CreateMutex never returns ERROR_ALREADY_EXISTS

Question

I am using CreateMutex to stop multiple application to run certain functions at the same time. These functions are in dll so can be called over by same application or separate applictions. This dll talks to hardware so I want to return 'busy' if another function is already running rather than have to wait on it. I thought best approach is to use CreateMutex instead combination of OpenMutex.

int FunctionExposedByDll()
{

    hMutexAPI = CreateMutex(0, 0, API_RUNNING_MUTEXT );

    if (!hMutexAPI )
    {
        DWORD dwErr = GetLastError();

        if (dwErr == ERROR_ALREADY_EXISTS )
            return MY_ERROR_BUSY;
    }

    // actual function here

    CloseHandle( hMutexAPI );

}

So if the mutex is already created, I should get ERROR_ALREADY_EXISTS which tells me system is executing an api and I should return. However the above code always return a valid mutex even if the prior function has not returned and mutex handle is not closed.

I also tried CreateMutexEx function and in that case when I try it the 2nd time it returns ERROR_ACCESS_DENIED when I am expecting ERROR_ALREADY_EXISTS. So my question is what do I need to do to get correct status that mutex already exist when it exist?

I am using windows 7

**Update**

Based on Rob K answer, I have changed the code to following:

int FunctionExposedByDll()
{

    hMutexAPI = CreateMutex(0, 0, API_RUNNING_MUTEXT );


    DWORD dwErr = GetLastError();

    if (dwErr == ERROR_ALREADY_EXISTS )
    {
        CloseHandle( hMutexAPI); // i have to call this but it contributes to first chance exception too!
        return MY_ERROR_BUSY;
    }

    // actual function here

    CloseHandle( hMutexAPI );

}

Now I am getting/reading the correct status of semaphore but releasing is an issue. If I don't release it when its in 'busy state', than I always get the ERROR_ALREADY_EXISTS even when the other API has finished. So CloseHandle() fixes that but creates another issue. When I return and close CloseHandle()from busy state and the first API completes later and want to close the handle, I get first chance exception. I don't see how can I avoid that!?

Answer 1

That's not the correct way to use CreateMutex(). CreateMutex() (almost) always succeeds in returning a valid handle to a mutex. From the MSDN doc which you linked: "If the mutex is a named mutex and the object existed before this function call, the return value is a handle to the existing object, GetLastError returns ERROR_ALREADY_EXISTS..."

Quoting again: "Two or more processes can call CreateMutex to create the same named mutex. The first process actually creates the mutex, and subsequent processes with sufficient access rights simply open a handle to the existing mutex. This enables multiple processes to get handles of the same mutex, while relieving the user of the responsibility of ensuring that the creating process is started first."

What you want to do is use CreateMutex() to open a handle to the mutex, and then use WaitForSingleObject() with a timeout value of 0 to try to take it. If the WaitForSingleObject() fails to take the mutex, then you return MY_ERROR_BUSY. If it does succeed to take the mutex, call ReleaseMutex() when you are done using it to unlock it.

ETA:

If WaitForSingleObject returns WAIT_OBJECT_0 or WAIT_ABANDONED, then you own the mutex (e.g. it becomes signalled) and you must call ReleaseMutex to give-up ownership (e.g. unsignal it) before calling CloseHandle.

If it returns WAIT_TIMEOUT, you do not own the mutex, and you can just call CloseHandle.

int FunctionExposedByDll()
{

    HANDLE hMutexAPI = CreateMutex(0, 0, API_RUNNING_MUTEXT );

    int rval = MY_ERROR_BUSY;    
    if ( hMutexAPI )
    {
        DWORD wait_success = WaitForSingleObject( hMutexAPI, 0 );
        if ( wait_success == WAIT_OBJECT_0 || wait_success == WAIT_ABANDONED )
        {


            // actual function here

            ReleaseMutex( hMutexAPI );
            rval = SUCCESS_VALUE;
        } 
        CloseHandle( hMutexAPI );
    }
    return rval;
}

You should take some time to get more familiar with the basic interprocess communication primitives, starting here: http://en.wikipedia.org/wiki/Mutex

ETA once more I figured C++ 11 may have added the IPC primitives to the standard library, and it appears they have:

http://en.cppreference.com/w/cpp/thread/mutex, 
http://en.cppreference.com/w/cpp/thread/lock_guard

If you're using a C++11 capable compiler (e.g. Visual Studio 2013), please use these instead of the system primitives.

And one more edit... As pointed out in a comment, the C++ standard library primitives are not usable for interprocess communication, only for synchronizing threads in the same process. (This is a great disappointment.)

Instead, use Boost.Interprocess if you can.

Answer 2

CreateMutex returns a valid handle anyway. That is, a test for !hMutexAPI fails and you never enter the clause.