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 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 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.

   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, 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.

NOTES

       System V semaphores (semget(2), semop(2), etc.) are an older semaphore API.  POSIX semaphores provide a simpler, and better designed inter-
       face  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),  shm_over-
       view(7)

COLOPHON

       This  page is part of release 4.15 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                                                               2017-05-03                                                     SEM_OVERVIEW(7)