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Thread: [RESOLVED] Problem with text in GetWindowText function

  1. #1
    Join Date
    Apr 2020
    Posts
    3

    [RESOLVED] Problem with text in GetWindowText function

    Hi,

    I am starting a simple WinApi project in Dev C++. I know it's an alo compiler, but I really like it. So, I made everything, window is working just fine, exept for a text message on MessageBox. When I type something like "Hello" in a text field, program displays Message box with some strange message (every time I start program it is a complitely diferent message) like: ž{}s.

    Here is the code of a program:
    Code:
    #include <windows.h>
    
    /* This is where all the input to the window goes to */
    LRESULT CALLBACK WndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam) {
    
        HWND TextBox, Button;
    
        
    	switch(Message) {
    		
    		case WM_CREATE:
    			TextBox=CreateWindow("EDIT", "", WS_BORDER|WS_CHILD|WS_VISIBLE|WS_VSCROLL|ES_MULTILINE|ES_AUTOVSCROLL , 10, 10, 400, 400, hwnd, NULL, NULL, NULL);
    			Button=CreateWindow("Button", "Dede", WS_VISIBLE|WS_CHILD|WS_BORDER, 420,20,70, 20, hwnd, (HMENU) 1, NULL, NULL);
    			
    			break;
    			
    		case WM_COMMAND:
    				switch(LOWORD(wParam))
    			{
    				case 1:
                        char textSaved[20];
                     //   char *t= &textSaved[0];
    					int gwtstat=GetWindowTextLength(TextBox);
    					GetWindowText(TextBox, textSaved, gwtstat);
    
    					::MessageBox(hwnd, textSaved, "Title", MB_OK);
    				break;
    			}
    		
    			 break;
    		/* Upon destruction, tell the main thread to stop */
    		case WM_DESTROY: {
    			PostQuitMessage(0);
    			break;
    		}
    		
    		/* All other messages (a lot of them) are processed using default procedures */
    		default:
    			return DefWindowProc(hwnd, Message, wParam, lParam);
    	}
    	return 0;
    }
    
    /* The 'main' function of Win32 GUI programs: this is where execution starts */
    int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) {
    	WNDCLASSEX wc; /* A properties struct of our window */
    	HWND hwnd; /* A 'HANDLE', hence the H, or a pointer to our window */
    	MSG msg; /* A temporary location for all messages */
    
    	/* zero out the struct and set the stuff we want to modify */
    	memset(&wc,0,sizeof(wc));
    	wc.cbSize		 = sizeof(WNDCLASSEX);
    	wc.lpfnWndProc	 = WndProc; /* This is where we will send messages to */
    	wc.hInstance	 = hInstance;
    	wc.hCursor		 = LoadCursor(NULL, IDC_ARROW);
    	
    	/* White, COLOR_WINDOW is just a #define for a system color, try Ctrl+Clicking it */
    	wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
    	wc.lpszClassName = "WindowClass";
    	wc.hIcon		 = LoadIcon(NULL, IDI_APPLICATION); /* Load a standard icon */
    	wc.hIconSm		 = LoadIcon(NULL, IDI_APPLICATION); /* use the name "A" to use the project icon */
    
    	if(!RegisterClassEx(&wc)) {
    		MessageBox(NULL, "Window Registration Failed!","Error!",MB_ICONEXCLAMATION|MB_OK);
    		return 0;
    	}
    
    	hwnd = CreateWindowEx(WS_EX_CLIENTEDGE,"WindowClass","Caption",WS_VISIBLE|WS_OVERLAPPEDWINDOW,
    		CW_USEDEFAULT, /* x */
    		CW_USEDEFAULT, /* y */
    		640, /* width */
    		480, /* height */
    		NULL,NULL,hInstance,NULL);
    
    	if(hwnd == NULL) {
    		MessageBox(NULL, "Window Creation Failed!","Error!",MB_ICONEXCLAMATION|MB_OK);
    		return 0;
    	}
    
    	/*
    		This is the heart of our program where all input is processed and 
    		sent to WndProc. Note that GetMessage blocks code flow until it receives something, so
    		this loop will not produce unreasonably high CPU usage
    	*/
    	while(GetMessage(&msg, NULL, 0, 0) > 0) { /* If no error is received... */
    		TranslateMessage(&msg); /* Translate key codes to chars if present */
    		DispatchMessage(&msg); /* Send it to WndProc */
    	}
    	return msg.wParam;
    }

    So, if anybody could help...tnx!
    Last edited by VictorN; April 26th, 2020 at 04:01 PM. Reason: added CODE tags

  2. #2
    VictorN's Avatar
    VictorN is offline Super Moderator Power Poster
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    Location
    Hanover Germany
    Posts
    19,797

    Re: Problem with text in GetWindowText function

    first, please use CODE tags when posting code snippets (well, this time I did it for you!)
    Second, do you compile/build your project as UNICODE or MBCS (ANSI)?
    Victor Nijegorodov

  3. #3
    Join Date
    Apr 2020
    Posts
    3

    Re: Problem with text in GetWindowText function

    Well, I don't know. I just build it. Can I change it somehow?

  4. #4
    2kaud's Avatar
    2kaud is offline Super Moderator Power Poster
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    Location
    England
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    Re: Problem with text in GetWindowText function

    Code:
    HWND TextBox, Button;
    A problem is here. These are initialised in WM_CREATE, but when WndProc() exits, these values are 'lost'. So when Wndproc() is entered again, these variables have unknown values (NOT the previously assigned values) - but are unlikely to be valid window handles. GetWindowText() is likely to fail (returns 0). As you aren't testing that the functions used have completed properly, you aren't handling errors. When you use a function that can fail, you always need to check for such failure. The MSDN documentation describes this. Likewise, the textSaved array isn't initialised either, so if GetWindowText() fails, then MessageBox() will display whatever happens to be at the memory locations used by textSaved - probably the garbage you are seeing displayed.

    In this case, any 'easy' solution is make TextBox and Button static so that their values remain between successive calls to WndProc().

    Code:
    static HWND TextBox = 0, Button = 0;
    A better way of coding of case 1 could be:

    Code:
    TCHAR textSaved[20] = {0};
    int gwtstat = GetWindowTextLength(TextBox);
    
    if (gwstat > 0)
        if ((gwstat = GetWindowText(TextBox, textSaved, gwtstat)) > 0)
            ::MessageBox(hwnd, textSaved, "Title", MB_OK);
    
    if (gwstat == 0)
        ::MessageBox(hwnd, "Problem obtaining window text", "Title", MB_OK);
    
    break;
    Using TCHAR rather than char should enable this to work with either UNICODE or MBCS coding.
    Last edited by 2kaud; April 27th, 2020 at 04:10 AM.
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++17 Compiler: Microsoft VS2019 (16.7.4)

  5. #5
    Join Date
    Apr 2020
    Posts
    3

    Re: Problem with text in GetWindowText function

    WOOOOOW MAN, Thanks a lot!!! IT WORKS!!!

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