#define _GNU_SOURCE
#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>

int vasprintf(char **s, const char *fmt, va_list ap)
{
    size_t l;
    *s = 0;
    FILE *f = open_memstream(s, &l);
    if (!f)
        return -1;

    if ((l = vfprintf(f, fmt, ap)) == -1) {
        free(*s);
        *s = 0;
    }
    fclose(f);
    return l;
}