Actually, using SendMessage to communicate between two threads, wherein each thread is using the same library, is inherently problematic, and can lead to deadlocks. The problem is that while one thread is holding on to the library lock, it might send a message to the other thread, and if that thread then calls the same library, the library can't acquire the same lock, since the lock is being held by the first thread. But the first thread will never release the lock, since it too is waiting for the second thread to reply to the SendMessage call. So the threads are waiting on each other, and you have a hang.
It's likely that this hang did not manifest itself in the previous version of the code, since that code did not include any thread-protection locking. So there were no deadlocks, but you had memory corruption instead. Now, the memory corruption should no longer occur, but you have the potential for deadlocks, if the threads interact in a dangerous way. And calling SendMessage would be considered interacting in a dangerous way.
Have you considered using PostMessage instead? Using PostMessage, the sending thread doesn't have to wait for the receiving thread, so the potential for deadlocks is greatly reduced.
Luis
