Call Back Functions

Question

Someone suggested to me to use Call Back Functions to implement a timer to run in the background while my Server application reads input from clients. I tried looking at explanations online, but was hoping if someone could give me a simpler analogy.

Thanks.

Answer 1

There are two separate ways to implement a timer using callbacks in Windows, SetTimer and timeSetEvent. The basics are:

1. SetTimer uses messages, even if you use a callback (the callback function is invoked as a result of processing a message). So SetTimer isn't viable if you don't run a message pump.

2. Callbacks are called by the operating system, which doesn't know a C++ "this" pointer from a hole in the ground, so your callback either has to be a global C-style function or a static member.

3. timeSetEvent is part of the "multimedia" timer family, and doesn't require a message pump. The observations about the callback function signature above still apply though. The lack of requirement for a message pump can be important if you're writing a console app though.

4. You might also consider threading and CreateWaitableTimer, but I don't use waitable timers very often so can't comment on them.

If you need to do work in the background, then threading can be a much more elegant way to address the problem. You don't have to divide the work up into chunks when you're threading (which you do if you're kicking the work from a timer). But of course your thread can't touch the GUI, so life can get a little complicated when you start threading. There's an intro to worker threads on my website here.

Answer 2

Analogy?

Take a look here for a brief explanation of callback functions:
What is a “callback” in C and how are they implemented?

Using a timer with a callback would be saying 'call function x every y seconds' and with a system that supports multitasking, that function would be called every y seconds in a second thread of execution, no matter what the original function might be doing.

Edit: As has been suggested in another answer, the system might not create a second thread for you, in which case you'd have to create the thread yourself and set up the callback from that thread.

Edit: In Windows, you can use the SetTimer function. It will post a WM_TIMER message to your window's message queue, which your message loop might handle itself or hand over to the default message procedure to call a callback function you've specified. I'm not sure what happens if you don't have a window, but give it a try.

Answer 3

Your question is fairly unclear, but it's probable they were suggesting you create a thread and run your function in that thread.

This can be done by subclassing a system-specific Thread class; by constructing the same class with some sort of call-back function as an argument; by creating a timer that invokes a callback function after some time limit... without a more specific question I can't give more specific advice.