[Bug c/55746] New: vfprintf behaves differently variable on stack vs const argument.

[Bug c/55746] New: vfprintf behaves differently variable on stack vs const argument.

[lbana at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55746

             Bug #: 55746
           Summary: vfprintf behaves differently variable on stack vs
                    const argument.
    Classification: Unclassified
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: lbana@hotmail.com


Here is the test code that behaves differently w.r.t vfprintf. In one case it
prints the string where as in other case it crashes.

#include <stdarg.h>
#include <stdio.h>
int main(int argc, const char **argv)
{
    const char *var = "Test VarArg\n";

    asm("nop");
    asm("nop");
    vfprintf(stdout, "TestMe\n", 0);
    asm("nop");
    asm("nop");
    fflush(stdout);
    asm("nop");
    asm("nop");
    vfprintf(stdout, var, 0);
    asm("nop");
    asm("nop");
    fflush(stdout);

    return 0;
}

#gcc -g -o t3 t3.c 
#./t3
TestMe
Segfault


In first case vfprintf is optimized with _fwrite in the second case it uses
_vfprintf and crashes probably in va_copy.

thanks in advance.
lb

[Bug c/55746] vfprintf behaves differently variable on stack vs const argument.

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55746

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-12-19 23:44:14 UTC ---
And this is not a bug really.  GCC is optimizing the first one to fwrite but
not the second one since it does not see the constant string in the first case.
 Also this code is undefined as the varargs that you passed to vfprintf is not
a real one.