When to use QueueUserAPC()?

QueueUserAPC is a neat tool that can often be a shortcut for some tasks that are otherwise handled with synchronization objects. It allows you to tell a particular thread to do something whenever it is convenient for that thread (i.e. when it finishes its current work and starts waiting on something).

Let's say you have a main thread and a worker thread. The worker thread opens a socket to a file server and starts downloading a 10GB file by calling recv() in a loop. The main thread wants to have the worker thread do something else in its downtime while it is waiting for net packets; it can queue a function to be run on the worker while it would otherwise be waiting and doing nothing.

You have to be careful with APCs, because as in the scenario I mentioned you would not want to make another blocking WinSock call (which would result in undefined behavior). You really have to be watching in order to find any good uses of this functionality because you can do the same thing in other ways. For example, by having the other thread check an event every time it is about to go to sleep, rather than giving it a function to run while it is waiting. Obviously the APC would be simpler in this scenario.

It is like when you have a call desk employee sitting and waiting for phone calls, and you give that person little tasks to do during their downtime. "Here, solve this Rubik's cube while you're waiting." Although, when a phone call comes in, the person would not put down the Rubik's cube to answer the phone (the APC has to return before the thread can go back to waiting).

QueueUserAPC is also useful if there is a single thread (Thread A) that is in charge of some data structure, and you want to perform some operation on the data structure from another thread (Thread B), but you don't want to have the synchronization overhead / complexity of trying to share that data between two threads. By having Thread B queue the operation to run on Thread A, which solely maintains that structure, you are executing any arbitrary function you want on that data without having to worry about synchronization.

It is just another tool like a thread pool. However with a thread pool you cannot send a task to a particular thread. You have no control over where the work is done. When you queue up a task that may end up creating a whole new thread. You may queue two tasks and they get done simultaneously on two different threads. With QueueUserAPC, you can be guaranteed that the tasks would get done in order and on the thread you designate.