CODEWITHSUNDEEP
clinuxforksemaphoreshared-memory
Wokers
Wokers

Reputation: 117

how to initiliaze unnamed semaphore with shared memory?

I know that unnamed semaphores should be in shared memory area, but I don't know how to implement because mmap() returns a pointer to the mapped area, and i am obligated to use sem_t pointer, but this time, i think it s not really safe. Here what i did, 

    sem_t *sem;
    char* name = "sharedSem";
    int fd;

    sem_init(sem, 1, 1);

    fd = shm_open(name, O_CREAT | O_RDWR, 0666);
    if(fd == -1)
    {
        perror("fail");
        exit(-1);
    }
    ftruncate(fd, sizeof(sem_t));
    sem = (sem_t*) mmap(0, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

When i use pointer, also i can't read the value of semaphore, i don't even know if it is possible..

Can i create an unnamed semaphore without sem_t pointer in shared memory area that children processes can also access?

Upvotes: 1

Views: 786

Answers (1)

Joseph Sible-Reinstate Monica
Joseph Sible-Reinstate Monica

Reputation: 48572

The problem is you're calling sem_init while sem is still not pointing anywhere in particular. Move that call to after you point it to the result of mmap (i.e., to the bottom of the snippet you posted).

Upvotes: 1

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