io.c:485
static VALUE
rb_io_seek(io, offset, whence)
    VALUE io, offset;
    int whence;
{
    OpenFile *fptr;
    off_t pos;

    GetOpenFile(io, fptr);
    pos = io_seek(fptr, NUM2OFFT(offset), whence);
    if (pos < 0) rb_sys_fail(fptr->path);
    clearerr(fptr->f);

    return INT2FIX(0);
}
