mutex race condition

This is a discussion on mutex race condition within the C Programming forums, part of the General Programming Boards category; I am having difficulty finding the race condition in this code. The tutorial i am doing indicates a semaphore implementation ...

  1. #1
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    mutex race condition

    I am having difficulty finding the race condition in this code. The tutorial i am doing indicates a semaphore implementation is needed to solve the race condition that cannot be solved by mutexes alone.


    "if the parent thread never gives up the CPU long enough for the child thread to run (eg. because input from stdin is instant)"


    Code:
    void *print(void* data)
    {
    
            myStruct *d = (myStruct *)data;
            pthread_mutex_lock(&(d->mutex_b ));
            pthread_mutex_lock(&(d->mutex_a ));
            pthread_mutex_destroy(&d->mutex_a);
    
            printf("buffer = %s\n", d->buffer);
    
            pthread_mutex_unlock(&(d->mutex_b ));
            pthread_mutex_destroy(&d->mutex_b);
    
            pthread_mutex_lock(&(d->mutex_c ));
            printf("exiting from child...\n");
            exit(EXIT_SUCCESS);
    }
    
    int main(void)
    {
            myStruct data;
            pthread_t thread;
            char c = ' ';
    
            pthread_mutex_init(&data.mutex_a, NULL);
            pthread_mutex_init(&data.mutex_b, NULL);
            pthread_mutex_init(&data.mutex_c, NULL);
    
            pthread_mutex_lock(&data.mutex_a);
            pthread_mutex_lock(&data.mutex_c);
    
            pthread_create(&thread, NULL, print, (void*)(&data));
    
            fgets(data.buffer, sizeof(data.buffer), stdin);
    
            pthread_mutex_unlock(&data.mutex_a);
            pthread_mutex_lock(&data.mutex_b);
    
            printf("press enter...\n");
    
            c = getch();
    
            if(c == 10)
            {
                    printf("you pressed enter\n");
                    pthread_mutex_unlock(&data.mutex_c);
                    pthread_mutex_destroy(&data.mutex_c);
    
            }
            else
            {
                    printf("next time press enter when prompted!\nnow exiting\n");
                    exit(EXIT_FAILURE);
            }
    
            pthread_join(thread, NULL);
    
    }
    
    ...
    Last edited by erasm; 09-20-2009 at 01:05 AM.

  2. #2
    ATH0 quzah's Avatar
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    With just a skim, you're double-locking _c, did you mean to?


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
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    Quote Originally Posted by quzah View Post
    With just a skim, you're double-locking _c, did you mean to?


    Quzah.
    Sorry i omitted some code.

  4. #4
    Malum in se abachler's Avatar
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    The only difference between a semaphore and a mutex -

    Quote Originally Posted by wikipedia
    In computer science, a semaphore is a protected variable or abstract data type which constitutes the classic method for restricting access to shared resources such as shared memory in a parallel programming environment.

    Mutual exclusion (often abbreviated to mutex) algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections. A critical section is a piece of code where a process or thread accesses a common resource.
    is that semaphores (at least in windows) can have more than one owner (as set during semaphore creation), while a mutex, is by definition only allowed one owner. An example of where a semaphore might be useful where a mutex would not is network communications, where you need to avoid deadlocking over the network resource, but also need to restrict the number of simultaneous sends to ensure quality of service.
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

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