SEM_OVERVIEW
Section: Linux Programmer's Manual (7)
Updated: 2020-06-09
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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;
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 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.
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.
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 -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.
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.
EXAMPLES
An example of the use of various POSIX semaphore functions is shown in
sem_wait(3).
SEE ALSO
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)
COLOPHON
This page is part of release 5.11 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
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- DESCRIPTION
-
- Versions
-
- Persistence
-
- Linking
-
- Accessing named semaphores via the filesystem
-
- NOTES
-
- EXAMPLES
-
- SEE ALSO
-
- COLOPHON
-
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