Question

I tried to explain it all in the title: I have a multithreaded C program which will have multiple windows calling one Window Procedure.

There is some processing done in the window procedure. Do I need to protect it, or will each call to the window procedure be separated in memory?

My instinct is that I don’t need a mutex, because they’re all local variables, is this wrong?

LRESULT APIENTRY EditSubclassProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) 
{   
    if (uMsg == WM_GETDLGCODE) 
        return DLGC_WANTALLKEYS;
    else if(uMsg == WM_CHAR)
    {
        if( (int) wParam == 13)
        {
            char* strCurrentCommand;

            unsigned long ulThisConversation = GetConversation(0, 0, hwnd, 0, 0);
            ...

I’m concerned with the local variables strCurrentCommand and ulThisConversation.

Answer 1

Local variables and parameters to functions go on the stack. Each thread gets its own stack, and each invocation of a function gets a space on the stack of the thread it’s running in for its parameters and local variables. So you’re fine.