shmat
void *shmat(int, const void *, int);
struct shmat_args shmat;
if ((shm_buf = shmat(shmid, NULL, 0)) == (void *) -1)
if ((shm_buf = shmat(sender_shmid, NULL, 0)) == (void *) -1)
if ((shm_buf = shmat(shmid, NULL, 0)) == (void *) -1)