PostQuitMessage

The PostQuitMessage function posts a message to Windows indicating that an application is requesting to terminate execution (quit). This function is typically used in response to a `WM_DESTROY` message.

VOID WINAPI PostQuitMessage(int nExitCode);

• nExitCode — Specifies an application-defined exit code. It must be the wParam parameter of the `WM_QUIT` message.

This function does not return a value.

• The PostQuitMessage function posts a `WM_QUIT` message to the application and returns immediately; the function simply indicates to the system that the application will request to quit some time in the future.
• When the application receives the `WM_QUIT` message, it should exit the message loop in the main function and return control to Windows.

#include <windows.h>
 
LRESULT CALLBACK __export WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch (msg)
    {
        case WM_DESTROY:
            PostQuitMessage(0);
            return 0;
    }
    return DefWindowProc(hwnd, msg, wParam, lParam);
}
 
int WINAPI WinMain(HANDLE hInstance, HANDLE hPrevInstance,
                   LPSTR lpCmdLine, int nCmdShow)
{
    MSG msg;
 
    /* ... window class registration and creation ... */
 
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
 
    return (int)msg.wParam;
}

EXTRN PostQuitMessage:FAR
 
WndProc PROC FAR
    cmp     ax, WM_DESTROY
    je      handle_destroy
    ; ... other messages ...
 
handle_destroy:
    push    0                    ; nExitCode = 0
    call    PostQuitMessage
    xor     ax, ax               ; return 0 (message handled)
    retf
WndProc ENDP
 
; Message loop that catches WM_QUIT
WinMain PROC FAR
    ; ... initialization ...
 
message_loop:
    lea     ax, msg
    push    ax                   ; lpMsg
    push    0                    ; hWnd
    push    0                    ; wMsgFilterMin
    push    0                    ; wMsgFilterMax
    call    GetMessage
    cmp     ax, 0
    je      exit_app             ; FALSE — WM_QUIT received
 
    lea     ax, msg
    push    ax
    call    TranslateMessage
    lea     ax, msg
    push    ax
    call    DispatchMessage
    jmp     message_loop
 
exit_app:
    mov     ax, msg.wParam       ; return exit code from WM_QUIT
    retf
WinMain ENDP

• MessageBox
• InitApp
• GetMessage