I want to wait on both a file descriptor and a mutex, what's the recommended way to do this?

Whenever one uses an event driven architecture, one is required to have a single mechanism to report event completion. On Linux, if one is using files, one is required to use something from the select or poll family meaning that one is stuck with using a pipe to initiate all none file related events.

Edit: Linux has eventfd and timerfd. These can be added to your epoll list and used to break out of the epoll_wait when either triggered from another thread or on a timer event respectively.

There is another option and that is signals. One can use fcntl modify the file descriptor such that a signal is emitted when the file descriptor becomes active. The signal handler may then push a file-ready message onto any type of queue of your choosing. This may be a simple semaphore or mutex/condvar driven queue. Since one is now no longer using select/poll, one no longer needs to use a pipe to queue none file based messages.

Health warning: I have not tried this and although I cannot see why it will not work, I don't really know the performance implications of the signal approach.

Edit: Manipulating a mutex in a signal handler is probably a very bad idea.

I've solved this exact problem using what you mention, pipe() and libevent (which wraps epoll). The worker thread writes a byte to its pipe FD when its output queue goes from empty to non-empty. That wakes up the main IO thread, which can then grab the worker thread's output. This works great is actually very simple to code.

You have the Linux tag so I am going to throw this out: POSIX Message Queues do all this, which should fulfill your "built-in" request if not your less desired cross-platform wish.

The thread-safe synchronization is built-in. You can have your worker threads block on read of the queue. Alternatively MQs can use mq_notify() to spawn a new thread (or signal an existing one) when there is a new item put in the queue. And since it looks like you are going to be using select(), MQ's identifier (mqd_t) can be used as a file descriptor with select.