bug#8675: error: token "@" is not valid in preprocessor expressions

[Paul Eggert <eggert@cs.ucla.edu>]

[Adding bug-gnulib to this thread.  For bug-gnulib readers, the scenario is
 in Emacs a "bzr update; make" failed with:

  ./unistd.h:1186:5: error: token "@" is not valid in preprocessor expressions

 because unistd.h was built with the old Makefile.
 Full thread at <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8675#31>.
]

On 05/18/11 05:39, Andreas Schwab wrote:
> If that does not work, then proper dependencies are missing.

I don't see how adding a dependency would fix this problem.  In this
case, lib/Makefile in turn depended on 'configure', 'configure.in',
'm4/longlong.m4', etc., etc., and one of these files got updated, so
lib/Makefile was regenerated; but as I understand it, the 'make' that was
(still) running was based on the out-of-date lib/Makefile, and it
generated the a bad lib/unistd.h.

Adding a dependency "unistd.h: Makefile" wouldn't fix this problem.

This sort of problem is not due to gnulib per se; it's inherent to any
project that uses makefiles.  In general, if a patch modifies a
makefile, or anything the makefile depends on, then you must
regenerate everything from scratch with a fresh 'make' invocation.

That being said, I've run into Christoph's problem myself more than
once, and it's a hassle, and it'd be nice to address it somehow.  How
about this idea?  I expect it would have worked around this problem.
Currently lib/Makefile contains something like this:

	unistd.h: <dependencies>
		<big-hairy-command>

Suppose we change this rule to look like this:

	unistd.h: Makefile <dependencies>
		case ' $? ,$(USING_NEW_MAKEFILE)' in \
		  *' Makefile '*,) \
		    exec $(MAKE) $(AM_MAKEFLAGS) USING_NEW_MAKEFILE=yes $@;; \
		esac; \
		<big-hairy-command>

This would be a gnulib change, so I'll CC: this to bug-gnulib.
A similar pattern would apply to every module that generates
a .h file with a big hairy command that uses 'make' variables.