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       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 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 by 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 semget(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  only  supported  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  -lrt
       to link against the real-time library, librt.

   Accessing named semaphores via the file system
       On  Linux,  named  semaphores  are  created  in  a virtual file system,
       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


       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)


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