SEM_OVERVIEW(7)             Linux Programmer's Manual            SEM_OVERVIEW(7)

       sem_overview - overview of POSIX semaphores

       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

              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

       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.

       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.

       POSIX named semaphores have kernel persistence: if not removed by
       sem_unlink(3), a semaphore will exist until the system is shut down.

       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.

       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.

       An example of the use of various POSIX semaphore functions is shown in

       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)

       This page is part of release 5.13 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

Linux                              2020-06-09                    SEM_OVERVIEW(7)