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27.9 Handling Tools that Produce Many Outputs

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 ‘Makefile’s.

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 That way, if ‘data.h’ is required and ‘data.foo’ is out of date, the dependency on ‘data.c’ will trigger the build.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 three minor problems in this setup. One is related to the timestamp ordering of ‘data.h’, ‘data.w’, ‘data.x’, and ‘data.c’. Another one is a race condition if a parallel make attempts to run multiple instances of the recover block at once. Finally, the recursive rule breaks ‘make -n’ when run with GNU make (as well as some other make implementations), as it may remove ‘data.h’ even when it should not (see How the MAKE Variable Works in The GNU Make Manual).

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
        @if test -f $@; then :; else \
## Recover from the removal of $@
          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

These solutions all still suffer from the third problem, namely that they break the promise that ‘make -n’ should not cause any actual changes to the tree. For those solutions that do not create lock files, it is possible to split the recover rules into two separate recipe commands, one of which does all work but the recursion, and the other invokes the recursive ‘$(MAKE)’. The solutions involving locking could act upon the contents of the ‘MAKEFLAGS’ variable, but parsing that portably is not easy (see The Make Macro MAKEFLAGS in The Autoconf Manual). Here is an example:

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 $@
        @dry=; for f in x $$MAKEFLAGS; do \
          case $$f in \
            *=*|--*);; \
            *n*) dry=:;; \
          esac; \
        done; \
        if test -f $@; then :; else \
          $$dry trap 'rm -rf elc-lock elc-stamp' 1 2 13 15; \
          if $$dry mkdir elc-lock 2>/dev/null; then \
## This code is being executed by the first process.
            $$dry rm -f elc-stamp; \
            $(MAKE) $(AM_MAKEFLAGS) elc-stamp; \
            $$dry 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 && test -z "$$dry"; do \
              sleep 1; \
            done; \
## Succeed if and only if the first process succeeded.
            $$dry 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 in 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 make.


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