Enumerating Windows Messages

messages.c

#include <windows.h> 
#include <stdio.h>
#include <stdint.h>

#pragma comment(lib, "USER32")

FARPROC NtUserPostMessage;

BOOL CALLBACK QueryWindowMessageProc(HWND Window, LPARAM Param)
{
    BOOL ReturnCode = TRUE;
    DWORD Result;
    CHAR Title[1024] = {0};

    GetWindowText(Window, Title, sizeof Title);

    for (DWORD MsgNum = 0; MsgNum <= UINT16_MAX; MsgNum++) {
        switch (MsgNum) {
            // Whitelisted messages.
            case WM_NULL:
            case WM_SIZE:
            case WM_MOVE:
            case WM_GETHOTKEY:
            case WM_GETICON:
            case WM_RENDERFORMAT:
            case WM_DRAWCLIPBOARD:
            case WM_CHANGECBCHAIN:
            case WM_DWMNCRENDERINGCHANGED:
            case WM_THEMECHANGED:
            case WM_GETTEXT:
            case WM_GETTEXTLENGTH:
            case WM_TOUCH:

            // WM_SYSMENU, used by explorer to make the right click menu on the previews.
            case 0x313:

            // Unknown, probably theme related.
            case 0x31B: 
                continue;
        }

        if (NtUserPostMessage(Window, MsgNum, 0, 0)) {
            fprintf(stderr, "Message %#x to \"%s\" was allowed!\n", MsgNum, Title);
            ReturnCode = FALSE;
        } else {
            Result = GetLastError();
            if (Result != ERROR_ACCESS_DENIED && Result != ERROR_INVALID_MESSAGE) {
                fprintf(stderr, "Message %#x to \"%s\" didn't work, but wasnt denied (%#x).\n",
                                MsgNum,
                                Title,
                                Result);
            }
        }
    }

    fprintf(stderr, "Window %p \"%s\" completed.\n", Window, Title);

    return ReturnCode;
}

int main(int argc, char **argv)
{
    ULONG_PTR WindowId;
    DWORD MessageId;
    DWORD ThreadId;
    CHAR Title[1024] = {0};
    CHAR Class[1024] = {0};

    // Windows will try to parse the data you provide to PostMessage() so it
    // can marshal it. I only want to know if the message is allowed, so
    // I'll use the system call directly and skip all the marshaling.
    //
    // This way I can just check if I got ERROR_ACCESS_DENIED and not worry about
    // exceptions interfering with the test results.
    NtUserPostMessage = GetProcAddress(LoadLibrary("WIN32U"), "NtUserPostMessage");

    if (argc != 2) {
        fprintf(stderr, "Usage: %s THREADID\n", *argv);
        return 1;
    }

    ThreadId = strtoul(*++argv, NULL, 0);

    EnumThreadWindows(ThreadId, QueryWindowMessageProc, 0);

    return 0;
}