Thread: clueless about the semaphore object

  1. #1
    Registered User
    Join Date
    Sep 2006

    clueless about the semaphore object

    hi i am trying to develop a program
    that can coordinate the access of a resource...

    my resource is an array of strings.. my producers will
    put strings into the array, my consumers will retrieve
    one string at a time then delete that one string from the array
    once they got it.

    i would like the consumers and producers to take turns
    in accessing the string array.

    eg.. only one consumer can be
    accessing the string array or only one producer can be
    adding into the string array at any given time.

    anyways, right now i am thinking of this strategy ...(after reading
    some pseudocodes and producer/consumer semaphore
    solutions online)

    	HANDLE hsem1; // gate to restrict access to string array.
    	HANDLE hsem2; // signal when string array full
    	HANDLE hsem3; // signal when string array empty
    	DWORD  dwProdWait;
    	DWORD  dwConsWait;
    	TCHAR sem1Name[] = TEXT("String array resource");
    	// create the gate for restricting access to the string array	
    	// initial count 0 to allow time for setting up string array
    	hsem1 = CreateSemaphore(NULL, 0, 1, sem1Name);
    	// increment semaphore so that string array can now be accessed
    	ReleaseSemaphore(hSemaphore, 1, NULL);
    	// producer try to enter the gate
    	dwProdWait = WaitForSingleObject(hsem1, 0L);
    	switch (dwProdWait)
    		case WAIT_OBJECT_0: // Semaphore object signaled
    		case WAIT_TIMEOUT: // Semaphore object non-signaled.
        i am dumbfounded right now.. how i can make 
    	the producer wait on the boxempty to be nonsignaled
        and how i can make the consumer wait
    	for boxfull to be nonsignaled..
        is this even a valid strategy right now ? please help thanks.

  2. #2
    Yes, my avatar is stolen anonytmouse's Avatar
    Join Date
    Dec 2002
    If you only want one thread at a time to access a resource, you can use a critical section (simplest), event, mutex or a semaphore. To wait on a semaphore just use a non-zero timeout. You can't wait for an object to be non-signalled, only signalled. A semphore is signalled when its count is not zero. Calling WaitForSingleObject with a semaphore decreases its count by one.
    WaitForSingleObject(hsem1, INFINITE);
      // Access resource here...
    ReleaseSemaphore(hsem1, 1, NULL);

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