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Postings from March 2013
On Fri, Mar 22, 2013 at 12:30:23PM -0700, Mark Martinec wrote:
> Perl include files (cv.h, inline.h, possibly others) use certain
> macros as expressions (like CvNAMED, MUTABLE_PTR), but these macros
> expand to "braced-group", which is not allowed in an expression
> according to ISO C - but is mercifully allowed by gcc.
>
> Some Perl modules use cc options -std=c99 -pedantic-errors
> and include these perl .h files, so their compilation fails
> with compile time errors like:
>
> .../5.17.9/mach/CORE/cv.h:194: error: ISO C forbids braced-groups within expressions
> .../5.17.9/mach/CORE/cv.h:196: error: ISO C forbids braced-groups within expressions
> .../5.17.9/mach/CORE/cv.h:201: error: ISO C forbids braced-groups within expressions
> .../5.17.9/mach/CORE/cv.h:202: error: ISO C forbids braced-groups within expressions
> .../5.17.9/mach/CORE/inline.h:23: error: ISO C forbids braced-groups within expressions
>
> Here is a source code example from cv.h near line 194:
>
> PERL_STATIC_INLINE GV *
> S_CvGV(CV *sv)
> {
> return CvNAMED(sv)
> ? 0
> : ((XPVCV*)MUTABLE_PTR(SvANY(sv)))->xcv_gv_u.xcv_gv;
> }
>
> which gets expanded by a C preprocessor into:
>
> S_CvGV(CV *sv)
> {
> return (((XPVCV*)({ void *_p = ((sv)->sv_any); _p; }))->xcv_flags & 0x8000)
> ? 0
> : ((XPVCV*)({ void *_p = ((sv)->sv_any); _p; }))->xcv_gv_u.xcv_gv;
> }
>
> which illustrates where "braced-group within expression"
> comes into play.
But these definitions are conditional:
#if defined(__GNUC__) && !defined(PERL_GCC_BRACE_GROUPS_FORBIDDEN)
# define MUTABLE_PTR(p) ({ void *_p = (p); _p; })
#else
# define MUTABLE_PTR(p) ((void *) (p))
#endif
Further:
#if defined(PERL_GCC_PEDANTIC) || \
(defined(__GNUC__) && defined(__cplusplus) && \
((__GNUC__ < 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ < 2))))
# ifndef PERL_GCC_BRACE_GROUPS_FORBIDDEN
# define PERL_GCC_BRACE_GROUPS_FORBIDDEN
# endif
#endif
> An example of such CPAN module is ZMQ::LibZMQ3. I have opened
> a ticket with this module ([rt.cpan.org #84124] suggesting to
> drop the -pedantic-errors option) - here is the author's reply:
>
> ( https://rt.cpan.org/Ticket/Display.html?id=84124 )
>
> Given some discussion on #p5p, I suspect that there is something wrong
> in the bleadperl isn't setting PERL_GCC_PEDANTIC. You might want to file
> this bug via perlbug. If they decide to release perl-5.18 without fixing
> this, I'll release a new version with manual PERL_GCC_PEDANTIC define.
>
>
> So I suggest such constructs are either avoided, or clearly documented
> that these include files do not comply with ISO C99 standard.
I'm not sure I understand this. Is the author suggesting that bleadperl
built with -pedantic-errors doesn't set PERL_GCC_PEDANTIC?
Because:
$ sh Configure -des -Dusedevel -Accflags="-pedantic-errors"
...
$ make
`sh cflags "optimize='-O2'" perlmini.o` -DPERL_IS_MINIPERL -DPERL_EXTERNAL_GLOB perlmini.c
CCCMD = cc -DPERL_CORE -c -pedantic-errors -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -Wall -DPERL_GCC_PEDANTIC
(note the -DPERL_GCC_PEDANTIC on the command-line)
Or, is the author's module being built on a non-pedantic perl, but it adds
-pedantic-errors to the command line when building the module's source
files? In which case, I don't think perl has ever added PERL_GCC_PEDANTIC
to the command line or any include file.
Do you have a definite pass / transition, i.e. this module builds under
5.X, but doesn't build under 5.Y?
--
I've often wanted to drown my troubles, but I can't get my wife to go
swimming.
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