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Do pthread mutexes work across threads if in shared memory?


By : 對沒錯
Date : October 15 2020, 01:20 AM
may help you . You need to tell the mutex to be process-shared when it's inited:
http://www.opengroup.org/onlinepubs/007908775/xsh/pthread_mutexattr_setpshared.html
code :


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Can shared memory be read and validated without mutexes?


By : user1884810
Date : March 29 2020, 07:55 AM
hope this fix your issue Joe Duffy gives the exact same algorithm and calls it: "A scalable reader/writer scheme with optimistic retry".
It works.

Relationship between shared memory concurrency algorithms and mutexes/semaphores


By : Avinash
Date : March 29 2020, 07:55 AM
hope this fix your issue I can't think of any circumstances where what you actually want is a busy wait. Busy waiting just consumes processor time without achieving anything. That's not to say that "busy wait" algorithms aren't useful (they are), but the "busy wait" part is not the desired property, it is just a necessary consequence of a property that is desired.
Peterson's lock algorithm and Lamport's bakery algorithm are fundamentally just implementations of the mutex concept. OS facilities provide implementations of the same concept, but with different trade-offs.

clarifications on full memory barriers involved by pthread mutexes


By : wapapet
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further The POSIX specification lists the functions that must "synchronize memory with respect to other threads", which includes functions like pthread_mutex_lock() and pthread_mutex_unlock().
In Appendix A.4.11 it is spelt out that functions that "synchronize memory":

How to use mutexes in a shared memory forked process?


By : Sanket Parghavi
Date : March 29 2020, 07:55 AM
this one helps. Since you don't have any mutexes, there's nothing to stop two processes from trying to increment the counter simultaneously, and since increment of an int is generally not atomic, this is an immediate race condition. Over and above the increment race, the routine increments and then reads the value, which is also not atomic, so subject to races.
In order to avoid races, you need some sort of mutex. You could use a semaphore, or you might be able to use a pthread mutex in shared memory (if you have pthreads available). Alternately you may be able use file locks, or you could write your own atomic assembly routines and call them. If you're using C11, you could even use the stuff in stdatomic.h. Lots of possible choices.
code :
    struct sembuf sem_ops = { 0, -1, SEM_UNDO };
    semop(semid, &sem_ops, 1);
    *shm++;
    rv = *shm;
    sem_ops.sem_op = 1;
    semop(semid, &sem_ops, 1);
    return rv;
}

Are variables declared in pthread's functions shared between threads?


By : Support RDX INDIA
Date : March 29 2020, 07:55 AM
I hope this helps . I am new to C and pthreads. Here's an example (which probably won't compile as I wrote it now) but it shows what I want to know: ,
Is int myLoop shared between the 3 threads?
code :
int myFunc(int *j)
{
    int q;
    // lots of code here
    myFunc (&q);
}
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