What is the different between API functions AllocConsole and AttachConsole(-1)?

Question

Could you please explain me, what is the different between API functions AllocConsole and AttachConsole(-1) ? I mean if AttachConsole gets ATTACH_PARENT_PROCESS(DWORD)-1.

Answer 1

Well, the fundamental difference is:

• AllocConsole() will create a new console (and attach to it)
• AttachConsole( ATTACH_PARENT_PROCESS /* -1 */) will not create a new console, it will attach to the existing console of the parent process.

In the first case you get a whole new console window, in the second case, you use an existing console window.

Of course, if you're already attached to a console (ie., you're a console mode program launched from cmd.exe) there's not much difference - you'll get an error with either API.

Also note that just because you detach from a console doesn't mean the detached console will be useful - for example, if you're a console process launched from a cmd window, that window essentially blocks until your process ends.

Some code to play with:

int main( int argc, char* argv[])
{
    int ch;
    BOOL bResult;

    printf( "default console\n");
    ch = getchar();

    bResult = FreeConsole();
    bResult = AllocConsole();    
    printf( "AllocConsole()\n");
    ch = getchar();

    bResult = FreeConsole();
    bResult = AttachConsole( ATTACH_PARENT_PROCESS);    
    printf( "AttachConsole( ATTACH_PARENT_PROCESS)\n");
    ch = getchar();

    return 0;
}

Answer 2

I don't think there's a function called CreateConsole, but there's AllocConsole.

Assuming that's what you meant, I think the difference is that AttachConsole(ATTACH_PARENT_PROCESS) can return ERROR_INVALID_HANDLE if the parent process doesn't have a console.

Try running this code from both a command prompt and Start -> Run:

#include <windows.h>
#pragma comment ( lib, "user32.lib" )

int main()
{
    BOOL b;
    char msg[1024];

    b = FreeConsole();
    sprintf(msg, "%d", b);
    MessageBox(NULL, msg, "FreeConsole", 0);

    b = AttachConsole(ATTACH_PARENT_PROCESS);
    sprintf(msg, "%d", b);
    MessageBox(NULL, msg, "AttachConsole", 0);

    return 0;
}

When run from a command prompt, two message boxes containing a 1 are displayed, meaning both calls succeeded. When run from Start -> Run, the first box contains 1 and the second contains 0, meaning that only the first call succeeded. The second one fails because explorer.exe (which is the parent of a process launched from Start -> Run) doesn't have a console.