foo.o bar.o
     foo.o : defs.h
     bar.o : defs.h test.h
     $(objects) : config.h

   This could be inserted or taken out without changing the rules that
really specify how to make the object files, making it a convenient form
to use if you wish to add the additional prerequisite intermittently.

   Another wrinkle is that the additional prerequisites could be
specified with a variable that you set with a command line argument to
'make' (*note Overriding Variables: Overriding.).  For example,

     extradeps=
     $(objects) : $(extradeps)

means that the command 'make extradeps=foo.h' will consider 'foo.h' as a
prerequisite of each object file, but plain 'make' will not.

   If none of the explicit rules for a target has a recipe, then 'make'
searches for an applicable implicit rule to find one *note Using
Implicit Rules: Implicit Rules.).