9.1.1. X Windows System interface

The X libraries used to implement X clients (such as Wine) do not work properly if multiple threads access the same display concurrently. It is possible to compile the X libraries to perform their own synchronization (initiated by calling XInitThreads()). However, Wine does not use this approach. Instead Wine performs its own synchronization using the wine_tsx11_lock() / wine_tsx11_unlock() functions. This locking protects library access with a critical section, and also arranges things so that X libraries compiled without -D_REENTRANT (eg. with global errno variable) will work with Wine.

In the past, all calls to X used to go through a wrapper called TSX...() (for "Thread Safe X ..."). While it is still being used in the code, it's inefficient as the lock is potentially acquired and released unnecessarily. New code should explicitly acquire the lock.