How to printf a size_t without warning in mingw-w64 gcc 7.1?

The problem is not the compiler but the C library. MinGW uses Microsoft's "Visual C Runtime" (msvcrt) which only conforms to c89 and it doesn't support the z format specifier.

Here's what you can do to safely print a size_t when using MinGW:

#include <inttypes.h>
#include <stdio.h>

#ifdef _WIN32
#  ifdef _WIN64
#    define PRI_SIZET PRIu64
#  else
#    define PRI_SIZET PRIu32
#  endif
#else
#  define PRI_SIZET "zu"
#endif

int main(void)
{
    size_t mySize = 24;

    printf("%" PRI_SIZET "\n", mySize);
}

On win64, you would get a warning with this code, because PRIu64 expands to the msvcrt-specific I64u format specifier. But you can silence this warning with the GCC flag -Wno-pedantic-ms-format.

Note that you need a similar trick for long long (here using PRIu64 on both 32bit and 64bit windows) because msvcrt doesn't know ll either.

edit: as pointed out by @M.M in a comment, you can instead link MinGW-provided alternative stdio functions that support C11 with #define __USE_MINGW_ANSI_STDIO 1. I prefer not to link extra code if I can get around the peculiarities of msvcrt, but that's of course a matter of taste.


PRIuPTR trick works (Cross platform format string for variables of type size_t?):

#include <stdio.h>
#include <inttypes.h>
int main()
{
    size_t a = (size_t)(1024ULL * 1024 * 1024 * 1024 + 13);
    printf("a                 = %" PRIuPTR "\n", a);
    printf("sizeof(size_t)    = %" PRIuPTR "\n", sizeof(size_t));
    printf("sizeof(uintptr_t) = %" PRIuPTR "\n", sizeof(uintptr_t));
    return 0;
}

Output x86:

a                 = 13
sizeof(size_t)    = 4
sizeof(uintptr_t) = 4

Output x64:

a                 = 1099511627789
sizeof(size_t)    = 8
sizeof(uintptr_t) = 8

The alternative solution as mentioned in comments is to toss in the __USE_MINGW_ANSI_STDIO compiler switch:

#define __USE_MINGW_ANSI_STDIO 1

#include <stdio.h>

int main(void)
{
    size_t a = 100;
    printf("a=%lu\n",a);
    printf("a=%llu\n",a);
    printf("a=%zu\n",a);
    printf("a=%I64u\n",a);
}

This makes the code compile as expected and gcc now gives the appropriate warnings: 

warning: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'size_t' [-Wformat=]  
warning: ISO C does not support the 'I' printf flag [-Wformat=]  
warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'size_t' [-Wformat=]

Alternatively you can define the macro on command line with -D__USE_MINGW_ANSI_STDIO=1

Tags:

C

Printf

Size T

Recent Posts