For and overview and related topics, see Introduction to PEXlib.
PEXlib functions usually do not check for invalid parameters. Applications should be careful to pass correct data to PEXlib. Incorrect values sent to the PEX server extension return an error event to the client. These errors are asynchronous, and so are difficult to trace to a particular Xlib or PEXlib function unless your application uses the XSync or XSynchronize function. However, use these functions prudently because they slow client programs significantly.
By default, when the server returns an error event, Xlib prints a message and the application does not continue. The application can register its own error handler by invoking the XSetErrorHandler function.
As with X11 requests, the explicit listing of Alloc errors only covers allocation of a very coarse level and is not intended to cover all cases of a server running out of allocation space during processing of a request.