Quick test of vsnprintf to make sure it is standards-compliant

test_vsnprintf.c

// This is a small C program that tests your version of vsnprintf to see if it is
// compliant with the C99 standard.
//
// The script gives successful results (compliant vsnprintf) on these environments:
// - MSYS2's MINGW64 environment (GCC 13.2.0)
//
// I also expect successful results on Linux, macOS and modern UCRT-based environments
// provided by Microsoft.
//
// From the MSDN documentation of vsnprintf:
// Beginning with the UCRT in Visual Studio 2015 and Windows 10, vsnprintf is no longer
// identical to _vsnprintf. The vsnprintf function conforms to the C99 standard;
// _vsnprintf is kept for backward compatibility with older code. The difference is
// that if you run out of buffer, vsnprintf null-terminates the end of the buffer
// and returns the number of characters that would have been required, while _vsnprintf
// doesn't null-terminate the buffer and returns -1. Also, _vsnprintf() includes one
// more character in the output because it doesn't null-terminate the buffer.

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

int main() {
  va_list ap;
  char s[4] = {};
  int r = vsnprintf(s, 2, "abcd", ap);
  printf("%d %s\n", r, s);  // should be "4 a"
}