If this is any help I threw in a printf and c->out.request_written and c->in.request_read went like: 2 - 2 ..... ..... 10 - 10 11 - 11 12 - 12 12 - 65536 Pretty dramatic jump. Dunno if this is useful or not, I really don't know how to debug this since I don't know xlib/xcl internals. Jeremy