How do you compare the content of an edit control to the text in a file?

Question

I am writing a simple text editor using the Win32 API and I am trying to write a function to compare the content of the file to the content of an edit control. I currently have this:

BOOL checkForModification (PCWSTR pszFileName, HWND hEdit) {
    BOOL bSuccess = FALSE;
    DWORD dwTextLength = GetWindowTextLengthA(hEdit);
    hFile = CreateFile(pszFileName, GENERIC_READ, 
        FILE_SHARE_READ, NULL,
        OPEN_EXISTING, 0, NULL);
    if (hFile != INVALID_HANDLE_VALUE) {
            DWORD dwFileSize;

            dwFileSize = GetFileSize(hFile, NULL);
            if (dwFileSize != 0xFFFFFFFF)
            {
                PSTR pszFileText;

                pszFileText = (PSTR)GlobalAlloc(GPTR, dwFileSize + 1);
                if (pszFileText != NULL) {
                    DWORD dwRead;

                    if (ReadFile(hFile, pszFileText, dwFileSize + 1, &dwRead, NULL))
                    {
                        bSuccess = TRUE;
                        pszFileText[dwFileSize] = 0; 

                        LPSTR pszEditText = (LPSTR)GlobalAlloc(GPTR, dwTextLength + 1);
                        GetWindowTextA(hEdit, pszEditText, dwTextLength);

                        int res = CompareStringA(LOCALE_SYSTEM_DEFAULT, NULL, pszFileText, -1, pszEditText, -1);
                        
                        if (res != CSTR_EQUAL) {
                            MessageBox(NULL, L"You changed the text!", L"Alert", MB_OK | MB_ICONINFORMATION);

                        }

                        GlobalFree(pszEditText);
                    }
                    else {
                        MessageBox(NULL, L"Oh no! Something went wrong!\nError code: 2", L"Error", MB_OK | MB_ICONERROR);
                    }
                    GlobalFree(pszFileText);
                }
            }
            CloseHandle(hFile);
        }
        else {
            MessageBox(NULL, L"Oh no! Something went wrong!\nError code: 1", L"Error", MB_OK | MB_ICONERROR);
        }
    
    return bSuccess;
}

The problem that I am having is that the result of CompareStringA is always returning CSTR_LESS_THAN, even when I don't change the text in the edit control. The encoding of the file is UTF-8. Why is this happening?

Answer 1

Seriously, use a debugger and test it with a file that contains simple text like ABCDE. You should be able to figure out the problem in less than 30 seconds just by inspecting a few variables!

You can trivially determine that the problem is not reading the documentation of function GetWindowTextA (https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getwindowtexta).

The size you pass include the null terminating charaters. Assuming that the edit also contains ABCDE, then the length is 5.

Calling GetWindowTextA(hEdit, pszEditText, dwTextLength); where dwTextLength will returns a buffer that contains ABCD plus the null character.

Obviously ABCD is before ABCDE using usual sorting rules.