CODEWITHSUNDEEP
c++winapivisual-c++
Jean Claud Van Who
Jean Claud Van Who

Reputation: 71

PostMessage does not trigger Message Handler

I have a VC++ code that uses PostMessage to trigger a function call. Although the function gets called eventually, I see that the function does not get called right after PostMessage is executed. It does not even get called after the function containing the PostMessage command completes execution. I noticed that a few more functions got called before the desired function was called.

I would like to know what determines when exactly the callback for a message gets called.

The message is a WM_USER message.

Upvotes: 3

Views: 2273

Answers (2)

Adrian McCarthy
Adrian McCarthy

Reputation: 48038

The Windows event-driven model in a nutshell (with many important details simplified away):

There is a "message pump," which is a loop that, on each iteration, pulls a message out of a queue and "dispatches" it by calling the "window procedure" associated with the specific window, passing it the details from the message.

If you're using a framework, it's likely that the message pump and the window procedure are provided for you and that the provided window procedure calls your handler functions in response to the messages it receives.

When a message is posted, it's added to the queue, and nothing else happens to it until the message pump gets to it. There may be messages ahead of it in the queue, and/or a bunch of other code may run before execution returns to the message pump.

When a message is sent, the message is not added to the queue. Instead, the sending code will call the window procedure directly. (Caveat: this is an oversimplification. Things are more complicated when sending a message to another thread.)

So if other functions are being called before the one you expect from your posted message, here are some possibilities:

  1. Other messages were already in the queue when you posted your message.

  2. Other messages are being sent rather than posted, so they're handled immediately, and only when they're done and the message pump eventually runs again will your posted message be handled.

  3. The message pump decided that other messages, which were posted after yours, are more important and therefore should be handled first. (This is not actually common. Certain types of messages, like WM_PAINT, are very low priority and are only generated when there's nothing in the queue, but I can't think of a case where a message takes priority over one that's already in the queue.)

  4. You're posting your message during initialization before the message pump even starts. For example, if you create a window, post a message to it, and then start the message pump, it's possible that there will be a series of messages in the queue related to the window creation ahead of your posted message.

Upvotes: 2

tenfour
tenfour

Reputation: 36906

Not all messages are equal. PostMessage doesn't simply add it to a queue data structure; the OS will do a lot of manipulation and prioritizing. Different messages get different priorities, and some messages like WM_PAINT, cannot be sent like this at all.

So there's no way to generalize "when will PostMessage invoke the message handler?" It depends on which message you're sending, how busy the target message queue is, and how the target message loop behaves.

I don't think there is any concise list of how messages are prioritized.

Upvotes: 1

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