[Bug c/37768] New: bogus warnings on x86_64-mingw32 due to attribute((format(printf))) breakage

["mikpe at it dot uu dot se" <gcc-bugzilla@gcc.gnu.org>]

When gcc is configured to generate code for x86_64-pc-mingw32, that is MinGW
for 64-bit Windows, attribute((format(printf))) is redefined by the backend to
be compatible with MSVC's runtime library, which differs significantly from
C99.

This is fine for calls that link to MSVC's library, but it breaks code that
uses private implementations of C99-compliant formatting routines, because the
backend redefines ALL uses of attribute((format(printf))) to mean MSVC's printf
not C99. The result is that C99-compliant code gets stray warnings and
inadequate printf format checking on x86_64-pc-mingw32.

The program below illustrates the issue. It declares a private C99-compliant
snprintf() implementation and invokes it with "%zu" and "%llx" formats. This
triggers the following bogus warnings on x86_64-pc-mingw32:

> x86_64-pc-mingw32-gcc -std=c99 -O -Wall -c badwarning.c
badwarning.c: In function 'main':
badwarning.c:16: warning: unknown conversion type character 'z' in format
badwarning.c:16: warning: unknown conversion type character 'l' in format
badwarning.c:16: warning: too many arguments for format

What I think the backend should do is to implement an "msprintf" format type,
and then Mingw-w64 should declare printf() et al using that not plain "printf".

/* badwarning.c */
#include <stddef.h>
#include <stdarg.h>

int  __attribute__((format(printf, 3, 4)))
    my_snprintf(char *buf, size_t n, const char *fmt, ...)
{
    /* invoke C99 compliant private vsnprintf() here */
    return 0;
}

int main(void)
{
    char buf[64];
    return my_snprintf(buf, sizeof buf, "%zu %llx",
                       sizeof buf, 0ULL);
}


-- 
           Summary: bogus warnings on x86_64-mingw32 due to
                    attribute((format(printf))) breakage
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mikpe at it dot uu dot se
GCC target triplet: x86_64-pc-mingw32


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37768