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       sem_overview - overview of POSIX semaphores


       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 semaphores.

       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

              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 semaphore 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.

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

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

       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, normally 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.


       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 semaphores.


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


       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), shm_overview(7)


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