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pthread_mutex_unlock not permitted in signal handler?

In trying to find a workaround to my previous message (destructors not being called on pthread_cancel), In this case, I have a set of pthread_cleanup statements which trigger a call to pthread_mutex_unlock on cancellation.

The trick is that on Tiger (Darwin 8.11.1), since I can't cancel out of a read()/sleep()/etc., I use a pthread_kill() to send a signal, and then the signal handler does pthread_testcancel(). Ugly as it is, this much has been working for a while now.

However, when I try to call pthread_mutex_unlock from the cleanup function that results from the eventual pthread_testcancel, it returns an error "Operation not permitted".

Is it *at all possible* to unlock a mutex following the interruption of a thread on Tiger? (I haven't tried this on Leopard yet -- since I don't need to do the signal hack there, hopefully this is moot) Basically, I'm reading from a socket, and need a way to kill the thread if there's no response. But I also need to maintain a lock on the socket while I'm waiting for a response so other tasks don't try to send their own queries. I'm polling relatively frequently, so I don't want to have to take the lock, launch a new thread, and monitor the other thread every time. (this is a real-time task with a hardware device)

Thoughts?

thanks
  -ethan
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