c - 屏幕截图 WinAPI - 释放内存

Question

一两天前我已经问过一个非常类似的问题，但我的问题不是很清楚，所以我尝试在这里重新表述:

我想要截取屏幕截图，为此我搜索并找到了以下代码:

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

inline int GetFilePointer(HANDLE FileHandle)
{
    return SetFilePointer(FileHandle, 0, 0, FILE_CURRENT);
}

extern _Bool SaveBMPFile(char* filePath, HBITMAP bitmap, HDC bitmapDC, int width, int height)
{
    _Bool Success = 0;
    HDC SurfDC = NULL;        // GDI-compatible device context for the surface
    HBITMAP OffscrBmp = NULL; // bitmap that is converted to a DIB
    HDC OffscrDC = NULL;      // offscreen DC that we can select OffscrBmp into
    LPBITMAPINFO lpbi = NULL; // bitmap format info; used by GetDIBits
    LPVOID lpvBits = NULL;    // pointer to bitmap bits array
    HANDLE BmpFile = INVALID_HANDLE_VALUE;    // destination .bmp file
    BITMAPFILEHEADER bmfh;  // .bmp file header

                            // We need an HBITMAP to convert it to a DIB:
    if ((OffscrBmp = CreateCompatibleBitmap(bitmapDC, width, height)) == NULL)
        return 0;

    // The bitmap is empty, so let's copy the contents of the surface to it.
    // For that we need to select it into a device context. We create one.
    if ((OffscrDC = CreateCompatibleDC(bitmapDC)) == NULL)
        return 0;

    // Select OffscrBmp into OffscrDC:
    HBITMAP OldBmp = (HBITMAP)SelectObject(OffscrDC, OffscrBmp);

    // Now we can copy the contents of the surface to the offscreen bitmap:
    BitBlt(OffscrDC, 0, 0, width, height, bitmapDC, 0, 0, SRCCOPY);

    // GetDIBits requires format info about the bitmap. We can have GetDIBits
    // fill a structure with that info if we pass a NULL pointer for lpvBits:
    // Reserve memory for bitmap info (BITMAPINFOHEADER + largest possible
    // palette):
    if ((lpbi = (LPBITMAPINFO)malloc(sizeof(BITMAPINFOHEADER) + 256 * sizeof(RGBQUAD))) == NULL)
        return 0;


    ZeroMemory(&lpbi->bmiHeader, sizeof(BITMAPINFOHEADER));
    lpbi->bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    // Get info but first de-select OffscrBmp because GetDIBits requires it:
    SelectObject(OffscrDC, OldBmp);
    if (!GetDIBits(OffscrDC, OffscrBmp, 0, height, NULL, lpbi, DIB_RGB_COLORS))
        return 0;

    // Reserve memory for bitmap bits:
    if ((lpvBits = malloc(lpbi->bmiHeader.biSizeImage)) == NULL)
        return 0;

    // Have GetDIBits convert OffscrBmp to a DIB (device-independent bitmap):
    if (!GetDIBits(OffscrDC, OffscrBmp, 0, height, lpvBits, lpbi, DIB_RGB_COLORS))
        return 0;


    //ANSI->Unicode
    LPCSTR szAnsi = filePath;
    int Size = MultiByteToWideChar(CP_ACP, 0, szAnsi, -1, NULL, 0);
    LPWSTR filename = malloc(sizeof(LPWSTR) * Size);
    MultiByteToWideChar(CP_ACP, 0, szAnsi, -1, filename, Size);
    // Create a file to save the DIB to:
    if ((BmpFile = CreateFile(filename,
        GENERIC_WRITE,
        0, NULL,
        CREATE_ALWAYS,
        FILE_ATTRIBUTE_NORMAL,
        NULL)) == INVALID_HANDLE_VALUE)

        return 0;

    DWORD Written;    // number of bytes written by WriteFile

                      // Write a file header to the file:
    bmfh.bfType = 19778;        // 'BM'
                                // bmfh.bfSize = ???        // we'll write that later
    bmfh.bfReserved1 = bmfh.bfReserved2 = 0;
    // bmfh.bfOffBits = ???     // we'll write that later
    if (!WriteFile(BmpFile, &bmfh, sizeof(bmfh), &Written, NULL))
        return 0;

    if (Written < sizeof(bmfh))
        return 0;

    // Write BITMAPINFOHEADER to the file:
    if (!WriteFile(BmpFile, &lpbi->bmiHeader, sizeof(BITMAPINFOHEADER), &Written, NULL))
        return 0;

    if (Written < sizeof(BITMAPINFOHEADER))
        return 0;

    // Calculate size of palette:
    int PalEntries;
    // 16-bit or 32-bit bitmaps require bit masks:
    if (lpbi->bmiHeader.biCompression == BI_BITFIELDS)
        PalEntries = 3;
    else
        // bitmap is palettized?
        PalEntries = (lpbi->bmiHeader.biBitCount <= 8) ?
        // 2^biBitCount palette entries max.:
        (int)(1 << lpbi->bmiHeader.biBitCount)
        // bitmap is TrueColor -> no palette:
        : 0;
    // If biClrUsed use only biClrUsed palette entries:
    if (lpbi->bmiHeader.biClrUsed)
        PalEntries = lpbi->bmiHeader.biClrUsed;

    // Write palette to the file:
    if (PalEntries) {
        if (!WriteFile(BmpFile, &lpbi->bmiColors, PalEntries * sizeof(RGBQUAD), &Written, NULL))
            return 0;

        if (Written < PalEntries * sizeof(RGBQUAD))
            return 0;
    }

    // The current position in the file (at the beginning of the bitmap bits)
    // will be saved to the BITMAPFILEHEADER:
    bmfh.bfOffBits = GetFilePointer(BmpFile);

    // Write bitmap bits to the file:
    if (!WriteFile(BmpFile, lpvBits, lpbi->bmiHeader.biSizeImage, &Written, NULL))
        return 0;

    if (Written < lpbi->bmiHeader.biSizeImage)
        return 0;

    // The current pos. in the file is the final file size and will be saved:
    bmfh.bfSize = GetFilePointer(BmpFile);

    // We have all the info for the file header. Save the updated version:
    SetFilePointer(BmpFile, 0, 0, FILE_BEGIN);
    if (!WriteFile(BmpFile, &bmfh, sizeof(bmfh), &Written, NULL))
        return 0;

    if (Written < sizeof(bmfh))
        return 0;

    return 1;
}



_Bool ScreenCapture(char* filePath, int xStart, int yStart, int width, int height)
{
    // get a DC compat. w/ the screen
    HDC hDc = CreateCompatibleDC(0);

    // make a bmp in memory to store the capture in
    HBITMAP hBmp = CreateCompatibleBitmap(GetDC(0), width, height);

    // join em up
    SelectObject(hDc, hBmp);

    // copy from the screen to my bitmap
    BitBlt(hDc, 0, 0, width, height, GetDC(0), xStart, yStart, SRCCOPY);

    // save my bitmap
    _Bool ret = SaveBMPFile(filePath, hBmp, hDc, width, height);

    // free the bitmap memory
    DeleteObject(hBmp);

    return ret;
}

main()
{
    ScreenCapture("screenshot.bmp", 0, 0, 1920, 1080);

    FILE* Screen = NULL;
    Screen = fopen("screenshot.bmp", "r"); //Error, the image is "used" somewhere...     

    return 0;
}

我不懂 WinAPI，但我在 Viual Studio 中看到，每次调用 ScreenCapture() 时，都会有未释放的 RAM，这是一个问题。

如果你们中的一些人知道错误出在哪里......

感谢帮助，谢谢:)

Answer 1

HBITMAP hBmp = CreateCompatibleBitmap(GetDC(0), width, height);

GetDC(0) 必须使用 ReleasedDC 进行清理。当您以这种方式编写 GetDC(0) 时，就无法清理。您的程序只有 10,000 个 GDI 句柄可用，因此这种资源泄漏可能是一个严重的问题。

你应该这样做:

HDC hdc_desktop = GetDC(HWND_DESKTOP);
HBITMAP hBmp = CreateCompatibleBitmap(hdc_desktop, width, height);
...
ReleaseDC(HWND_DESKTOP, hdc_desktop);

HWND_DESKTOP 只是 0，为了清楚起见，我使用了它。 GetDC 返回一个句柄 hdc_desktop，可以在最后清理。

DeleteObject(hBmp) 是一个小错误，因为 hBmp 当前已在 hDc 中选择，无法删除。很多程序员都会犯这个错误，因此较新的 Windows 版本(我认为自 XP 以来)已经预料到了这一点，因此 Windows 修复了此错误(至少在这种情况下)，但您确实应该保存旧位图的句柄，恢复旧位图，然后删除新位图。

GetDIBits 还存在另一个小错误。文档说调用该函数时不得在 dc 中选择 hbitmap。但 Windows 再次习惯于看到此错误并修复它。

您的SaveBMPFile可以被简化。您没有保存调色板位图，您可以完全忽略调色板。

不相关的问题:您正在使用 Unicode 函数，因此您使用 UTF-16 字符串文字:L"screenshot.bmp" 而不是 "screenshot.bmp"

extern _Bool SaveBMPFile(const wchar_t* filePath, 
    HDC memdc, HBITMAP hbitmap, int width, int height)
{
    _Bool success = 0;
    WORD bpp = 24; //or 32 for 32-bit bitmap
    DWORD size = ((width * bpp + 31) / 32) * 4 * height;

    BITMAPFILEHEADER filehdr = { 0 };
    filehdr.bfType = 19778;
    filehdr.bfSize = 54 + size;
    filehdr.bfOffBits = 54; 
    //54 = 14 + 40, sizeof BITMAPFILEHEADER & BITMAPINFOHEADER

    BITMAPINFOHEADER infohdr = { sizeof(infohdr) };
    infohdr.biWidth = width;
    infohdr.biHeight = height;
    infohdr.biPlanes = 1;
    infohdr.biBitCount = bpp;

    BYTE *bits = malloc(size);
    GetDIBits(memdc, hbitmap, 0, height, bits, (BITMAPINFO*)&infohdr, DIB_RGB_COLORS);

    HANDLE hfile = CreateFileW(filePath,
        GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
    if(hfile != INVALID_HANDLE_VALUE)
    {
        DWORD temp;
        WriteFile(hfile, &filehdr, 14, &temp, NULL);
        WriteFile(hfile, &infohdr, 40, &temp, NULL);
        WriteFile(hfile, bits, size, &temp, NULL);
        CloseHandle(hfile);
        success = 1;
    }

    free(bits);    
    return success;
}

_Bool ScreenCapture(const wchar_t* filePath, int x, int y, int width, int height)
{
    HDC hdc = GetDC(HWND_DESKTOP);
    HDC memdc = CreateCompatibleDC(hdc);
    HBITMAP hbitmap = CreateCompatibleBitmap(hdc, width, height);
    HBITMAP oldbitmap = SelectObject(memdc, hbitmap);
    BitBlt(memdc, 0, 0, width, height, hdc, x, y, SRCCOPY);
    SelectObject(memdc, oldbitmap);

    _Bool ret = SaveBMPFile(filePath, memdc, hbitmap, width, height);

    DeleteObject(hbitmap);
    DeleteDC(memdc);
    ReleaseDC(HWND_DESKTOP, hdc);

    return ret;
}

int main(void)
{
    ScreenCapture(L"screenshot.bmp", 0, 0, 
        GetSystemMetrics(SM_CXFULLSCREEN), GetSystemMetrics(SM_CYFULLSCREEN));
    return 0;
}

您可以循环运行它并在 Taskman 中观察 GDI 句柄以确保没有泄漏。