NAME
semoverview - 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 (sempost(3)); and decrement the semaphore value by one (semwait(3)). If the value of a semaphore is currently zero, then a semwait(3) operation will block until the value becomes greater than zero. POSIX semaphores come in two forms: named semaphores and unnamed sema‐ phores. Named semaphores A named semaphore is identified by a name of the form /somename;
that is, a null-terminated string of up to NAMEMAX-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 semopen(3). The semopen(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 sempost(3) and semwait(3). When a process has finished using the semaphore, it can use semclose(3) to close the semaphore. When all processes have finished using the semaphore, it can be removed from the system using semunlink(3).
Unnamed semaphores (memory-based semaphores) An unnamed semaphore does not have a name. Instead the sema‐ phore 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 shmopen(3)). Before being used, an unnamed semaphore must be initialized using seminit(3). It can then be operated on using sempost(3) and semwait(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 semdestroy(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 sema‐ phores. 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 semunlink(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 file system On Linux, named semaphores are created in a virtual file system, nor‐ mally mounted under /dev/shm, with names of the form sem.somename.
(This is the reason that semaphore names are limited to NAMEMAX-4 rather than NAMEMAX 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. CONFORMING TO
POSIX.1-2001. 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 sema‐ phores. EXAMPLE An example of the use of various POSIX semaphore functions is shown in semwait(3). SEE ALSO semclose(3), semdestroy(3), semgetvalue(3), seminit(3), semopen(3), sempost(3), semunlink(3), semwait(3), pthreads(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-05-13 SEMOVERVIEW(7)