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

Question

I am using the mingw-w64 (x64) fork of minGW as prepared on nuwen.net. This is from the 7.1 version of gcc :

gcc --version
gcc (GCC) 7.1.0

I am compiling this program:

#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);
}

with warnings and c11 standard:

gcc -Wall -Wextra -Wpedantic -std=c11 test_size_t.c

and I get these warnings:

   test_size_t.c: In function 'main':
    test_size_t.c:6:14: warning: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'size_t {aka long long unsigned int}' [-Wformat=]
      printf("a=%lu\n",a);
                ~~^
                %I64u
    test_size_t.c:6:14: warning: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'size_t {aka long long unsigned int}' [-Wformat=]
      printf("a=%lu\n",a);
                ~~^
                %I64u
    test_size_t.c:7:14: warning: unknown conversion type character 'l' in format [-Wformat=]
      printf("a=%llu\n",a);
                  ^
    test_size_t.c:7:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%llu\n",a);
             ^~~~~~~~~~
    test_size_t.c:7:14: warning: unknown conversion type character 'l' in format [-Wformat=]
      printf("a=%llu\n",a);
                  ^
    test_size_t.c:7:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%llu\n",a);
             ^~~~~~~~~~
    test_size_t.c:8:13: warning: unknown conversion type character 'z' in format [-Wformat=]
      printf("a=%zu\n",a);
                 ^
    test_size_t.c:8:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%zu\n",a);
             ^~~~~~~~~
    test_size_t.c:8:13: warning: unknown conversion type character 'z' in format [-Wformat=]
      printf("a=%zu\n",a);
                 ^
    test_size_t.c:8:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%zu\n",a);
             ^~~~~~~~~
    test_size_t.c:9:9: warning: ISO C does not support the 'I64' ms_printf length modifier [-Wformat=]
      printf("a=%I64u\n",a);
         ^~~~~~~~~~~
    test_size_t.c:9:9: warning: ISO C does not support the 'I64' ms_printf length modifier [-Wformat=]

I would like to printf a size_t without warning but don't know the correct format specifier in this situation.

Answer 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.

Answer 2

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=]