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-.TH SEM_OVERVIEW 7 2012-05-13 "Linux" "Linux Programmer's Manual"
-.SH NAME
-sem_overview \- overview of POSIX semaphores
-.SH 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
-.RB ( sem_post (3));
-and decrement the semaphore value by one
-.RB ( sem_wait (3)).
-If the value of a semaphore is currently zero, then a
-.BR sem_wait (3)
-operation will block until the value becomes greater than zero.
-
-POSIX semaphores come in two forms: named semaphores and
-unnamed semaphores.
-.TP
-.B Named semaphores
-A named semaphore is identified by a name of the form
-.IR /somename ;
-that is, a null-terminated string of up to
-.BI NAME_MAX \-4
-(i.e., 251) characters consisting of an initial slash,
-.\" glibc allows the initial slash to be omitted, and makes
-.\" multiple initial slashes equivalent to a single slash.
-.\" This differs from the implementation of POSIX message queues.
-followed by one or more characters, none of which are slashes.
-.\" glibc allows subdirectory components in the name, in which
-.\" case the subdirectory tree must exist under /dev/shm, and
-.\" the fist subdirectory component must exist as the name
-.\" sem.name, and all of the subdirectory components must allow the
-.\" required permissions if a user wants to create a semaphore
-.\" object in a subdirectory.
-Two processes can operate on the same named semaphore by passing
-the same name to
-.BR sem_open (3).
-
-The
-.BR 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
-.BR sem_post (3)
-and
-.BR sem_wait (3).
-When a process has finished using the semaphore, it can use
-.BR sem_close (3)
-to close the semaphore.
-When all processes have finished using the semaphore,
-it can be removed from the system using
-.BR sem_unlink (3).
-.TP
-.B 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
-.IR "thread-shared semaphore" )
-or processes (a
-.IR "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
-.BR shmget (2),
-or a POSIX shared memory object built created using
-.BR shm_open (3)).
-
-Before being used, an unnamed semaphore must be initialized using
-.BR sem_init (3).
-It can then be operated on using
-.BR sem_post (3)
-and
-.BR 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
-.BR sem_destroy (3).
-.PP
-The remainder of this section describes some specific details
-of the Linux implementation of POSIX semaphores.
-.SS 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.
-.SS Persistence
-POSIX named semaphores have kernel persistence:
-if not removed by
-.BR sem_unlink (3),
-a semaphore will exist until the system is shut down.
-.SS Linking
-Programs using the POSIX semaphores API must be compiled with
-.I cc \-pthread
-to link against the real-time library,
-.IR librt .
-.SS Accessing named semaphores via the filesystem
-On Linux, named semaphores are created in a virtual filesystem,
-normally mounted under
-.IR /dev/shm ,
-with names of the form
-.IR \fBsem.\fPsomename .
-(This is the reason that semaphore names are limited to
-.BI NAME_MAX \-4
-rather than
-.B 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.
-.SH CONFORMING TO
-POSIX.1-2001.
-.SH NOTES
-System V semaphores
-.RB ( semget (2),
-.BR 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.
-.SH EXAMPLE
-An example of the use of various POSIX semaphore functions is shown in
-.BR sem_wait (3).
-.SH SEE ALSO
-.BR sem_close (3),
-.BR sem_destroy (3),
-.BR sem_getvalue (3),
-.BR sem_init (3),
-.BR sem_open (3),
-.BR sem_post (3),
-.BR sem_unlink (3),
-.BR sem_wait (3),
-.BR pthreads (7)
-.SH COLOPHON
-This page is part of release 3.79 of the Linux
-.I man-pages
-project.
-A description of the project,
-information about reporting bugs,
-and the latest version of this page,
-can be found at
-\%http://www.kernel.org/doc/man\-pages/.
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