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This section describes a 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 Makefiles.
Suppose we have a program called foo that will read one file called data.foo and produce two files named data.c and data.h. We want to write a Makefile rule that captures this one-to-two dependency.
The naive rule is incorrect:
# This is incorrect.
data.c data.h: data.foo
foo data.foo
What the above rule really says is that data.c and data.h each depend on data.foo, and can each be built by running ‘foo data.foo’. In other words it is equivalent to:
# We do not want this.
data.c: data.foo
foo data.foo
data.h: data.foo
foo data.foo
which means that foo can be run twice. Usually it will not be run twice, because make implementations are smart enough to check for the existence of the second file after the first one has been built; they will therefore detect that it already exists. However there are a few situations where it can run twice anyway:
A solution that works with parallel make but not with phony dependencies is the following:
data.c data.h: data.foo
foo data.foo
data.h: data.c
The above rules are equivalent to
data.c: data.foo
foo data.foo
data.h: data.foo data.c
foo data.foo
therefore a parallel make will have to serialize the builds of data.c and 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:
# There is still a problem with this one.
data.c: data.foo
foo data.foo
data.h: data.c
The idea is that ‘foo data.foo’ is run only when data.c needs to be updated, but we further state that data.h depends upon data.c. That way, if data.h is required and data.foo is out of date, the dependency on data.c will trigger the build.
This is almost perfect, but suppose we have built data.h and data.c, and then we erase data.h. Then, running ‘make data.h’ will not rebuild data.h. The above rules just state that data.c must be up-to-date with respect to data.foo, and this is already the case.
What we need is a rule that forces a rebuild when data.h is missing. Here it is:
data.c: data.foo
foo data.foo
data.h: data.c
## Recover from the removal of $@
@if test -f $@; then :; else \
rm -f data.c; \
$(MAKE) $(AM_MAKEFLAGS) data.c; \
fi
The above scheme can be extended to handle more outputs and more inputs. One of the outputs is selected to serve as a witness to the successful completion of the command, it depends upon all inputs, and all other outputs depend upon it. For instance, if foo should additionally read data.bar and also produce data.w and data.x, we would write:
data.c: data.foo data.bar
foo data.foo data.bar
data.h data.w data.x: data.c
## Recover from the removal of $@
@if test -f $@; then :; else \
rm -f data.c; \
$(MAKE) $(AM_MAKEFLAGS) data.c; \
fi
However there are now two minor problems in this setup. One is related to the timestamp ordering of data.h, data.w, data.x, and data.c. The other one is a race condition if a parallel make attempts to run multiple instances of the recover block at once.
Let us deal with the first problem. foo outputs four files,
but we do not know in which order these files are created. Suppose
that data.h is created before data.c. Then we have a
weird situation. The next time make is run, data.h
will appear older than data.c, the second rule will be
triggered, a shell will be started to execute the ‘if...fi’
command, but actually it will just execute the then branch,
that is: nothing. In other words, because the witness we selected is
not the first file created by foo, make will start
a shell to do nothing each time it is run.
A simple riposte is to fix the timestamps when this happens.
data.c: data.foo data.bar
foo data.foo data.bar
data.h data.w data.x: data.c
@if test -f $@; then \
touch $@; \
else \
## Recover from the removal of $@
rm -f data.c; \
$(MAKE) $(AM_MAKEFLAGS) data.c; \
fi
Another solution is to use a different and dedicated file as witness, rather than using any of foo's outputs.
data.stamp: data.foo data.bar
@rm -f data.tmp
@touch data.tmp
foo data.foo data.bar
@mv -f data.tmp $@
data.c data.h data.w data.x: data.stamp
## Recover from the removal of $@
@if test -f $@; then :; else \
rm -f data.stamp; \
$(MAKE) $(AM_MAKEFLAGS) data.stamp; \
fi
data.tmp is created before foo is run, so it has a timestamp older than output files output by foo. It is then renamed to data.stamp after foo has run, because we do not want to update data.stamp if foo fails.
This solution still suffers from the second problem: the race condition in the recover rule. If, after a successful build, a user erases data.c and data.h, and runs ‘make -j’, then make may start both recover rules in parallel. If the two instances of the rule execute ‘$(MAKE) $(AM_MAKEFLAGS) data.stamp’ concurrently the build is likely to fail (for instance, the two rules will create data.tmp, but only one can rename it).
Admittedly, such a weird situation does not arise during ordinary
builds. It occurs only when the build tree is mutilated. Here
data.c and data.h have been explicitly removed without
also removing data.stamp and the other output files.
make clean; make will always recover from these situations even
with parallel makes, so you may decide that the recover rule is solely
to help non-parallel make users and leave things as-is. Fixing this
requires some locking mechanism to ensure only one instance of the
recover rule rebuilds data.stamp. One could imagine something
along the following lines.
data.c data.h data.w data.x: data.stamp
## Recover from the removal of $@
@if test -f $@; then :; else \
trap 'rm -rf data.lock data.stamp' 1 2 13 15; \
## mkdir is a portable test-and-set
if mkdir data.lock 2>/dev/null; then \
## This code is being executed by the first process.
rm -f data.stamp; \
$(MAKE) $(AM_MAKEFLAGS) data.stamp; \
result=$$?; rm -rf data.lock; exit $$result; \
else \
## This code is being executed by the follower processes.
## Wait until the first process is done.
while test -d data.lock; do sleep 1; done; \
## Succeed if and only if the first process succeeded.
test -f data.stamp; \
fi; \
fi
Using a dedicated witness, like data.stamp, is very handy when
the list of output files is not known beforehand. As an illustration,
consider the following rules to compile many *.el files into
*.elc files in a single command. It does not matter how
ELFILES is defined (as long as it is not empty: empty targets
are not accepted by POSIX).
ELFILES = one.el two.el three.el ...
ELCFILES = $(ELFILES:=c)
elc-stamp: $(ELFILES)
@rm -f elc-temp
@touch elc-temp
$(elisp_comp) $(ELFILES)
@mv -f elc-temp $@
$(ELCFILES): elc-stamp
## Recover from the removal of $@
@if test -f $@; then :; else \
trap 'rm -rf elc-lock elc-stamp' 1 2 13 15; \
if mkdir elc-lock 2>/dev/null; then \
## This code is being executed by the first process.
rm -f elc-stamp; \
$(MAKE) $(AM_MAKEFLAGS) elc-stamp; \
rmdir elc-lock; \
else \
## This code is being executed by the follower processes.
## Wait until the first process is done.
while test -d elc-lock; do sleep 1; done; \
## Succeed if and only if the first process succeeded.
test -f elc-stamp; exit $$?; \
fi; \
fi
For completeness it should be noted that GNU make is able to express rules with multiple output files using pattern rules (see Pattern Rule Examples). We do not discuss pattern rules here because they are not portable, but they can be convenient in packages that assume GNU make.