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POSIX Semaphores

How

POSIX semaphores provide three variations:

• Unnamed semaphores (sem_init(2), sem_destroy(2)):
  
  
  • Thread shared: only shared by the threads in a single process; used for thread synchronisation.
    
    
  • Process shared; used for interprocess synchronization.
    
    
    • Semaphore is placed at agreed location in a region of shared memory.
      
      
    • Especially convenient for related (fork(2)) processes, since child inherits parent's references to shared memory regions/segments that parent created.
    
    
    
• Named semaphores (sem_open(), sem_close()):
  
  
  • Has a name(!) -- /name
    
    
  • Processes can share semaphore by opening same name.
    
    
  • Interprocess synchronization.
    
    
  • For unrelated processes, a named semaphore is simpler than setting up an unnamed semaphore in shared memory.
    
    

Linux 2.4 only supported unnamed thread-shared semaphores.

A Linux 2.6 system with a glibc that provides NPTL (glibc 2.3+) supports all flavours of POSIX semaphores.

(C) 2006, Michael Kerrisk