One can use poll/select when writing a server that can service multiple clients all in the same thread. select and poll, however need a file descriptor to work. For this reason, I am uncertain how to perform simple asynchronous operations, like implementing a simple callback to break up a long running operation or a delayed callback without exiting the select/poll loop. How does one go about doing this? Ideally, I would like to do this without resorting to spawning new threads.
In a nutshell, I am looking for a mechanism with which I can perform ALL asynchronous operations. The windows WaitForMultipleObjects or Symbian TRequestStatus seems a much more suited to generalized asynchronous operations.
For arbitrary callbacks, maintain a POSIX pipe (see pipe(2)). When you want to do a deferred call, write a struct consisting of a function pointer and optional context pointer to the write end. The read end is just another input for select. If it selects readable, read the same struct, and call the function with the context as argument.
For timed callbacks, maintain a list in order of due time. Entries in the list are structs of e.g. { due time (as interval since previous callback); function pointer; optional context pointer }. If this list is empty, block forever in select(). Otherwise, timeout when the first event is due. Before each call to select, recalculate the first event’s due time.
Hide the details behind a reasonable interface.