title: where does sem_open shared memory on osx
body: <p>I am currently porting a C lib for iOS, and need to change original sem_init() to sem_open() because
  iOS not support sem_init()</p>
  
  <p>I have seen some discussions telling that sem_open() will open a shared memory file in system, now I want to check if sem_open() a semaphore successfully by verifying the shared memory file.</p>
  
  <p>Have tried to find in /dev/shm, but there is no shm directory, but sem_open not returns "SEM_FAILED", so where does the shared memory file go?</p>
  
  <p>Thanks</p>