sem_overview(7) — Linux manual page


sem_overview(7)     Miscellaneous Information Manual     sem_overview(7)

NAME         top

       sem_overview - overview of POSIX semaphores

DESCRIPTION         top

       POSIX semaphores allow processes and threads to synchronize their

       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

       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.

       Before Linux 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

       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         top

       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         top

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

SEE ALSO         top

       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)

Linux man-pages (unreleased)     (date)                  sem_overview(7)

Pages that refer to this page: eventfd(2)execve(2)intro(2)semctl(2)semget(2)semop(2)sem_close(3)sem_destroy(3)sem_getvalue(3)sem_init(3)sem_open(3)sem_post(3)sem_unlink(3)sem_wait(3)tmpfs(5)shm_overview(7)sysvipc(7)