#define __STDC_WANT_LIB_EXT1__ 1

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

int main()
{
#if defined __STDC_LIB_EXT1__
    printf("Optional functions are defined.\n");
#else
    printf("Optional functions are not defined.\n");
#endif

    char str[100] = "Hello, world";
    printf("len: %d", strnlen(str, sizeof(str)));
}