27.5 Files left in build directory after distclean

This is a diagnostic you might encounter while running ‘make distcheck’.

As explained in Checking the Distribution, ‘make distcheck’ attempts to build and check your package for errors like this one.

‘make distcheck’ will perform a VPATH build of your package (see section Parallel Build Trees (a.k.a. VPATH Builds)), and then call ‘make distclean’. Files left in the build directory after ‘make distclean’ has run are listed after this error.

This diagnostic really covers two kinds of errors:

• files that are forgotten by distclean;
• distributed files that are erroneously rebuilt.

The former left-over files are not distributed, so the fix is to mark them for cleaning (see section What Gets Cleaned), this is obvious and doesn't deserve more explanations.

The latter bug is not always easy to understand and fix, so let's proceed with an example. Suppose our package contains a program for which we want to build a man page using help2man. GNU help2man produces simple manual pages from the ‘--help’ and ‘--version’ output of other commands (see (help2man)Top section `Overview' in The Help2man Manual). Because we don't want to force our users to install help2man, we decide to distribute the generated man page using the following setup.

# This Makefile.am is bogus.
bin_PROGRAMS = foo
foo_SOURCES = foo.c
dist_man_MANS = foo.1

foo.1: foo$(EXEEXT)
        help2man --output=foo.1 ./foo$(EXEEXT)

This will effectively distribute the man page. However, ‘make distcheck’ will fail with:

ERROR: files left in build directory after distclean:
./foo.1

Why was ‘foo.1’ rebuilt? Because although distributed, ‘foo.1’ depends on a non-distributed built file: ‘foo$(EXEEXT)’. ‘foo$(EXEEXT)’ is built by the user, so it will always appear to be newer than the distributed ‘foo.1’.

‘make distcheck’ caught an inconsistency in our package. Our intent was to distribute ‘foo.1’ so users do not need to install help2man, however since this rule causes this file to be always rebuilt, users do need help2man. Either we should ensure that ‘foo.1’ is not rebuilt by users, or there is no point in distributing ‘foo.1’.

More generally, the rule is that distributed files should never depend on non-distributed built files. If you distribute something generated, distribute its sources.

One way to fix the above example, while still distributing ‘foo.1’ is to not depend on ‘foo$(EXEEXT)’. For instance, assuming foo --version and foo --help do not change unless ‘foo.c’ or ‘configure.ac’ change, we could write the following ‘Makefile.am’:

bin_PROGRAMS = foo
foo_SOURCES = foo.c
dist_man_MANS = foo.1

foo.1: foo.c $(top_srcdir)/configure.ac
        $(MAKE) $(AM_MAKEFLAGS) foo$(EXEEXT)
        help2man --output=foo.1 ./foo$(EXEEXT)

This way, ‘foo.1’ will not get rebuilt every time ‘foo$(EXEEXT)’ changes. The make call makes sure ‘foo$(EXEEXT)’ is up-to-date before help2man. Another way to ensure this would be to use separate directories for binaries and man pages, and set SUBDIRS so that binaries are built before man pages.

We could also decide not to distribute ‘foo.1’. In this case it's fine to have ‘foo.1’ dependent upon ‘foo$(EXEEXT)’, since both will have to be rebuilt. However it would be impossible to build the package in a cross-compilation, because building ‘foo.1’ involves an execution of ‘foo$(EXEEXT)’.

Another context where such errors are common is when distributed files are built by tools that are built by the package. The pattern is similar:

distributed-file: built-tools distributed-sources
        build-command

should be changed to

distributed-file: distributed-sources
        $(MAKE) $(AM_MAKEFLAGS) built-tools
        build-command

or you could choose not to distribute ‘distributed-file’, if cross-compilation does not matter.

The points made through these examples are worth a summary:

For desperate cases, it's always possible to disable this check by setting distcleancheck_listfiles as documented in Checking the Distribution. Make sure you do understand the reason why ‘make distcheck’ complains before you do this. distcleancheck_listfiles is a way to hide errors, not to fix them. You can always do better.