Unable to write to AllocConsole()

Question

I have a cpp dll in which I want to print text to console after injection. I used AllocConsole() to create console since its the easiest way, but I can't write in console.

When I try printf() nothing happens. Also I tried std::cout<<""; but no result as well.

BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
        AllocConsole();
        printf("Injected");
        CreateThread(NULL, NULL, (LPTHREAD_START_ROUTINE)main, NULL, NULL, NULL);
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}

Answer 1

After allocating a new console via AllocConsole() , you need to re-open the standard streams ( stdout , stderr , stdin ) before you can use them.

You can do so by using freopen (in newer versions of Visual Studio you need to use freopen_s ) Example:

FILE *fDummy;
freopen_s(&fDummy, "CONIN$", "r", stdin);
freopen_s(&fDummy, "CONOUT$", "w", stderr);
freopen_s(&fDummy, "CONOUT$", "w", stdout);

If you want to use the deprecated freopen you can disable the warning by #define ing _CRT_SECURE_NO_WARNINGS .

If you also want to use the wide-character streams ( std::wcout , std::wcerr , etc...), you need to call SetStdHandle() to set a new output handle for your process. You can get the required file handle for this by calling CreateFile() with CONOUT$ / CONIN$ as file name:

HANDLE hConOut = CreateFile(_T("CONOUT$"), GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
SetStdHandle(STD_OUTPUT_HANDLE, hConOut);

Additionally, if you tried to use one of the streams before re-opening them, they will have the std::ios_base::badbit and std::ios_base::failbit set in their iostate , so subsequent writes / reads will be ignored.
You can reset the stream state with .clear() , after which you can read/write from/to the stream again:

std::cout.clear();
std::cin.clear();

Heres a full example of re-opening all the streams after AllocConsole() :

void CreateConsole()
{
    if (!AllocConsole()) {
        // Add some error handling here.
        // You can call GetLastError() to get more info about the error.
        return;
    }

    // std::cout, std::clog, std::cerr, std::cin
    FILE* fDummy;
    freopen_s(&fDummy, "CONOUT$", "w", stdout);
    freopen_s(&fDummy, "CONOUT$", "w", stderr);
    freopen_s(&fDummy, "CONIN$", "r", stdin);
    std::cout.clear();
    std::clog.clear();
    std::cerr.clear();
    std::cin.clear();

    // std::wcout, std::wclog, std::wcerr, std::wcin
    HANDLE hConOut = CreateFile(_T("CONOUT$"), GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
    HANDLE hConIn = CreateFile(_T("CONIN$"), GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
    SetStdHandle(STD_OUTPUT_HANDLE, hConOut);
    SetStdHandle(STD_ERROR_HANDLE, hConOut);
    SetStdHandle(STD_INPUT_HANDLE, hConIn);
    std::wcout.clear();
    std::wclog.clear();
    std::wcerr.clear();
    std::wcin.clear();
}