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| -rw-r--r-- | doc/automake.texi | 68 |
1 files changed, 24 insertions, 44 deletions
diff --git a/doc/automake.texi b/doc/automake.texi index 495d80ad6..47f49a49b 100644 --- a/doc/automake.texi +++ b/doc/automake.texi @@ -12862,6 +12862,13 @@ This section describes a @command{make} idiom that can be used when a tool produces multiple output files. It is not specific to Automake and can be used in ordinary @file{Makefile}s. +First, however: GNU @command{make} is able to express rules with +multiple output files using pattern rules (@pxref{Pattern Examples, , +Pattern Rule Examples, make, The GNU Make Manual}). We do not discuss +pattern rules here because they are not portable, but if you're able +to assume GNU @command{make}, they are typically more convenient than +any of the below approaches. + Suppose we have a program called @command{foo} that will read one file called @file{data.foo} and produce two files named @file{data.c} and @file{data.h}. We want to write a @file{Makefile} rule that captures @@ -12905,36 +12912,14 @@ Another case is when the dependency (here @file{data.foo}) is (or depends upon) a phony target. @end itemize -A solution that works with parallel @command{make} but not with -phony dependencies is the following: - -@example -data.c data.h: data.foo - foo data.foo -data.h: data.c -@end example - -@noindent -The above rules are equivalent to - -@example -data.c: data.foo - foo data.foo -data.h: data.foo data.c - foo data.foo -@end example +Ideally, we want a scheme that will support any number of output +files, and that works with parallel @command{make} invocations, and +that does nothing when @samp{make -n} is run. It is apparently not +possible to achieve a perfect solution. Even an acceptable solution +for the majority of cases gets complicated, so we will take it step by +step. -@noindent -therefore a parallel @command{make} will have to serialize the builds -of @file{data.c} and @file{data.h}, and will detect that the second is -no longer needed once the first is over. - -Using this pattern is probably enough for most cases. However it does -not scale easily to more output files (in this scheme all output files -must be totally ordered by the dependency relation), so we will -explore a more complicated solution. - -Another idea is to write the following: +One idea is to write the following: @example # There is still a problem with this one. @@ -12960,6 +12945,7 @@ What we need is a rule that forces a rebuild when @file{data.h} is missing. Here it is: @example +# More or less works, but not easy to generalize. data.c: data.foo foo data.foo data.h: data.c @@ -12971,6 +12957,7 @@ data.h: data.c It is tempting to use a single test as follows: @example +# This breaks make -n. data.h: data.c ## Recover from the removal of $@@ @@if test -f $@@; then :; else \ @@ -13003,14 +12990,14 @@ data.h data.w data.x: data.c @@test -f $@@ || $(MAKE) $(AM_MAKEFLAGS) data.c @end example -However there are now three minor problems in this setup. One is related -to the timestamp ordering of @file{data.h}, @file{data.w}, -@file{data.x}, and @file{data.c}. Another one is a race condition -if a parallel @command{make} attempts to run multiple instances of the -recover block at once. Finally, the recursive rule breaks @samp{make -n} -when run with GNU @command{make} (as well as some other @command{make} -implementations), as it may remove @file{data.h} even when it should not -(@pxref{MAKE Variable, , How the @code{MAKE} Variable Works, make, +However there are now three problems in this setup. One is related to +the timestamp ordering of @file{data.h}, @file{data.w}, @file{data.x}, +and @file{data.c}. A second is a race condition if a parallel +@command{make} attempts to run multiple instances of the recover block +at once. Finally, the recursive rule breaks @samp{make -n} when run +with GNU @command{make} (as well as some other @command{make} +implementations), as it may remove @file{data.h} even when it should +not (@pxref{MAKE Variable, , How the @code{MAKE} Variable Works, make, The GNU Make Manual}). Let us deal with the first problem. @command{foo} outputs four files, @@ -13181,13 +13168,6 @@ $(ELCFILES): elc-stamp fi @end example -For completeness it should be noted that GNU @command{make} is able to -express rules with multiple output files using pattern rules -(@pxref{Pattern Examples, , Pattern Rule Examples, make, The GNU Make -Manual}). We do not discuss pattern rules here because they are not -portable, but they can be convenient in packages that assume GNU -@command{make}. - @node Hard-Coded Install Paths @section Installing to Hard-Coded Locations |