SEM_OVERVIEW(7)		   Linux Programmer's Manual	       SEM_OVERVIEW(7)



NAME
       sem_overview - overview of POSIX semaphores

DESCRIPTION
       POSIX  semaphores  allow	 processes  and	 threads  to synchronize their
       actions.

       A semaphore is an integer whose value is never allowed  to  fall	 below
       zero.   Two  operations	can  be performed on semaphores: increment the
       semaphore value by one (sem_post(3)); and decrement the semaphore value
       by  one	(sem_wait(3)).	If the value of a semaphore is currently zero,
       then a sem_wait(3) operation will block until the value becomes greater
       than zero.

       POSIX  semaphores come in two forms: named semaphores and unnamed sema-
       phores.

       Named semaphores
	      A named semaphore is identified by a name of the form /somename;
	      that  is,	 a  null-terminated  string of up to NAME_MAX-4 (i.e.,
	      251) characters consisting of an initial slash, followed by  one
	      or  more	characters,  none of which are slashes.	 Two processes
	      can operate on the same named semaphore by passing the same name
	      to sem_open(3).

	      The  sem_open(3) function creates a new named semaphore or opens
	      an existing named	 semaphore.   After  the  semaphore  has  been
	      opened, it can be operated on using sem_post(3) and sem_wait(3).
	      When a process has finished using	 the  semaphore,  it  can  use
	      sem_close(3)  to	close  the semaphore.  When all processes have
	      finished using the semaphore, it can be removed from the	system
	      using sem_unlink(3).

       Unnamed semaphores (memory-based semaphores)
	      An  unnamed  semaphore  does not have a name.  Instead the sema-
	      phore is placed in a region of memory  that  is  shared  between
	      multiple	threads	 (a  thread-shared  semaphore) or processes (a
	      process-shared semaphore).  A thread-shared semaphore is	placed
	      in  an  area  of memory shared between the threads of a process,
	      for example, a global variable.  A process-shared semaphore must
	      be  placed  in  a	 shared memory region (e.g., a System V shared
	      memory segment created using shmget(2), or a POSIX shared memory
	      object built created using shm_open(3)).

	      Before  being  used,  an	unnamed	 semaphore must be initialized
	      using sem_init(3).  It can then be operated on using sem_post(3)
	      and  sem_wait(3).	 When the semaphore is no longer required, and
	      before the memory in which it is	located	 is  deallocated,  the
	      semaphore should be destroyed using sem_destroy(3).

       The  remainder  of  this section describes some specific details of the
       Linux implementation of POSIX semaphores.

   Versions
       Prior to kernel 2.6, Linux supported only unnamed, thread-shared	 sema-
       phores.	 On a system with Linux 2.6 and a glibc that provides the NPTL
       threading implementation, a complete implementation of POSIX semaphores
       is provided.

   Persistence
       POSIX  named  semaphores	 have  kernel  persistence:  if not removed by
       sem_unlink(3), a semaphore will exist until the system is shut down.

   Linking
       Programs using the POSIX	 semaphores  API  must	be  compiled  with  cc
       -pthread to link against the real-time library, librt.

   Accessing named semaphores via the filesystem
       On  Linux,  named  semaphores are created in a virtual filesystem, nor-
       mally mounted under /dev/shm, with  names  of  the  form	 sem.somename.
       (This  is  the  reason  that  semaphore names are limited to NAME_MAX-4
       rather than NAME_MAX characters.)

       Since Linux 2.6.19, ACLs can be placed on files under  this  directory,
       to control object permissions on a per-user and per-group basis.

NOTES
       System  V semaphores (semget(2), semop(2), etc.) are an older semaphore
       API.  POSIX semaphores provide a simpler, and better designed interface
       than  System  V semaphores; on the other hand POSIX semaphores are less
       widely available (especially on older  systems)	than  System  V	 sema-
       phores.

EXAMPLE
       An  example of the use of various POSIX semaphore functions is shown in
       sem_wait(3).

SEE ALSO
       sem_close(3),	 sem_destroy(3),     sem_getvalue(3),	  sem_init(3),
       sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7)

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2015-08-08		       SEM_OVERVIEW(7)