Weird behaviour when global variables are added in another file

Question

Here's my simple program:

#include "stdafx.h"
#include<Windows.h>

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);
void initBackBuffer(HWND hwnd);

HDC hBackDC = NULL;
HBITMAP hBackBitmap = NULL;

const int WIDTH = 512;
const int HEIGHT = 512;

DWORD screenBuffer[WIDTH * HEIGHT];


void draw(HWND hwnd) {
    HDC hWinDC = GetDC(hwnd);

    SetBitmapBits(hBackBitmap, HEIGHT * WIDTH * sizeof(DWORD), (const void*)(screenBuffer));
    BitBlt(hWinDC, 0, 0, WIDTH, HEIGHT, hBackDC, 0, 0, SRCCOPY);
    ReleaseDC(hwnd, hWinDC);
}

int WINAPI wWinMain(HINSTANCE hInstace, HINSTANCE hPrevInstace, LPWSTR lpCmdLine, int nCmdShow) {
    memset(screenBuffer, 0, sizeof(screenBuffer));
    MSG msg = { 0 };
    WNDCLASS wnd = { 0 };

    wnd.lpfnWndProc = WndProc;
    wnd.hInstance = hInstace;
    wnd.lpszClassName = L"Window";

    if (!RegisterClass(&wnd)) {
        return 0;
    }

    HWND hwnd = CreateWindowEx(NULL, wnd.lpszClassName, L"Window",
        WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, WIDTH, HEIGHT, NULL, NULL, hInstace, NULL);

    if (!hwnd) {
        return 0;
    }

    ShowWindow(hwnd, nCmdShow);
    UpdateWindow(hwnd);

    for (int i = 0; i <= 512; i++) {
        screenBuffer[i * WIDTH + 0] = 0x00FF0000;
    }

    while (true) {
        if(PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {
            if (msg.message == WM_QUIT) {
                break;
            }

            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }


        draw(hwnd);
    }

    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam){

    switch (msg){
        case WM_CREATE:
            initBackBuffer(hwnd);
            break;
        case WM_DESTROY:
            DeleteDC(hBackDC);
            DeleteObject(hBackBitmap);
            PostQuitMessage(0);
            break;
    }
    return DefWindowProc(hwnd, msg, wParam, lParam);
}

void initBackBuffer(HWND hwnd) {
    HDC hWinDC = GetDC(hwnd);

    hBackDC = CreateCompatibleDC(hWinDC);
    hBackBitmap = CreateCompatibleBitmap(hWinDC, WIDTH, HEIGHT); 
    SetBitmapBits(hBackBitmap, HEIGHT * WIDTH * sizeof(DWORD), (const void*)(screenBuffer));

    SelectObject(hBackDC, hBackBitmap);
    ReleaseDC(hwnd, hWinDC);
}

The output is as expected.

I moved

const int WIDTH = 512;
const int HEIGHT = 512;

DWORD screenBuffer[WIDTH * HEIGHT]; 

into Global.h and I added #include "Global.h" in my main file.

Main File :

#include "stdafx.h"
#include<Windows.h>
#include "Global.h"

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);
void initBackBuffer(HWND hwnd);

HDC hBackDC = NULL;
HBITMAP hBackBitmap = NULL;


void draw(HWND hwnd) {
    HDC hWinDC = GetDC(hwnd);

    SetBitmapBits(hBackBitmap, HEIGHT * WIDTH * sizeof(DWORD), (const void*)(screenBuffer));
    BitBlt(hWinDC, 0, 0, WIDTH, HEIGHT, hBackDC, 0, 0, SRCCOPY);
    ReleaseDC(hwnd, hWinDC);
}

int WINAPI wWinMain(HINSTANCE hInstace, HINSTANCE hPrevInstace, LPWSTR lpCmdLine, int nCmdShow) {
    memset(screenBuffer, 0, sizeof(screenBuffer));
    MSG msg = { 0 };
    WNDCLASS wnd = { 0 };

    wnd.lpfnWndProc = WndProc;
    wnd.hInstance = hInstace;
    wnd.lpszClassName = L"Window";

    if (!RegisterClass(&wnd)) {
        return 0;
    }

    HWND hwnd = CreateWindowEx(NULL, wnd.lpszClassName, L"Window",
        WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, WIDTH, HEIGHT, NULL, NULL, hInstace, NULL);

    if (!hwnd) {
        return 0;
    }

    ShowWindow(hwnd, nCmdShow);
    UpdateWindow(hwnd);

    for (int i = 0; i <= 512; i++) {
        screenBuffer[i * WIDTH + 0] = 0x00FF0000;
    }

    while (true) {
        if(PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {
            if (msg.message == WM_QUIT) {
                break;
            }

            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }


        draw(hwnd);
    }

    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam){

    switch (msg){
        case WM_CREATE:
            initBackBuffer(hwnd);
            break;
        case WM_DESTROY:
            DeleteDC(hBackDC);
            DeleteObject(hBackBitmap);
            PostQuitMessage(0);
            break;
    }
    return DefWindowProc(hwnd, msg, wParam, lParam);
}

void initBackBuffer(HWND hwnd) {
    HDC hWinDC = GetDC(hwnd);

    hBackDC = CreateCompatibleDC(hWinDC);
    hBackBitmap = CreateCompatibleBitmap(hWinDC, WIDTH, HEIGHT); 
    SetBitmapBits(hBackBitmap, HEIGHT * WIDTH * sizeof(DWORD), (const void*)(screenBuffer));

    SelectObject(hBackDC, hBackBitmap);
    ReleaseDC(hwnd, hWinDC);
}

Global.h

#pragma once


const int WIDTH = 512;
const int HEIGHT = 512;

DWORD screenBuffer[WIDTH * HEIGHT];

I get a erroneous white window.

I don't understand why this is happening because the compiler will anyway copy the contents of Global.h in to main file, so both variants should produce same results.

What is the cause of this problem ?

Answer 1

There is a bug here:

const int WIDTH = 512;
const int HEIGHT = 512;
DWORD screenBuffer[WIDTH * HEIGHT];
void foo()
{
    for (int i = 0; i <= 512; i++) {
        screenBuffer[i * WIDTH + 0] = 0x00FF0000;
    }
}

This should be should be i < 512. Otherwise it overwrites a random memory location, this can result in an error in a different location, or no error if you are lucky. Debugger may report a nonsensical error, or no error at all. If screenBuffer was created on stack, debugger may give "heap corruption" error.

Consider using std::vector to avoid this problem in future.

vector<int> vec;
vec[vec.size()] = 0;//<- debug error

^{Side note: SetDIBitsToDevice or StretchDIBits will set bits directly:}

void draw(HWND hwnd) 
{
    BITMAPINFO bi;
    bi.bmiHeader.biSize = sizeof(bi.bmiHeader);
    bi.bmiHeader.biBitCount = 32;
    bi.bmiHeader.biWidth = WIDTH;
    bi.bmiHeader.biHeight = HEIGHT;
    bi.bmiHeader.biPlanes = 1;
    bi.bmiHeader.biCompression = BI_RGB;
    HDC hdc = GetDC(hwnd);
    SetDIBitsToDevice(hdc, 0, 0, WIDTH, HEIGHT, 0, 0, 0, HEIGHT, screenBuffer,
        &bi, DIB_RGB_COLORS);
    ReleaseDC(hwnd, hdc);
}