Win32 - Appending text to an Edit Control

Trying to append text to an edit control inside a dialog box. I can't get _tcscat_s to append correctly. It crashes and says something about the buffer being too small or something about a null terminated string.

int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow )
{
    return DialogBox( hInstance, MAKEINTRESOURCE( IDD_MAIN ), NULL, DlgProc );
}

BOOL CALLBACK DlgProc( HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam ) 
{
    switch( Message )
    {
        case WM_INITDIALOG:
            OpenAndReadFile( hwnd );
            return TRUE;
        case WM_COMMAND:
            switch( LOWORD( wParam ) )
            {
                case IDSTART:
                    EndDialog( hwnd, IDSTART );
                    break;
                case IDQUIT:
                    EndDialog( hwnd, IDQUIT );
                    break;
            }
            break;
        case WM_CLOSE:
            EndDialog( hwnd, 0 );
            break;
        default:
            return FALSE;
    }
    return TRUE;
}

BOOL OpenAndReadFile( const HWND &hwnd ) 
{   
    // Open the file

    HANDLE hFile;
    hFile = CreateFile( TEXT( "sites.txt" ),    // file to open
                        GENERIC_READ,           // open for reading
                        FILE_SHARE_READ,        // share for reading
                        NULL,                   // default security
                        OPEN_EXISTING,          // existing file only
                        FILE_ATTRIBUTE_NORMAL,  // normal file
                        NULL );                 // no attr. template

    if ( hFile == INVALID_HANDLE_VALUE )
    {
        SetDlgItemText( hwnd, IDC_OUTPUT, TEXT( "Error: File could not be opened\r\n" ) );
        return FALSE;
    }
    else
        SetDlgItemText( hwnd, IDC_OUTPUT, TEXT( "sites.txt opened\r\n" ) );

    AppendText( hwnd, TEXT("TEXT") );

    // Read data from file

    const DWORD BUFFERSIZE = GetFileSize( hFile, NULL );
    char *ReadBuffer = new char [BUFFERSIZE]();
    DWORD dwBytesRead = 0;

    // read one character less than the buffer size to save room for the
    // terminate NULL character.
    //if ( FALSE == ReadFile( hFile, ReadBuffer, BUFFERSIZE - 1, &dwBytesRead, NULL ) )
    {

    }

    return TRUE;
}

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get size to determine buffer size
    int outLength = GetWindowTextLength( GetDlgItem( hwnd, IDC_OUTPUT ) );

    // create buffer to hold current text in edit control
    TCHAR * buf = ( TCHAR * ) GlobalAlloc( GPTR, outLength + 1 );

    // get existing text from edit control and put into buffer
    GetDlgItemText( hwnd, IDC_OUTPUT, buf, outLength + 1 );

    // append the newText to the buffer
    _tcscat_s( buf, outLength + 1, newText );

    // Set the text in the dialog
    SetDlgItemText( hwnd, IDC_OUTPUT, buf );
}

GetWindowTextLength() returns the number of TCHAR elements in the text, but GlobalAlloc() expects a byte count instead. If you are compiling for Unicode, TCHAR is 2 bytes, not 1 byte, but you are not taking that into account. You are also not allocating the buffer large enough to hold both the existing text and the new text being appended. You are also leaking the memory that you allocate.

Try this:

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get edit control from dialog
    HWND hwndOutput = GetDlgItem( hwnd, IDC_OUTPUT );

    // get new length to determine buffer size
    int outLength = GetWindowTextLength( hwndOutput ) + lstrlen(newText) + 1;

    // create buffer to hold current and new text
    TCHAR * buf = ( TCHAR * ) GlobalAlloc( GPTR, outLength * sizeof(TCHAR) );
    if (!buf) return;

    // get existing text from edit control and put into buffer
    GetWindowText( hwndOutput, buf, outLength );

    // append the newText to the buffer
    _tcscat_s( buf, outLength, newText );

    // Set the text in the edit control
    SetWindowText( hwndOutput, buf );

    // free the buffer
    GlobalFree( buf );
}

Alternatively:

#include <vector>

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get edit control from dialog
    HWND hwndOutput = GetDlgItem( hwnd, IDC_OUTPUT );

    // get new length to determine buffer size
    int outLength = GetWindowTextLength( hwndOutput ) + lstrlen(newText) + 1;

    // create buffer to hold current and new text
    std::vector<TCHAR> buf( outLength );
    TCHAR *pbuf = &buf[0];

    // get existing text from edit control and put into buffer
    GetWindowText( hwndOutput, pbuf, outLength );

    // append the newText to the buffer
    _tcscat_s( pbuf, outLength, newText );

    // Set the text in the edit control
    SetWindowText( hwndOutput, pbuf );
}

With that said, getting the window's current text into memory, appending to it, and then replacing the window's text is a very inefficient way to append text to an edit control. Use the EM_REPLACESEL message instead:

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get edit control from dialog
    HWND hwndOutput = GetDlgItem( hwnd, IDC_OUTPUT );

    // get the current selection
    DWORD StartPos, EndPos;
    SendMessage( hwndOutput, EM_GETSEL, reinterpret_cast<WPARAM>(&StartPos), reinterpret_cast<WPARAM>(&EndPos) );

    // move the caret to the end of the text
    int outLength = GetWindowTextLength( hwndOutput );
    SendMessage( hwndOutput, EM_SETSEL, outLength, outLength );

    // insert the text at the new caret position
    SendMessage( hwndOutput, EM_REPLACESEL, TRUE, reinterpret_cast<LPARAM>(newText) );

    // restore the previous selection
    SendMessage( hwndOutput, EM_SETSEL, StartPos, EndPos );
}