R – Where do you place POSIX semaphores when using POSIX shared memory

A couple of questions:

• Are you running ipcs as the same user that created the shared memory/semaphores (or as the superuser)?
• Are you running ipcs while the program is running? (Are you sure it's not removing them when it exits?)

Update:

Actually, after reading this thread I'm not sure ipcs is supposed to be able to show the POSIX semaphores. I tried your sample code (with some edits to fix compile errors) and you can see the shared memory segment in the /dev/shm directory.

It's easy to share named POSIX semaphores

• Choose a name for your semaphore

  #define SNAME "/mysem"
  

• Use sem_open with O_CREAT in the process that creates them

  sem_t *sem = sem_open(SNAME, O_CREAT, 0644, 3); /* Initial value is 3. */
  

• Open semaphores in the other processes

  sem_t *sem = sem_open(SEM_NAME, 0); /* Open a preexisting semaphore. */
  

If you insist on using shared memory, it's certainly possible.

int fd = shm_open("shmname", O_CREAT, O_RDWR);
ftruncate(fd, sizeof(sem_t));
sem_t *sem = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE,
    MAP_SHARED, fd, 0);

sem_init(sem, 1, 1);

I haven't tested the above so it could be completely bonkers.