fseek and open_memstream

Why when I uncomment fseek line buf become empty?
#define _GNU_SOURCE
#include <stdio.h>

int main() {
char *buf = 0;
int size = 0;
FILE* f = open_memstream(&buf, &size);
fwrite("test", 1, 4, f);
// fseek(f, 0, SEEK_SET);
fclose(f);
fprintf(stderr, buf);
}
--
Dmitry
.

Relevant Pages