CODEWITHSUNDEEP
c++linuxpthreads
Jonas
Jonas

Reputation: 1373

pthread_mutex based inter process communication not working

I am currently trying to sinchronize two processess in Linux using the pthread_mutex model.

Here's the code I am working on:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

using namespace std;

int main (int argc, char **argv)
{
    printf ("starting process\n");

    if (_POSIX_THREAD_PROCESS_SHARED == -1) {
        printf ("shared mutex is not supported!\r\n");
    }

    pthread_mutexattr_t attr;
    pthread_mutex_t shm_mutex;


    if (pthread_mutexattr_init(&attr) != 0)
        printf ("init attr error!\r\n");
    if (pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_NORMAL) != 0)
        printf ("set type error!\r\n");
    if (pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED) != 0)
        printf ("set shared error!\r\n");

    int value;
    if (pthread_mutexattr_getpshared(&attr, &value) != 0 || value != PTHREAD_PROCESS_SHARED) {
        printf ("mutex is not shared!\r\n");
    }

    if (pthread_mutex_init(&shm_mutex, &attr) != 0)
        printf ("mutex init error!\r\n");

    for (int i=0; i < 10; i++) {

        if (pthread_mutex_lock(&shm_mutex) != 0)
            printf ("lock error!\r\n");

        printf ("begin run %d\r\n", i);
        sleep(10);
        printf ("end run %d\r\n", i);

        if (pthread_mutex_unlock(&shm_mutex) != 0)
            printf ("unlock error!\r\n");

        sleep(1); // sleep 1 second
    }

    pthread_mutex_destroy(&shm_mutex);
    pthread_mutexattr_destroy(&attr);

    return 0;
}

When I run two separate processes, the begin/end logic doesm't work.

Is there anything wrong with the code?

Upvotes: 1

Views: 182

Answers (1)

Olivier
Olivier

Reputation: 1199

You're supposed to allocate shm_mutex in shared memory yourself. See man shm_overview. The flag merely means you're allowed to do this with the mutex.

See this reference for more detailed information.

Upvotes: 3

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