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Overriding Part of Another Makefile
Sometimes it is useful to have a makefile that is mostly just like
another makefile. You can often use the `include' directive to include
one in the other, and add more targets or variable definitions.
However, if the two makefiles give different commands for the same
target, `make' will not let you just do this. But there is another way.
In the containing makefile (the one that wants to include the other),
you can use a match-anything pattern rule to say that to remake any
target that cannot be made from the information in the containing
makefile, `make' should look in another makefile. Note: Pattern
Rules, for more information on pattern rules.
For example, if you have a makefile called `Makefile' that says how
to make the target `foo' (and other targets), you can write a makefile
called `GNUmakefile' that contains:
frobnicate > foo
@$(MAKE) -f Makefile $@
If you say `make foo', `make' will find `GNUmakefile', read it, and
see that to make `foo', it needs to run the command `frobnicate > foo'.
If you say `make bar', `make' will find no way to make `bar' in
`GNUmakefile', so it will use the commands from the pattern rule: `make
-f Makefile bar'. If `Makefile' provides a rule for updating `bar',
`make' will apply the rule. And likewise for any other target that
`GNUmakefile' does not say how to make.
The way this works is that the pattern rule has a pattern of just
`%', so it matches any target whatever. The rule specifies a
dependency `force', to guarantee that the commands will be run even if
the target file already exists. We give `force' target empty commands
to prevent `make' from searching for an implicit rule to build
it--otherwise it would apply the same match-anything rule to `force'
itself and create a dependency loop!
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