We help IT Professionals succeed at work.

Why can't I write to MFC GUI console?

thj asked
I've developed a C++ Delaunay/Voronoi triangulation library which runs fine on several flavors of Unix as well as in NT
and Win95 console applications. The debug version of the library writes error and diagnastic information to STDOUT and STDERR as formatted text through cout and cerr.
Now I have an MFC GUI application to visualize the triangulations. I want to be able to allocate a console and have the diagnostic output from the triangulation library go to the console window without having to modify the library (so it still runs on Unix).
The application is a standard MFC app created with AppWizard. I can create the console window using AllocConsole()  ( in InitInstance() in the App ), but nothing gets written to it.
I tried to redirect STD_OUTPUT_HANDLE and STD_ERROR_HANDLE using SetStdHandle() but can't seem to get a handle to the newly created console's standard output and standard error. The docs for CreateFile() say to use CON and CONOUT&. What the hell are they? I grepped and searched for these but turned up nothing. What's going on here?
I noticed that before I created the console, GetStdHandle() returned 0 (no console) and after the call to AllocConsole() STD_OUTPUT_HANDLE was 7 (0x00000007). Is this a valid handle? (Doesn't work if it is!)

The docs for AllocConsole() say that the call sets up
(redirects) the std io handles. Why doesn't it work?

I include iostream.h header in the library so that it works with Unix and in console applications. When I build the MFC application I have to use the /NODEFAULTLIB switch to get it to link properly. I also noticed that when I run it from the build menu in devstudio, there are 2 io libraries loaded and one is relocated in memory (forget the exact message). Could this be the problem?

What am I doing wrong? Am I going about this all wrong? Should I be using CreateProsess() instead?
Any help would be appreciated.
Watch Question



Adjusted points to 100

Microsoft's C stdio library initialize routine needs stdin, stdout, stderr HANDLE to be created at first to be used.

In this case, Console handle is created later than stdio init routine. There is no way to output to stdout or stderr.

The following will guide you to use low level and high level stdio

1. call your CWinApp subclass as CTheApp.
2. include stdio.h at the top of CTheApp constructor.
3. define following above CTheApp's constructor.
    extern      "C" {
    void __cdecl _ioinit (void) ;
    } ;
4. add following to your CTheApp's constructor.
    HANDLE hHandle;
    BOOL bRet;
    // Create Console if not have
    hHandle = GetStdHandle(STD_INPUT_HANDLE);
    hHandle = GetStdHandle(STD_OUTPUT_HANDLE);
    hHandle = GetStdHandle(STD_ERROR_HANDLE);
    if ( hHandle == 0 ) {
        bRet = AllocConsole();

        hHandle = GetStdHandle(STD_INPUT_HANDLE);
        hHandle = GetStdHandle(STD_OUTPUT_HANDLE);
        hHandle = GetStdHandle(STD_ERROR_HANDLE);

        // reinitialize io subsystem
        _ioinit ();

        // reinitialize FILE stdio
        _iob[0]._file = 0;
        _iob[1]._file = 1;
        _iob[2]._file = 2;

    // TODO : add other initialize code

5. Change project's setting to use MFC static library.
    This is needed to use _ioinit. Indeed use static C library.

I tested using VC4.2 and windows NT 4.0

-Call AllocConsole() in InitInstance() as you're doing now.-Call a new function in your dll called InitIOStream() (see below for implementation)-cout and cerr should now work in the dll************test.dll************#include <iostream.h>
#include <io.h>
#include <FCNTL.H>
#include <fstream.h>
#include <windows.h>

void __declspec(dllexport) InitIOStream()
    int fd;    //get file descriptor of console's stderr    fd = _open_osfhandle((long) GetStdHandle        STD_ERROR_HANDLE),_O_TEXT);    //create unbuffered file stream based on stderr
    static ofstream mycerr(fd, NULL, 0);
    //get file descriptor of console's stdout    fd = _open_osfhandle((long) GetStdHandle        (STD_OUTPUT_HANDLE),_O_TEXT);    //create unbuffered file stream based on stdout    static ofstream mycout(fd, NULL, 0);
   //re-assign cerr and cout to our new streams    cerr = mycerr;
    cout = mycout;
    cerr << "cerr initialized" << endl;
    cout << "cout initialized" << endl;

void __declspec(dllexport) TestCerr()
    cerr << "hello from cerr in a dll" << endl;

void __declspec(dllexport) TestCout()
    cout << "hello from cout in a dll" << endl;

thj -

I'm currently looking for a Win2K triangulation DLL.  Could you send me details of your one?