Packages made with Autoconf and Automake ship with some generated files like configure or Makefile.in. These files were generated on the developer’s machine and are distributed so that end-users do not have to install the maintainer tools required to rebuild them. Other generated files like Lex scanners, Yacc parsers, or Info documentation are usually distributed on similar grounds.
Automake output generates rules in Makefiles to rebuild these files.
make will run
autoconf to rebuild
configure whenever configure.ac is changed. This makes
development safer by ensuring a configure is never out-of-date
with respect to configure.ac.
As generated files shipped in packages are up-to-date, and because
tar preserves times-tamps, these rebuild rules are not
triggered when a user unpacks and builds a package.
Unless you use CVS keywords (in which case files must be updated at commit time), CVS preserves timestamp during ‘cvs commit’ and ‘cvs import -d’ operations.
When you check out a file using ‘cvs checkout’ its timestamp is set to that of the revision that is being checked out.
cvs update, files will have the date of the
update, not the original timestamp of this revision. This is meant to
make sure that
make notices that sources files have been updated.
This timestamp shift is troublesome when both sources and generated
files are kept under CVS. Because CVS processes files in lexical
order, configure.ac will appear newer than configure
cvs update that updates both files, even if
configure was newer than configure.ac when it was
checked in. Calling
make will then trigger a spurious rebuild
There are basically two clans amongst maintainers: those who keep all distributed files under CVS, including generated files, and those who keep generated files out of CVS.
cvs updateto update their copy, instead of
cvs checkoutto fetch a fresh one, timestamps will be inaccurate. Some rebuild rules will be triggered and attempt to run developer tools such as
Calls to such tools are all wrapped into a call to the
script discussed later (see maintainer-mode), so that the user will
see more descriptive warnings about missing or out-of-date tools, and
possible suggestions about how to obtain them, rather than just some
“command not found” error, or (worse) some obscure message from some
older version of the required tool they happen to have installed.
Maintainers interested in keeping their package buildable from a CVS
checkout even for those users that lack maintainer-specific tools might
want to provide a helper script (or to enhance their existing bootstrap
script) to fix the timestamps after a
cvs update or a
git checkout, to prevent spurious
rebuilds. In case of a project committing the Autotools-generated
files, as well as the generated .info files, such a script might
look something like this:
#!/bin/sh # fix-timestamp.sh: prevents useless rebuilds after "cvs update" sleep 1 # aclocal-generated aclocal.m4 depends on locally-installed # '.m4' macro files, as well as on 'configure.ac' touch aclocal.m4 sleep 1 # autoconf-generated configure depends on aclocal.m4 and on # configure.ac touch configure # so does autoheader-generated config.h.in touch config.h.in # and all the automake-generated Makefile.in files touch `find . -name Makefile.in -print` # finally, the makeinfo-generated '.info' files depend on the # corresponding '.texi' files touch doc/*.info
AM_MAINTAINER_MODE, which will disable all of these rebuild rules by default. This is further discussed in maintainer-mode.
For instance, suppose a developer has modified Makefile.am and has rebuilt Makefile.in, and then decides to do a last-minute change to Makefile.am right before checking in both files (without rebuilding Makefile.in to account for the change).
This last change to Makefile.am makes the copy of Makefile.in out-of-date. Since CVS processes files alphabetically, when another developer ‘cvs update’s his or her tree, Makefile.in will happen to be newer than Makefile.am. This other developer will not see that Makefile.in is out-of-date.
One way to get CVS and
make working peacefully is to never
store generated files in CVS, i.e., do not CVS-control files that
are Makefile targets (also called derived files).
This way developers are not annoyed by changes to generated files. It does not matter if they all have different versions (assuming they are compatible, of course). And finally, timestamps are not lost; changes to sources files can’t be missed as in the Makefile.am/Makefile.in example discussed earlier.
The drawback is that the CVS repository is not an exact copy of what is distributed and that users now need to install various development tools (maybe even specific versions) before they can build a checkout. But, after all, CVS’s job is versioning, not distribution.
Allowing developers to use different versions of their tools can also hide bugs during distributed development. Indeed, developers will be using (hence testing) their own generated files, instead of the generated files that will be actually released. The developer who prepares the tarball might be using a version of the tool that produces bogus output (for instance a non-portable C file), something other developers could have noticed if they weren’t using their own versions of this tool.
Another class of files not discussed here (because they do not cause
timestamp issues) are files that are shipped with a package, but
maintained elsewhere. For instance, tools like
autopoint (from Gettext) or
Libtool), will install or update files in your package.
These files, whether they are kept under CVS or not, raise similar concerns about version mismatch between developers’ tools. The Gettext manual has a section about this; see Integrating with CVS in GNU gettext tools.