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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 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).