#407 printf bug in mingw64

[Lelanthran]

Strange bug when using both %zu and %s in the same formatting string:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main (void)
{
   const char const input[] = "Hello World";
   char *str = malloc (33517 + 1);
   for (size_t i=0; i<33517; i++) {
      str[i % sizeof input] = input [i % sizeof input];
   }
   str[33517] = 0;
   printf ("%zu bytes\n%s\n\n", strlen (str), str);
   return 0;
}

The output never appears (it crashes and windows displays a dialog box) so I've included the gdb output:

bash$ gcc -std=c99 -DUSE_MINGW_ANSI_STDIO=1 -o test.exe mingw64-printf-problem.c
bash$ gdb ./test
GNU gdb (GDB) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-w64-mingw32".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./test...done.
(gdb) run
Starting program: C:\tmp\test.exe
[New Thread 3076.0x69c]

Program received signal SIGSEGV, Segmentation fault.
0x000007feff95716e in msvcrt!_beginthreadex ()
   from C:\Windows\system32\msvcrt.dll
(gdb) bt
#0  0x000007feff95716e in msvcrt!_beginthreadex ()
   from C:\Windows\system32\msvcrt.dll
#1  0x000007feff957a1d in printf () from C:\Windows\system32\msvcrt.dll
#2  0x00000000004015ce in main ()
(gdb)

Removing either the %zu (and its argument) or the %s (and its argument) makes it rnu successfully, although instead of the string length a zu is displayed instead.

[Ozkan Sezer]

I think you missed two leading underscores from -DUSE_MINGW_ANSI_STDIO=1, i.e. you need "-D__USE_MINGW_ANSI_STDIO=1"

[Lelanthran]

Thanks, that's fixed it. The example code I saw did not have two leading underscores to tell mingw64 to use its own runtime

[Ozkan Sezer]

OK then, closing.

If the example code is from our site/documentation, then please point us to it so that we can correct it.

[Lelanthran]

When I searched the tickets for printf bugs with respect to %zu, one of the first relevant results was this: https://sourceforge.net/p/mingw-w64/bugs/377/

This is where I got the #define without the two leading underscores.

Once again, thanks for helping me.