sem_overview

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 interface
       than System V semaphores; on the other hand POSIX semaphores are less
       widely available (especially on older systems) than System V
       semaphores.

EXAMPLES
       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_overview(7)

COLOPHON
       This page is part of release 5.08 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                             2020-06-09                   SEM_OVERVIEW(7)