The documentation for Windows message queues says that
To avoid the overhead of creating a message queue for non-GUI threads, all threads are created initially without a message queue. The system creates a thread-specific message queue only when the thread makes its first call to one of the specific user functions.
A customer was unable to observe the documented behavior. According to their experiments, they found that they were able to post a message to a thread immediately upon its creation!
DWORD CALLBACK ThreadProc(void* parameter)
{
// This succeeds (?)
BOOL success =
PostThreadMessage(GetCurrentThreadId(), WM_APP, 314, 159);
// So does this (!)
MSG msg;
success = PeekMessage(&msg, nullptr, 0, 0, PM_REMOVE);
// And it's our message (!)
if (msg.message == WM_APP) {
OutputDebugStringW(L"We received the message (?)\r\n");
}
return 0;
}
What’s going on here? We were able to post a message to the thread despite it never having created a message queue.
What’s going on here is that PostThreadMessage is itself a message queue creation function!
So at the time you call PostThreadMessage, the thread does not have a message queue. When you call it, the system says, “First, I need to create a message queue for the current thread if it doesn’t have one.” This is what creates the message queue. And then we post the thread message into that queue.
If you want to see PostThreadMessage fail due to the lack of a message queue in the destination thread, then use it to post a message into the message queue of some other thread.
static auto ready = CreateEvent(nullptr, TRUE, FALSE, nullptr);
static auto exit = CreateEvent(nullptr, TRUE, FALSE, nullptr);
// Start a thread and wait for it to start.
DWORD id;
auto thread = CreateThread(nullptr, 0, [](void*) {
SetEvent(ready);
return WaitForSingleObject(exit, INFINITE);
}, nullptr, 0, &id);
WaitForSingleObject(ready, INFINITE);
// Now that it has started, try to post it a message.
auto success = PostThreadMessage(id, WM_APP, 314, 159);
// Tell the thread to exit.
SetEvent(exit);
In this case, the PostThreadMessage fails because the thread has not yet created a queue.
The customer’s experiment failed because it made the thread post a thread message to itself, and the act of posting a thread message creates the message queue. To observe the absence of a message queue, you have to do the post from another thread.
So which functions create a message queue?
The functions that are guaranteed to create a message queue are PeekMessage, GetMessage, and CreateWindow (or other functions that create windows like DialogBox). There may be other functions that also create a message queue as a side effect (like PostThreadMessage), but you shouldn’t rely on them.
Would it be better to create message queue explicitly like WinCreateMsgQueue in OS/2?
It is pretty rare that a message queue is created when you don't mean to create one. It is also important to note that the biggest protection here, using up a lot of memory by filling up a message queue, is still in place because it isn't possible to create a message queue accidentally for another thread. The only times that I know of that a message queue is accidentally created for the current thread...
I ran into a similar thing with GetGUIThreadInfo.
I was looking for a way to simplify figuring out if a thread was a GUI thread. IsGUIThread existed for the current thread, but I was looking for a more general way to test this. After I found that GetGUIThreadInfo would fail for other threads, I tested it on the current thread. To my surprise, it succeeded.
In the end, I used IsGUIThread for the current thread...