How does the message loop use threads?

Question

I'm somewhat confused and wondering if I've been misinformed, in a separate post I was told "New threads are only created when you make them explicitly. C++ programs are by default single threaded." When I open my program that doesn't explicitly create new threads in ollydbg I noticed multiple times that there are often 2 threads running. I wanted to understand how the message loop works without stopping up execution, the explanation I got was very insufficient at explaining how it works.

Does the message loop create a new thread or does it take up the main thread? If it takes the main thread does it do so after everything else has been executed regardless of code order? If it doesn't do this but still takes up the main thread does it spawn a new thread so that the program can execute instead of getting stuck in the message loop?

EDIT: Solved most of my questions with experimentation. The message loop occupies the main thread and any code after the code:

while (GetMessage (&messages, NULL, 0, 0))
{
    TranslateMessage(&messages);
    DispatchMessage(&messages);
}
return messages.wParam;

Will not execute unless something special is done to cause it to execute because the program is stuck in the message loop. Putting an infinite loop in a window procedure that gets executed causes the program to crash. I still don't understand the mystery of the multiple threads when in olly to the degree I would prefer though.

Answer 1

Perhaps the place to start is to realize that "the message loop" isn't a thing as such; it's really just something that a thread does.

Threads in windows generally fall into one of two categories: those that own UI, and those that do background work (eg network operations).

A simple UI app typically has just one thread, which is a UI thread. For the UI to work, the thread needs to wait until there's some input to handle (mouse click, keyboard input, etc), handle the input (eg. update the state and redraw the window), and then go back to waiting for more input. This whole act of "wait for input, process it, repeat" is the message loop. (Also worth mentioning at this stage is the message queue: each thread has its own input queue which stores up the input messages for a thread; and the act of a thread "waiting for input" is really about checking if there's anything in the queue, and if not, waiting till there is.) In win32 speak, if a thread is actively processing input this way, it's also said to be "pumping messages".

A typical simple windows app's mainline code will first do basic initialization, create the main window, and then do the wait-for-input-and-process-it message loop. It does this usually until the user closes the main window, at which point the thread exits the loop, and carries on executing the code that comes afterwards, which is usually cleanup code.

A common architecture in windows apps is to have a main UI thread - usually this is the main thread - and it creates and owns all the UI, and has a message loop that dispatches messages for all of the UI that the thread created. If an app needs to do something that could potentially block, such as reading from a socket, a worker thread is often used for that purpose: you don't want the UI thread to block (eg. while waiting for input from a socket), as it wouldn't be processing input during that time and the UI would end up being unresponsive.

You could write an app that had more than one UI thread in it - and each thread that creates windows would then need its own message loop - but it's a fairly advanced technique and not all that useful for most basic apps.

The other threads you are seeing are likely some sort of helper threads that are created by Windows to do background tasks; and for the most part, you can ignore them. If you initialize COM, for example, windows may end up creating a worker thread to manage some COM internal stuff, and it may also create some invisible HWNDs too.

Answer 2

Typically the thread that starts the program only runs the message loop, taking up the main thread. Anything not part of handling messages or updating the UI is typically done by other threads. The additional thread that you see even if your application doesn't create any threads could be created by a library or the operating system. Windows will create threads inside your process to handle things like dispatching events to your message loop.