Switch statement usage - C

Question

Hello,

I have a thread function on Process B that contains a switch to perform certain operations based on the results of an event sent from Process A, these are stored as two elements in an array.

I set the first element to the event which signals when Process A has data to send and I have the second element set to the event which indicates when Process A has closed.

I have began to implement the functionality for the switch statement but I'm not getting the results as I expect.

Consider the following:

//Thread function
        DWORD WINAPI ThreadFunc(LPVOID passedHandle)
        {    
            for(i = 0; i < 2; i++)
            {
                ghEvents[i] = OpenEvent(EVENT_ALL_ACCESS, FALSE, TEXT("Global\\ProducerEvents"));
                if(ghEvents[i] == NULL)
                {
                    getlasterror = GetLastError();
                }
            }
            dwProducerEventResult = WaitForMultipleObjects(
                2,
                ghEvents,
                FALSE,
                INFINITE);

        switch (dwProducerEventResult) 
        {
        case WAIT_OBJECT_0 + 0: 
            {
                //Producer sent data
                //unpackedHandle = *((HWND*)passedHandle);

                MessageBox(NULL,L"Test",L"Test",MB_OK);
                break;
            }
        case WAIT_OBJECT_0 + 1:
            {
                //Producer closed
                ExitProcess(1);
                break;
            }
        default: 
            return;
        }

}

As you can see if the event in the first array is signalled Process B should display a simple message box, if the second array is signalled the application should close.

When I actually close Process A, Process B displays the message box instead.

If I leave the first case blank (Do nothing) both applications close as they should.

Furthermore Process B sends data an error is thrown (When I comment out the unpacking):

Have I implemented my switch statement incorrectly? I though I handled the unpacking of the HWND correctly too, any suggestions?

Thanks for your time.

Edit:

The example I'm following: Here

Event creation in Process A:

for (i = 0; i < 2; i++) 
            { 
                ghEvents[i] = CreateEvent( 
                    NULL,   // default security attributes
                    TRUE,  // auto-reset event object
                    FALSE,  // initial state is nonsignaled
                    TEXT("Global\\ProducerEvents"));  // unnamed object

                if (ghEvents[i] == NULL) 
                { 
                    printf("CreateEvent error: %d\n", GetLastError() ); 
                    ExitProcess(0); 
                } 
        }

Answer 1

Why are you opening the same event twice?

ghEvents[i] = OpenEvent(EVENT_ALL_ACCESS, FALSE, TEXT("Global\\ProducerEvents"));

Shouldn't there be two different events?

This is consistent with the behavior you are reporting

When I actually close Process A, Process B displays the message box instead.

Because when you close process A the same event is signalled that enters the switch as WAIT_OBJECT_0 + 0

To sum up, create two different events - one for when process A is closing and the other to signal the message box to be shown.

The code below will only work for i <= 0 <= 9 But the main point is that the cycle iteration must create an event with a different name.

Process A:

LPTSTR eventName[] = _T("Global\\ProducerEventi");
for (i = 0; i < 2; i++) 
{ 
    eventName[20] = '0' - i; // this will change last i character with '0' or '1'
    ghEvents[i] = CreateEvent(NULL, TRUE, FALSE, eventName);
 ...

Process B:

LPTSTR eventName[] = _T("Global\\ProducerEventi");
for(i = 0; i < 2; i++)
{
    eventName[20] = '0' - i; // this will change last i character with '0' or '1'
    ghEvents[i] = OpenEvent(EVENT_ALL_ACCESS, FALSE, eventName);
    ...

Answer 2

it is not about case/switch. you are trying to dereference invalid pointer.

Answer 3

It looks like you're trying to dereference a HWND, hence the crash.

Answer 4

Assuming you're calling ThreadFunc like this:

ThreadFunc(hwndSomeWindow);

In which case, the cast should be:

unpackedHandle = (HWND) passedHandle;

Your code would have been right if the call were like this:

ThreadFunc(&hwndSomeWindow);