Share array of Semaphores between two processes | linux C

Question

Everyone, I'm here to ask you how can I share an array of semaphores between 2 process??Since I created an array with semget(..) but I can't use shmat(..) on it !

With shared memory segment I usually use shmget() first and then shmat(..) so the child process can access to it. But How it works with an array of semaphores?

I can't find any similar method able to attach() !!!

Here I Use the semget() to create an array of 5 semaphores:

/* allocate semaphores */
    if ((semid = semget(IPC_PRIVATE,5,IPC_CREAT|0666)) == -1) {
        printf("shmget() fallita sem id\n");
        perror("semget()");
        //releaseAll(bufferid,Tid,Did,semid);
        exit(-4);

Here in another process I try to attach this array before use it (but i'm using shmat and seems not working..)

sem_t* addr1;

    if((addr1=(sem_t*)shmat(semid,NULL,0))==-1){
        printf("shmat() fallita sem id\n");
        perror("shmat() for content");

        exit(-1);// +1 per \0 finale
    }

You don't need shared memory at all. Just don't use IPC_PRIVATE, use a real key returned by ftok instead, in both processes. See e.g. here. — n. m. could be an AI
– n. m. could be an AI, Commented Dec 21, 2013 at 19:33

Duck · Accepted Answer · 2013-12-21 20:19:41Z

4

There are two kinds of semaphores available on linux: sysV and POSIX. You are using sysV semaphores.

sysV semaphores are maintained in the kernel so you don't need to allocate space for them or put them in shared memory to share them. But you do need a way for every process using them to find them. The mechanism to do that is key_t type.

The function ftok takes a pathname and an id and returns a key_t. This allows every process specifying the proper path and id to generate the same key_t.

semget takes the generated key_t, the number of requested semaphores, and some permissions flags and returns the semaphore identifier, that is the semid. The semid is used in all subsequent calls to semctl and semop.

An alternative, when you don't need to share the identity of the semaphore with other processes is to call semget with IPC_PRIVATE instead of a generated key_t. You are implicitly acknowledging by the use of IPC_PRIVATE that no unrelated processes need to know the returned semid and thus access the semaphore. This is useful for single processes with multiple threads that share memory and thus the semid; or for related processes that create the semaphores before forking but share a copy of the semid after the fork.

So your problem seems to be that you have two unrelated processes that need to share some semaphores. For that to work you need to use the ftok mechanism to generate the key so everyone involved can find them.

edited Dec 21, 2013 at 20:19

answered Dec 21, 2013 at 20:14

Duck

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2 Comments

user244050 Over a year ago

what happen if I want to use IPc-Private and i try to do some operation on the array direclty using the Id ? It works or I Have to use ftok? why?

Duck Over a year ago

I take back my previous comment. I don't think so. The second process would still have to call semget to establish permissions so even if you could pass it the semid it should fail with EACCES if it tried to use semop or semctl. And if it called semget itself with IPC_PRIVATE it would just get a different semid and not share the existing semaphore.

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