C.5. C.5 Writing Extension Stubs

All X requests always contain the length of the request, expressed as a 16-bit quantity of 32 bits. This means that a single request can be no more than 256K bytes in length. Some servers may not support single requests of such a length. The value of dpy->max_request_size contains the maximum length as defined by the server implementation. For further information, see "X Window System Protocol."

C.5.1. C.5.1 Requests, Replies, and Xproto.h

The <Xproto.h> file contains three sets of definitions that are of interest to the stub implementor: request names, request structures, and reply structures.