sem_close vs sem_unlink when process terminates

Question

I have multiple processes communicating using a semaphore.

sem_open("\name",O_CREATE,S_IRWXU|S_IRWXG,10);

I have 2 questions w.r.t following:

1. Now, when one process dies suddenly without calling sem_close or sem_unlink. What happens to the remaining processes ?

2. If sem_close() is called, however sem_unlink() is not called for 1 process. All the other processes, call sem_close() and sem_unlink(). Does the semaphore still exists, after all the processes exit ?

Answer 1

In answer to your questions:

1. when a process dies, voluntarily or unvoluntarily, the sem_close operation occurs automatically.
2. No, the semaphore does not exist if all processes have exited which had the semaphore opened and at least one process has called sem_unlink.

To expand a bit: sem_unlink removes the named semaphore, and will destruct the semaphore once its reference count is 0. That is after all process that opened it have called sem_close or have exited.

References: Book - Unix Networking Programming-Interprocess Communication by W.Richard Stevens, vol 2, ch10

Answer 2

Late answer but nevertheless I find this information missing in the other answer:

• sem_close is associated with the semaphore and its count
  • Semaphore's reference count would be decremented when called
  • Even when count drops to zero semaphore is still kept alive
  • Another process can sem_open with same name, restoring the count back to 1 (or greater with subsequent sem_opens)
• sem_unlink is associated with a semaphore's name
  • Name is no longer taken when called; name is relinquished
  • Semaphores still open (i.e. without sem_close calls) are kept alive until ref. count drops to zero
  • When count drops to zero an unlinked semaphore is destroyed
  • If a process sem_opens with the same, now-unliked name, it'd get a completely new (different) semaphore

Though a semaphore associated with a name is sem_closed, the semaphore lives and name stays taken until it's sem_unlinked.