source: vendor/3.6.9/lib/replace/test/snprintf.c

void foo(const char *format, ...)
{
        va_list ap;
        int len;
        char buf[20];
        long long l = 1234567890;
        l *= 100;

        va_start(ap, format);
        len = vsnprintf(buf, 0, format, ap);
        va_end(ap);
        if (len != 5) exit(1);

        va_start(ap, format);
        len = vsnprintf(0, 0, format, ap);
        va_end(ap);
        if (len != 5) exit(2);

        if (snprintf(buf, 3, "hello") != 5 || strcmp(buf, "he") != 0) exit(3);

        if (snprintf(buf, 20, "%lld", l) != 12 || strcmp(buf, "123456789000") != 0) exit(4);
        if (snprintf(buf, 20, "%zu", 123456789) != 9 || strcmp(buf, "123456789") != 0) exit(5);
        if (snprintf(buf, 20, "%2\$d %1\$d", 3, 4) != 3 || strcmp(buf, "4 3") != 0) exit(6);
        if (snprintf(buf, 20, "%s", 0) < 3) exit(7);

        printf("1");
        exit(0);
}
main() { foo("hello"); }