C++ Timers in Unix

This looks like a simple wrapper around select(2). The class keeps a list of callbacks, I guess separate for read, write, and timer expiration. Then there's something like a dispatch or wait call somewhere there that packs given file descriptors into sets, calculates minimum timeout, and invokes select with these arguments. When select returns, the wrapper probably goes over read set first, invoking read callback, then write set, then looks if any of the timers have expired and invokes those callbacks. This all might happen on the same thread, or on separate threads depending on the implementation of the wrapper.

You should read up on select and poll - they are very handy.The general term is IO demultiplexing.

A readable event means that data is available for reading on a particular file descriptor without blocking, and a writable event means that you can write to a particular file descriptor without blocking. These are most often used with sockets and pipes. See the select() manual page for details on these.

A timer event means that a previously created timer has expired. If the library is using select() or poll(), the library itself has to keep track of timers since these functions accept a single timeout. The library must calculate the time remaining until the first timer expires, and use that for the timeout parameter. Another approach is to use timer_create(), or an older variant like setitimer() or alarm() to receive notification via a signal.

You can determine which mechanism is being used at the OS layer using a tool like strace (Linux) or truss (Solaris). These tools trace the actual system calls that are being made by the program.

At a guess, the call to create() stores the function pointer somewhere. Then, when the timer goes off, it calls the function you specified via that pointer. But as this is not a Standard C++ function, you should really read the docs or look at the source to find out for sure.

Regarding your other questions, I don't see mention of a priority list, and select() is a sort of general purpose event multiplexer.