#include <stdio.h>
#include <assert.h>


main(int argc, char *argv[])
{
        FILE *f;
        int rv, v[4];


        f = fopen(argv[1], "r+");
        assert(f != NULL);


        rv = fseek(f, 16, SEEK_SET);
        assert (rv != -1);


        printf("%ld\n", ftell(f));


        rv = fread(v, sizeof v, 1, f);
        assert (rv != -1);


        printf("%ld\n", ftell(f));


        v[0] = 0;
        rv = fwrite(&v[0], sizeof v[0], 1, f);


        printf("%ld\n", ftell(f));
}


--------------------------------------

dd if=/dev/zero of=bar bs=1024 count=1
./a.out bar