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POInterProcess communication -- Locking Mutex in shared memory
 

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    <p>I have 2 processes that will execute the same code:</p> <pre><code>#include &lt;stdlib.h&gt; #include &lt;stdio.h&gt; #include &lt;unistd.h&gt; #include &lt;string.h&gt; #include &lt;semaphore.h&gt; #include &lt;sys/types.h&gt; #include &lt;sys/ipc.h&gt; #include &lt;sys/shm.h&gt; #include &lt;sys/wait.h&gt; #include &lt;pthread.h&gt; #ifndef _POSIX_THREAD_PROCESS_SHARED #error This system does not support process shared mutex #endif struct shm_content { pthread_mutex_t mutex; }; pthread_mutex_t *mptr; //Mutex Pointer pthread_mutexattr_t matr; //Mutex Attribute int shared_mem_id; //shared memory Id int *mp_shared_mem_ptr; //shared memory ptr -- pointing to mutex int main (void) { int rtn; size_t shm_size; /* initialize shared memory segment */ shm_size = 1*sizeof(pthread_mutex_t); if ((shared_mem_id = shmget(IPC_PRIVATE, shm_size, 0660)) &lt; 0) { perror("shmget"), exit(1) ; } if ((mp_shared_mem_ptr = (int *)shmat(shared_mem_id, (void *)0, 0)) == NULL) { perror("shmat"), exit(1); } //Offset to find the location of the mutex variable in the shared memory shm_content* pcontent = reinterpret_cast&lt;shm_content*&gt;(mp_shared_mem_ptr); mptr = &amp;(pcontent-&gt;mutex); // Setup Mutex if (rtn = pthread_mutexattr_init(&amp;matr)) { fprintf(stderr,"pthreas_mutexattr_init: %s",strerror(rtn)),exit(1); } if (rtn = pthread_mutexattr_setpshared(&amp;matr,PTHREAD_PROCESS_SHARED)) { fprintf(stderr,"pthread_mutexattr_setpshared %s",strerror(rtn)),exit(1); } if (rtn = pthread_mutex_init(mptr, &amp;matr)) { fprintf(stderr,"pthread_mutex_init %s",strerror(rtn)), exit(1); } // Lock mutex and then wait for signal to relase mutex printf("child mutex lock \n"); pthread_mutex_lock( mptr ); printf("child mutex locked\n"); int i = 0; // :) //busy wait while (i&lt;10) { printf("Busy Wait!!! I AM PROCESS 1\n"); //in the second process will change this line to : //printf("Busy Wait!!! I AM PROCESS 2\n"); sleep(2); } printf("child mutex unlock\n"); pthread_mutex_unlock( mptr ); printf("child mutex unlocked\n"); } </code></pre> <p>I am expecting that the second process will only be able to acquire the mutex once the first process releases it, but now it seams that there's 2 copies of the mutex and each process can lock its own.</p> <p>Any ideas?</p>
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    1. COthe code above wont run, i is undefined for example, I take it the aim is to have process 1 and 2 share information. Also, are you by any chance an ex C coder? the code above is very C-ish (for lack of a better term) I only ask as I often revert to type too, and code in C-style when meaning to code C++
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    2. COFixed the code :) the above code will run (g++ -o temp temp.cpp -pthread). Still I am not getting the behavior?
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    3. COWhere are the threads created?
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