为什么将窗口样式设定为CS_OWNDC,在GetDC后还要ReleaseDC

HisinWang 2011-06-28 10:52:57
Windows程序设计中指出：
在注册窗口类时将CS_OWNDC标志作为窗口类样式的一部分，那么每个基于这个窗口类创建的窗口都有它的私有设备环境。
但又说：即使是使用了CS_OWNDC样式，设备环境句柄在退出窗口前也应该被释放。
这里我始终都想不通，既然用GetDC获得的HDC指向窗口的私有设备环境，那怎么释放呢，释放掉那这个私有设备环境及即释放了其占据的内存，不可能啊,那之后再GetDCz又是怎么回事呢？
ReleaseDC到底做了什么啊？

如果没有将窗口样式设定为CS_OWNDC，那么GetDC就是创建一个DC对象，并返回指向这个对象的指针，当然必须配对的使用ReleaseDC以释放内存。但对于上面提到的CS_OWNDC样式我就没有办法理解了！
望高手指点！
感觉DC很玄啊，很多东西都没法理解！

THKS......
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wocow3 2011-06-30
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uses a significant portion of 64K GDI heap
说的是GDI heap是64K
HisinWang 2011-06-30
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[Quote=引用 7 楼 wocow3 的回复:]
uses a significant portion of 64K GDI heap
说的是GDI heap是64K
[/Quote]
正解！谢谢！
没看仔细！
wocow3 2011-06-29
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对CS_OWNDC窗口风格的DC调用ReleaseDC实际上并不会清理DC，也不会释放其占据的内存。线程的DC cache会将标记CS_OWNDC的DC是特殊的DC，获取CS_OWNDC窗口的DC总是返回它的这个特殊DC，而一般的窗口则返随意获取一个非CS_OWNDC的DC，并每次重置其属性。
“即使是使用了CS_OWNDC样式，设备环境句柄在退出窗口前也应该被释放。”的意图只是让你的代码在CS_OWNDC和非CS_OWNDC下保持一致。
HisinWang 2011-06-29
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我一直在想：为什么对于common DC要retrieved from a pool of contexts maintained by the system。用的时候新建不就可以了吗。为什么还要限定个数！难道它很占内存！
终于在MSDN上找到答案了：
Windows 95/98/Me: Although the CS_OWNDC style is convenient, use it carefully, because each device context uses a significant portion of 64K GDI heap.
原来DC是很占内存的：64K！
HisinWang 2011-06-29
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[Quote=引用 3 楼 wocow3 的回复:]
对CS_OWNDC窗口风格的DC调用ReleaseDC实际上并不会清理DC，也不会释放其占据的内存。线程的DC cache会将标记CS_OWNDC的DC是特殊的DC，获取CS_OWNDC窗口的DC总是返回它的这个特殊DC，而一般的窗口则返随意获取一个非CS_OWNDC的DC，并每次重置其属性。
“即使是使用了CS_OWNDC样式，设备环境句柄在退出窗口前也应该被释放。”的意图只是让你的代码在CS……
[/Quote]
我查看MSDN上关于ReleaseDC的说明：
The ReleaseDC function releases a device context (DC), freeing it for use by other applications. The effect of the ReleaseDC function depends on the type of DC. It frees only common and window DCs. It has no effect on class or private DCs.
红色部分所说也就是3楼这位仁兄的见解！
HisinWang 2011-06-29
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受益匪浅！
我查看CSDN上的GetDC函数的说明：
Remarks:
The GetDC function retrieves a common, class, or private DC depending on the class style of the specified window. For class and private DCs, GetDC leaves the previously assigned attributes unchanged. However, for common DCs, GetDC assigns default attributes to the DC each time it is retrieved. For example, the default font is System, which is a bitmap font. Because of this, the handle for a common DC returned by GetDC does not tell you what font, color, or brush was used when the window was drawn.

Note that the handle to the DC can only be used by a single thread at any one time.

After painting with a common DC, the ReleaseDC function must be called to release the DC. Class and private DCs do not have to be released. ReleaseDC must be called from the same thread that called GetDC. The number of DCs is limited only by available memory.

GetDC函数返回指定窗口客户区域DC的句柄。至于返回的DC类型是common DC，class DC还是private DC取决于指定窗口的class style即窗口所属注册窗口类的style.
当WNDCLASS结构体中style包含CS_CLASSDC时GetDC返回的就是class DC的句柄。所有基于这个窗口类的窗口实例共享这个DC。
当WNDCLASS结构体中style包含CS_OWNDC时GetDC返回的就是private DC的句柄。每个基于这个窗口类的窗口实例都有唯一的属于它自己的这个DC。
当WNDCLASS结构体中style不包含CS_CLASSDC以及CS_OWNDC时GetDC返回的就是common DC。
Eleven 2011-06-28
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注意看最后一段话~
Eleven 2011-06-28
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MSDN上有这么一段：
Classes and Device Contexts
A device context is a special set of values that applications use for drawing in the client area of their windows. The system requires a device context for each window on the display but allows some flexibility in how the system stores and treats that device context.

If no device-context style is explicitly given, the system assumes each window uses a device context retrieved from a pool of contexts maintained by the system. In such cases, each window must retrieve and initialize the device context before painting and free it after painting.

To avoid retrieving a device context each time it needs to paint inside a window, an application can specify the CS_OWNDC style for the window class. This class style directs the system to create a private device context — that is, to allocate a unique device context for each window in the class. The application need only retrieve the context once and then use it for all subsequent painting.
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/* * This Code Was Created By Jeff Molofee 2000 * A HUGE Thanks To Fredric Echols For Cleaning Up * And Optimizing The Base Code, Making It More Flexible! * If You've Found This Code Useful, Please Let Me Know. * Visit My Site At nehe.gamedev.net */ #include > #include // Header File For Windows #include // Header File For The OpenGL32 Library #include // Header File For The GLu32 Library #include // Header File For The Glaux Library HDC hDC=NULL; // Private GDI Device Context HGLRC hRC=NULL; // Permanent Rendering Context HWND hWnd=NULL; // Holds Our Window Handle HINSTANCE hInstance; // Holds The Instance Of The Application int ax[10000]; bool keys[256]; // Array Used For The Keyboard Routine bool active=TRUE; // Window Active Flag Set To TRUE By Default bool fullscreen=TRUE; // Fullscreen Flag Set To Fullscreen Mode By Default int a=0; int b=0; int c=0; LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM); // Declaration For WndProc GLvoid ReSizeGLScene(GLsizei width, GLsizei height) // Resize And Initialize The GL Window { if (height==0) // Prevent A Divide By Zero By { height=1; // Making Height Equal One } glViewport(0,0,width,height); // Reset The Current Viewport glMatrixMode(GL_PROJECTION); // Select The Projection Matrix glLoadIdentity(); // Reset The Projection Matrix // Calculate The Aspect Ratio Of The Window gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,0.1f,1000.0f); glMatrixMode(GL_MODELVIEW); // Select The Modelview Matrix glLoadIdentity(); // Reset The Modelview Matrix } int InitGL(GLvoid) // All Setup For OpenGL Goes Here { int axa,axb,axc; axa=axb=axc=0; while(axa<=9999) { ax[axa]=1; axa++; } glShadeModel(GL_SMOOTH); // Enable Smooth Shading glClearColor(0.0f, 0.0f, 0.0f, 0.5f); // Black Background glClearDepth(1.0f); // Depth Buffer Setup glEnable(GL_DEPTH_TEST); // Enables Depth Testing glDepthFunc(GL_LEQUAL); // The Type Of Depth Testing To Do glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST); // Really Nice Perspective Calculations return TRUE; // Initialization Went OK } int DrawGLScene(GLvoid) // Here's Where We Do All The Drawing { int ba,bb,bc,bd; glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // Clear Screen And Depth Buffer glLoadIdentity(); // Reset The Current Modelview Matrix glTranslatef(0.0f,0.0f,-300.0f); // Move Left 1.5 Units And Into The Screen 6.0 bc=bb=ba=0; glColor3f(0.0,1.0,0.0); while(bc<=9999) { if(ax[bc]==1) { ba=bc%100; bb=(bc-ba)/100; glBegin(GL_QUADS); // Draw A Quad glVertex3f(-0.5f+ba, -0.5f+bb, -1.0f); // Top Left glVertex3f( -0.5f+ba, 0.5f+bb, -1.0f); // Top Right glVertex3f( 0.5f+ba,0.5f+bb, -1.0f); // Bottom Right glVertex3f(0.5f+ba,-0.5f+bb, -1.0f); // Bottom Left glEnd(); // Done Drawing The Quad } bc++; } return TRUE; // Keep Going } GLvoid KillGLWindow(GLvoid) // Properly Kill The Window { if (fullscreen) // Are We In Fullscreen Mode? { ChangeDisplaySettings(NULL,0); // If So Switch Back To The Desktop ShowCursor(TRUE); // Show Mouse Pointer } if (hRC) // Do We Have A Rendering Context? { if (!wglMakeCurrent(NULL,NULL)) // Are We Able To Release The DC And RC Contexts? { MessageBox(NULL,"Release Of DC And RC Failed.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION); } if (!wglDeleteContext(hRC)) // Are We Able To Delete The RC? { MessageBox(NULL,"Release Rendering Context Failed.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION); } hRC=NULL; // Set RC To NULL } if (hDC && !ReleaseDC(hWnd,hDC)) // Are We Able To Release The DC { MessageBox(NULL,"Release Device Context Failed.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION); hDC=NULL; // Set DC To NULL } if (hWnd && !DestroyWindow(hWnd)) // Are We Able To Destroy The Window? { MessageBox(NULL,"Could Not Release hWnd.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION); hWnd=NULL; // Set hWnd To NULL } if (!UnregisterClass("OpenGL",hInstance)) // Are We Able To Unregister Class { MessageBox(NULL,"Could Not Unregister Class.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION); hInstance=NULL; // Set hInstance To NULL } } /* This Code Creates Our OpenGL Window. Parameters Are: * * title - Title To Appear At The Top Of The Window * * width - Width Of The GL Window Or Fullscreen Mode * * height - Height Of The GL Window Or Fullscreen Mode * * bits - Number Of Bits To Use For Color (8/16/24/32) * * fullscreenflag - Use Fullscreen Mode (TRUE) Or Windowed Mode (FALSE) */ BOOL CreateGLWindow(char* title, int width, int height, int bits, bool fullscreenflag) { GLuint PixelFormat; // Holds The Results After Searching For A Match WNDCLASS wc; // Windows Class Structure DWORD dwExStyle; // Window Extended Style DWORD dwStyle; // Window Style RECT WindowRect; // Grabs Rectangle Upper Left / Lower Right Values WindowRect.left=(long)0; // Set Left Value To 0 WindowRect.right=(long)width; // Set Right Value To Requested Width WindowRect.top=(long)0; // Set Top Value To 0 WindowRect.bottom=(long)height; // Set Bottom Value To Requested Height fullscreen=fullscreenflag; // Set The Global Fullscreen Flag hInstance = GetModuleHandle(NULL); // Grab An Instance For Our Window wc.style = CS_HREDRAW | CS_VREDRAW | CS_OWNDC; // Redraw On Size, And Own DC For Window. wc.lpfnWndProc = (WNDPROC) WndProc; // WndProc Handles Messages wc.cbClsExtra = 0; // No Extra Window Data wc.cbWndExtra = 0; // No Extra Window Data wc.hInstance = hInstance; // Set The Instance wc.hIcon = LoadIcon(NULL, IDI_WINLOGO); // Load The Default Icon wc.hCursor = LoadCursor(NULL, IDC_ARROW); // Load The Arrow Pointer wc.hbrBackground = NULL; // No Background Required For GL wc.lpszMenuName = NULL; // We Don't Want A Menu wc.lpszClassName = "OpenGL"; // Set The Class Name if (!RegisterClass(&wc)) // Attempt To Register The Window Class { MessageBox(NULL,"Failed To Register The Window Class.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } if (fullscreen) // Attempt Fullscreen Mode? { DEVMODE dmScreenSettings; // Device Mode memset(&dmScreenSettings,0,sizeof(dmScreenSettings)); // Makes Sure Memory's Cleared dmScreenSettings.dmSize=sizeof(dmScreenSettings); // Size Of The Devmode Structure dmScreenSettings.dmPelsWidth = width; // Selected Screen Width dmScreenSettings.dmPelsHeight = height; // Selected Screen Height dmScreenSettings.dmBitsPerPel = bits; // Selected Bits Per Pixel dmScreenSettings.dmFields=DM_BITSPERPEL|DM_PELSWIDTH|DM_PELSHEIGHT; // Try To Set Selected Mode And Get Results. NOTE: CDS_FULLSCREEN Gets Rid Of Start Bar. if (ChangeDisplaySettings(&dmScreenSettings,CDS_FULLSCREEN)!=DISP_CHANGE_SUCCESSFUL) { // If The Mode Fails, Offer Two Options. Quit Or Use Windowed Mode. if (MessageBox(NULL,"The Requested Fullscreen Mode Is Not Supported By\nYour Video Card. Use Windowed Mode Instead?","NeHe GL",MB_YESNO|MB_ICONEXCLAMATION)==IDYES) { fullscreen=FALSE; // Windowed Mode Selected. Fullscreen = FALSE } else { // Pop Up A Message Box Letting User Know The Program Is Closing. MessageBox(NULL,"Program Will Now Close.","ERROR",MB_OK|MB_ICONSTOP); return FALSE; // Return FALSE } } } if (fullscreen) // Are We Still In Fullscreen Mode? { dwExStyle=WS_EX_APPWINDOW; // Window Extended Style dwStyle=WS_POPUP; // Windows Style ShowCursor(FALSE); // Hide Mouse Pointer } else { dwExStyle=WS_EX_APPWINDOW | WS_EX_WINDOWEDGE; // Window Extended Style dwStyle=WS_OVERLAPPEDWINDOW; // Windows Style } AdjustWindowRectEx(&WindowRect, dwStyle, FALSE, dwExStyle); // Adjust Window To True Requested Size // Create The Window if (!(hWnd=CreateWindowEx( dwExStyle, // Extended Style For The Window "OpenGL", // Class Name title, // Window Title dwStyle | // Defined Window Style WS_CLIPSIBLINGS | // Required Window Style WS_CLIPCHILDREN, // Required Window Style 0, 0, // Window Position WindowRect.right-WindowRect.left, // Calculate Window Width WindowRect.bottom-WindowRect.top, // Calculate Window Height NULL, // No Parent Window NULL, // No Menu hInstance, // Instance NULL))) // Dont Pass Anything To WM_CREATE { KillGLWindow(); // Reset The Display MessageBox(NULL,"Window Creation Error.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } static PIXELFORMATDESCRIPTOR pfd= // pfd Tells Windows How We Want Things To Be { sizeof(PIXELFORMATDESCRIPTOR), // Size Of This Pixel Format Descriptor 1, // Version Number PFD_DRAW_TO_WINDOW | // Format Must Support Window PFD_SUPPORT_OPENGL | // Format Must Support OpenGL PFD_DOUBLEBUFFER, // Must Support Double Buffering PFD_TYPE_RGBA, // Request An RGBA Format bits, // Select Our Color Depth 0, 0, 0, 0, 0, 0, // Color Bits Ignored 0, // No Alpha Buffer 0, // Shift Bit Ignored 0, // No Accumulation Buffer 0, 0, 0, 0, // Accumulation Bits Ignored 16, // 16Bit Z-Buffer (Depth Buffer) 0, // No Stencil Buffer 0, // No Auxiliary Buffer PFD_MAIN_PLANE, // Main Drawing Layer 0, // Reserved 0, 0, 0 // Layer Masks Ignored }; if (!(hDC=GetDC(hWnd))) // Did We Get A Device Context? { KillGLWindow(); // Reset The Display MessageBox(NULL,"Can't Create A GL Device Context.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } if (!(PixelFormat=ChoosePixelFormat(hDC,&pfd))) // Did Windows Find A Matching Pixel Format? { KillGLWindow(); // Reset The Display MessageBox(NULL,"Can't Find A Suitable PixelFormat.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } if(!SetPixelFormat(hDC,PixelFormat,&pfd)) // Are We Able To Set The Pixel Format? { KillGLWindow(); // Reset The Display MessageBox(NULL,"Can't Set The PixelFormat.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } if (!(hRC=wglCreateContext(hDC))) // Are We Able To Get A Rendering Context? { KillGLWindow(); // Reset The Display MessageBox(NULL,"Can't Create A GL Rendering Context.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } if(!wglMakeCurrent(hDC,hRC)) // Try To Activate The Rendering Context { KillGLWindow(); // Reset The Display MessageBox(NULL,"Can't Activate The GL Rendering Context.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } ShowWindow(hWnd,SW_SHOW); // Show The Window SetForegroundWindow(hWnd); // Slightly Higher Priority SetFocus(hWnd); // Sets Keyboard Focus To The Window ReSizeGLScene(width, height); // Set Up Our Perspective GL Screen if (!InitGL()) // Initialize Our Newly Created GL Window { KillGLWindow(); // Reset The Display MessageBox(NULL,"Initialization Failed.","ERROR",MB_OK|MB_ICONEXCLAMATION); return FALSE; // Return FALSE } return TRUE; // Success } LRESULT CALLBACK WndProc( HWND hWnd, // Handle For This Window UINT uMsg, // Message For This Window WPARAM wParam, // Additional Message Information LPARAM lParam) // Additional Message Information { FILE *F; switch (uMsg) // Check For Windows Messages { case WM_ACTIVATE: // Watch For Window Activate Message { if (!HIWORD(wParam)) // Check Minimization State { active=TRUE; // Program Is Active } else { active=FALSE; // Program Is No Longer Active } return 0; // Return To The Message Loop } case WM_SYSCOMMAND: { switch (wParam) { case SC_SCREENSAVE: case SC_MONITORPOWER: return 0; } break; } case WM_CLOSE: // Did We Receive A Close Message? { PostQuitMessage(0); // Send A Quit Message return 0; // Jump Back } case WM_CHAR: // Is A Key Being Held Down? { switch (wParam) { case 'w': { F=fopen("e:/a.gertt","rb"); fread(ax,sizeof(int),10000,F); fclose(F); break; } } return 0; } case WM_KEYUP: // Has A Key Been Released? { return 0; // Jump Back } case WM_SIZE: // Resize The OpenGL Window { ReSizeGLScene(LOWORD(lParam),HIWORD(lParam)); // LoWord=Width, HiWord=Height return 0; // Jump Back } } // Pass All Unhandled Messages To DefWindowProc return DefWindowProc(hWnd,uMsg,wParam,lParam); } int WINAPI WinMain( HINSTANCE hInstance, // Instance HINSTANCE hPrevInstance, // Previous Instance LPSTR lpCmdLine, // Command Line Parameters int nCmdShow) // Window Show State { MSG msg; // Windows Message Structure BOOL done=FALSE; // Bool Variable To Exit Loop // Ask The User Which Screen Mode They Prefer if (MessageBox(NULL,"Would You Like To Run In Fullscreen Mode?", "Start FullScreen?",MB_YESNO|MB_ICONQUESTION)==IDNO) { fullscreen=FALSE; // Windowed Mode } // Create Our OpenGL Window if (!CreateGLWindow("NeHe's First Polygon Tutorial",640,480,16,fullscreen)) { return 0; // Quit If Window Was Not Created } while(!done) // Loop That Runs While done=FALSE { if (PeekMessage(&msg,NULL,0,0,PM_REMOVE)) // Is There A Message Waiting? { if (msg.message==WM_QUIT) // Have We Received A Quit Message? { done=TRUE; // If So done=TRUE } else // If Not, Deal With Window Messages { TranslateMessage(&msg); // Translate The Message DispatchMessage(&msg); // Dispatch The Message } } else // If There Are No Messages { // Draw The Scene. Watch For ESC Key And Quit Messages From DrawGLScene() if ((active && !DrawGLScene()) || keys[VK_ESCAPE]) // Active? Was There A Quit Received? { done=TRUE; // ESC or DrawGLScene Signalled A Quit } else // Not Time To Quit, Update Screen { SwapBuffers(hDC); // Swap Buffers (Double Buffering) } if (keys[VK_F1]) // Is F1 Being Pressed? { keys[VK_F1]=FALSE; // If So Make Key FALSE KillGLWindow(); // Kill Our Current Window fullscreen=!fullscreen; // Toggle Fullscreen / Windowed Mode // Recreate Our OpenGL Window if (!CreateGLWindow("NeHe's First Polygon Tutorial",800,600,32,fullscreen)) { return 0; // Quit If Window Was Not Created } } } } // Shutdown KillGLWindow(); // Kill The Window return (msg.wParam); // Exit The Program }
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