Yay, glibc!

ftell-glibc

// C libraries print the file size.
// well, except glibc, that always prints 0.

#include <stdio.h>

int main(void)
{
    FILE *fp = fopen("/tmp/aaa", "a+");
    printf("%ld\n", ftell(fp));
    fputc('x', fp);
    fclose(fp);
    return 0;    
}