Server Grabs and getting stuck in _XReply

December 18, 2013

Here’s a quick reference to anyone in the future who ever gets stuck debugging some external library being stuck in _XReply

> #0 _XReply
> #1 ???
> #2 some_library_do_thing
> \# 3 my_app_do_thing

Check if your app is using server grabs (XGrabServer, XUngrabServer).

Some third party libraries open up new X Server connections in the same process as yours. They shouldn’t do this, but they do anyways (graphics drivers are notorious culprits for this).

If you are holding a server grab and then call into a third party library which makes a synchronous call on its own X connection you will get a deadlock. This is because of how server grabs work - they prevent all connections from reading or writing any data until your connection has released the server grab.

When using server grabs (please don’t unless you really need atomic semantics around a series of X calls), always try to limit them to the closest possible subset of protocol requests you actually need atomic semantics around. Don’t just wrap some entire function in a server grab. Especially not if you plan to be calling callbacks which you don’t control or calling into libraries that are subject to change.

Also remember that XUngrabServer is not a synchronous operation - you will need to call XSync after using it in order to completely release the grab for other connections.

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Written by Sam Spilsbury an Australian PhD student living in Helsinki.