通过信号量访问共享内存

Question

I have a different number of process which should write on the shared memory that I already defined. I'm trying by using system V semaphores. The semaphore ( sem_access ) is initialized at 1.

    sem_access.sem_op = -1;
    semop(sem_access_id, &sem_access, 1);

    info->index++;
    printf ("INDEX= %d\n", info->index);

    sem_access.sem_op = 1;
    semop(sem_access_id, &sem_access, 1);

The problem is that the printf prints always the same number, without increment the index. PS: Sorry for my bad english, but I'm spanish. edit: if I insert a sleep(1) it works well, but i really I don't want the sleep!

Answer 1

provide some more info about your code. What problem you are getting ? Does your process get stuck once resources acquired or not waiting ?

man page of semop() says :

1) If sem_op is less than zero , the process must have alter permission on the semaphore set . So if sem_access.sem_op = -1 ; when other process will make this value 0 or greater than 0 then only this process will access shared resources.

process 1 : sem_access.sem_op = -1; //this process will wait until condition not true

process 2 : since in 1st process sem_op value is -1, this process needs to make sem_op value as 0 or 1 then only other process will get executed, So do like this

sem_access.sem_op = 1;

as man page of semop() says, If sem_op is a positive integer, the operation adds this value to the semaphore value (semval) . now sem_op becomes -1 + 1 = 0, refer again semop() man pages "This is a " wait-for-zero " operation: if semval is zero, the operation can immediately proceed".

I hope you got some idea to resolve problem.