SetTimer() pitfalls

The problem with timer messages as you are trying to use them is that they are low priority messages. Actually they are fake messages. Timers are associated with an underlying kernel timer object - when the message loop detects the kernel timer is signalled it simply marks the current threads message queue with a flag indicating that the next call to GetMessage - WHEN THERE ARE NO OTHER MESSAGES TO PROCESS - should synthesise a WM_TIMER message just in time and return it.

With potentially lots of timer objects, its not at all obvious that the system will fairly signal timer messages for all the timers equally, and any system load can entirely prevent the generation of WM_TIMER messages for long periods of time.

If you are in control of the message loop, you could use maintain your own list of timer events (along with GetTickCount timestamps when they should occur) and MSGWaitForMultipleObject - instead of GetMessage to wait for messages. Use the dwTimeout parameter to provide the smallest interval - from now - until the next timer should be signalled. So it will return from waiting for messages each time you have a timer to process.

And/Or you could use waitable timers - either on a GUI thread with MSGWaitForMultipleObjects, or just on a worker thread, to access the lower level timing functionality directly.

The biggest SetTimer() pitfall is that actually it is USER object (despite the fact it's not listed in MSDN USER objects list) hence it falls under Windows USER objects limitation - by default max 10000 objects per process, max 65535 objects per session (all running processes).

This can be easily proven by simple test - just call SetTimer() (parameters don't care, both windowed and windowless act the same way) and see USER objects count increased in Task Manager.

Also see ReactOS ntuser.h source and this article. Both of them state that TYPE_TIMER is one of USER handle types.

So beware - creating a bunch of timers could exhaust your system resources and make your process crash or even entire system unresponsive.