On Fri, Dec 31, 2021 at 7:44 AM Christian Ehrhardt <ehrha...@genua.de>
wrote:
> Here at genua, trying to build libpcap sometimes breaks in
> libpcap with the following error message:
>
> | Using $< in a non-suffix rule context is a GNUmake idiom \
> | (/data/git/ehrhardt/genuos/os/mk/bsd.dep.mk:47)
>
> The bug is in bsd.dep.mk where ${.IMPSRC} (aka $<) is used
> in a context where it is not neccessarily defined by OpenBSD make.
>
> Below is a diff to Use ${.ALLSRC:M*.y} instead of ${.IMPSRC} as
> the input file for yacc.
>
> The issue is with the rule for the grammar.h file that is generated
> by yacc from grammar.c. You can easily reproduce the bug with the
> following steps:
> - build libpcap from scratch: cd .../src/lib/libpcap && make clean all
> - remove the generated grammar.h file: rm obj*/grammar.h
> - build libpcap again (incremental build): make
> In normal builds this does not trigger as grammar.h is implicitly
> generated by the rule for grammar.c and when make checks for
> dependencies it simply finds grammar.h uptodate. However,
> incremental or parallel builds might decide to make grammar.h
> from grammar.y.
>
> Now, why is this only a problem for grammar.h but not for
> grammar.c? The answer to this question is burried deeply in
> OpenBSD's mk files.
>
> The snippet in bsd.dep.mk that triggers the error is a single
> rule statement that generates foo.c and foo.h from foo.y with a
> call to yacc -d. The rule is generated with a loop, i.e. it is not
> a prefix rule. However, a prefix rule context is required for
> the use of ${.IMPSRC} aka $<. For the .c file such a prefix
> rule is provided by bsd.sys.mk and this rule is in scope when
> make evaluates the yacc rule. However, for .h file generation
> from a .y file there is no such prefix rule defined in any of
> the Makefiles. Even if it were the .h suffix is missing from
> .SUFFIXES and the rule would not be considered.
>
> NOTE: The obvious way to fix this would be to use $f instead of
> ${.IMPSRC}. However, this does not work as $f is then missing
> the path prefix and yacc won't find it if an obj directory is
> used. This is probably the reason for the use of ${.IMPSRC} in
> the first place.
>
Ah, nice analysis! The suggested fix looks safe to me (can't see how a .c
could have two .y direct prerequisites, so this can't be ambiguous).
ok guenther@
