io.c:501
static VALUE
rb_io_seek_m(argc, argv, io)
    int argc;
    VALUE *argv;
    VALUE io;
{
    VALUE offset, ptrname;
    int whence = SEEK_SET;

    if (rb_scan_args(argc, argv, "11", &offset, &ptrname) == 2) {
	whence = NUM2INT(ptrname);
    }

    return rb_io_seek(io, offset, whence);
}
