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sem_overview (7)
>> sem_overview (7) ( Linux man: Макропакеты и соглашения )
NAME
sem_overview - 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
(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 semaphores.
Named semaphores
A named semaphore is identified by a name of the form
/somename.
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.
Versions
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.
Persistence
POSIX named semaphores have kernel persistence:
if not removed by
sem_unlink(3),
a semaphore will exist until the system is shut down.
Linking
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.name.
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 semaphores.
EXAMPLE
An example of the use of various POSIX semaphore functions is shown in
sem_wait(3).
This page is part of release 3.14 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/.
