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

static void    vsnprintf_test(char **str, size_t *alloc_len, size_t *offset, const char *fmt, ...)
{
        va_list args;

        if (NULL == *str)
        {
                va_start(args, fmt);
                printf("Calling vsnprintf()\n");
                *alloc_len = vsnprintf(NULL, 0, fmt, args) + 2; /* '\0' + one byte to prevent the operation retry */
                printf("After vsnprintf()\n");
                va_end(args);
                *offset = 0;
        }
}

int main(void)
{
        char            *msg = NULL;
        size_t          msg_alloc = 0, msg_offset = 0;

        vsnprintf_test(&msg, &msg_alloc, &msg_offset, "%s", "ABC");
}