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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 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 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 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 by 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 semget(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 only supported 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 pro-
       vided.

   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 -lrt to link against the real-time library,
       librt.

   Accessing named semaphores via the file system
       On Linux, named semaphores are created in a virtual file system, 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.

CONFORMING TO
       POSIX.1-2001.

NOTES
       System  V  semaphores  (semget(2), semop(2), etc.) are an older semaphore API.  POSIX semaphores provide a sim-
       pler, 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.

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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-02-25                   SEM_OVERVIEW(7)