Subj : Re: Win32 PostMessage Question
To   : comp.programming.threads
From : robertwessel2
Date : Wed Jul 06 2005 03:11 pm

Cool Guy wrote:
> From a *worker thread*, I'm posting messages to the *GUI thread* of my
> application (via PostMessage).
>
> If I send two such messages in a row (both from the same worker thread), is
> the first one *guaranteed* to be in front of the second one in the message
> queue that receives these messages?
>
> e.g.:
>
>     (in worker thread)
>     {
>         PostMessage(...);  // post *first*  message to GUI thread
>         PostMessage(...);  // post *second* message to GUI thread
>     }
>
> Is the first message here guaranteed to appear ahead of the second message
> in the receiving message queue?
>
> I assume so but I'm asking just to make sure.


Short answer: yes.  Long answer: First, since you're not checking the
return codes from the posts, the first message might silently not enter
the target's message queue at all.  Second, there are certain system
generated messages (WM_TIMER, WM_PAINT, and a few others), that have
special behavior, and can do some quite odd things if you actually try
to post them yourself (which you're not supposed to).  Third, the
target application has some control over how messages are removed from
the queue, which doesn't impact the order in which they're put into the
queue, but can certainly impact the order in which the receiving
application sees them.  Finally, an application doesn't necessarily
have to finish processing one message before retrieving another (if
there's an embedded message pump somewhere), which doesn't change the
order, but does create overlap in the sense that the second message can
be processed after the processing of the first message starts but
before it completes.

But if you stick with user messages, or messages you're normally
allowed to send, and assume typical behaviors for the target
application, and you don't overflow the target queues, they'll arrive
in order.

.