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

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

[mikpe at it dot uu dot se]

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

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

[joseph at codesourcery dot com]

------- Comment #1 from joseph at codesourcery dot com  2008-10-08 16:15 -------
Subject: Re:   New: bogus warnings on x86_64-mingw32 due to
 attribute((format(printf))) breakage

On Wed, 8 Oct 2008, mikpe at it dot uu dot se wrote:

> 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.

Correct.  This is the conclusion we eventually reached, that "printf" 
would be for the system formats (e.g. for a function wrapping a call to a 
system function), that "gnu_printf" would be for the formats accepted by 
the GNU C Library (including GNU extensions) on all platforms and that 
"ms_printf" would be for the MSVC formats, on Windows platforms only.

> 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:

Use gnu_printf for such an implementation.


-- 


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

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

[mikpe at it dot uu dot se]

------- Comment #2 from mikpe at it dot uu dot se  2008-10-09 07:04 -------
> > 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:
> 
> Use gnu_printf for such an implementation.

Thanks. Using gnu_printf (new in 4.4) solves the issues I had.


-- 

mikpe at it dot uu dot se changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


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