How to do asynchronous inter thread communication on platform with pthreads

Question

For example on windows there is MsgWaitForMultipleObjects that lets you asynchronously wait for windows messages, socket events, asynchronous io (IOCompletionRoutine), AND mutex handles.

On Unix you have select/poll that gives you everything except possibility to break out when some pthread_mutex is unlocked.

The story:

I have application that has main thread that does something with multiple sockets, pipes or files. Now from time to time there is a side job (a db transaction) that might take a longer time and if done synchronously in main thread would disrupt normal servicing of sockets. So I want to do the db operation in separate thread. That thread would wait on some mutex when idle until main thread decides to give it some job and unlocks the mutex so db thread can grab it. The problem is how the db thread can notify back the main thread that it has finished the job. Main thread has to process sockets, so it cannot afford sleeping in pthread_mutex_lock. Doing periodic pthread_mutex_trylock is the last I would want to do. Currently I consider using a pipe, but is this the better way?

Answer 1

I don't know how well it applies to your specific problem, but posix has message queues.

Answer 2

Using a pipe is a good idea here. Make sure that no other process has the write end of the pipe open, and then select() or poll() in the main thread the read end for reading. Once your worker thread is done with the work, close() the write end. The select() in the main thread wakes up immediately.

I don't think waiting on a mutex and something else would be possible, because on Linux, mutexes are implemented with the futex(2) system call, which doesn't support file descriptors.