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(Overriding Makefiles): Don't suggest using .DEFAULT; that loses if the

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target file exists.  Instead recommend the combination of a match-anything
rule and a force target.
This commit is contained in:
Roland McGrath 1994-05-11 01:42:43 +00:00
parent e83c7d40cd
commit c25db14264
1 changed files with 17 additions and 9 deletions
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make.texinfo
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@ -1247,13 +1247,12 @@ include one in the other, and add more targets or variable definitions.
However, if the two makefiles give different commands for the same However, if the two makefiles give different commands for the same
target, @code{make} will not let you just do this. But there is another way. target, @code{make} will not let you just do this. But there is another way.
@cindex @code{.DEFAULT}, used to override @cindex match-anything rule, used to override
In the containing makefile (the one that wants to include the other), In the containing makefile (the one that wants to include the other),
you can use the @code{.DEFAULT} special target to say that to remake you can use a match-anything pattern rule to say that to remake any
any target that cannot be made from the information in the containing target that cannot be made from the information in the containing
makefile, @code{make} should look in another makefile. makefile, @code{make} should look in another makefile.
@xref{Last Resort, , Defining Last-Resort Default Rules}, @xref{Pattern Rules}, for more information on pattern rules.
for more information on @code{.DEFAULT}.
For example, if you have a makefile called @file{Makefile} that says how For example, if you have a makefile called @file{Makefile} that says how
to make the target @samp{foo} (and other targets), you can write a to make the target @samp{foo} (and other targets), you can write a
@ -1263,18 +1262,27 @@ makefile called @file{GNUmakefile} that contains:
foo: foo:
frobnicate > foo frobnicate > foo
.DEFAULT: %: force
@@$(MAKE) -f Makefile $@@ @@$(MAKE) -f Makefile $@@
force: ;
@end example @end example
If you say @samp{make foo}, @code{make} will find @file{GNUmakefile}, If you say @samp{make foo}, @code{make} will find @file{GNUmakefile},
read it, and see that to make @file{foo}, it needs to run the command read it, and see that to make @file{foo}, it needs to run the command
@samp{frobnicate > foo}. If you say @samp{make bar}, @code{make} will @samp{frobnicate > foo}. If you say @samp{make bar}, @code{make} will
find no way to make @file{bar} in @file{GNUmakefile}, so it will use the find no way to make @file{bar} in @file{GNUmakefile}, so it will use the
commands from @code{.DEFAULT}: @samp{make -f Makefile bar}. If commands from the pattern rule: @samp{make -f Makefile bar}. If
@file{Makefile} provides a rule for updating @file{bar}, @code{make} @file{Makefile} provides a rule for updating @file{bar}, @code{make}
will apply the rule. And likewise for any other target that will apply the rule. And likewise for any other target that
@file{GNUmakefile} does not say how to make.@refill @file{GNUmakefile} does not say how to make.
The way this works is that the pattern rule has a pattern of just
@samp{%}, so it matches any target whatever. The rule specifies a
dependency @file{force}, to guarantee that the commands will be run even
if the target file already exists. We give @file{force} target empty
commands to prevent @code{make} from searching for an implicit rule to
build it---otherwise it would apply the same match-anything rule to
@file{force} itself and create a dependency loop!
@node Rules, Commands, Makefiles, Top @node Rules, Commands, Makefiles, Top
@chapter Writing Rules @chapter Writing Rules