SEM_OVERVIEW(7) Linux Programmer's Manual SEM_OVERVIEW(7)NAMEsem_overview - Overview of POSIX semaphores
DESCRIPTION
POSIX semaphores allow processes and threads to synchronise 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 sema‐
phores.
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 sem‐
aphore. 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 initialised 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).
LINUX SPECIFIC DETAILS
Versions
Prior to kernel 2.6, Linux only supported 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
sem_unlink(), 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, nor‐
mally mounted under /dev/shm, with names of the form sem.name.
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
sem_wait(3).
SEE ALSOsem_close(3), sem_destroy(3), sem_init(3), sem_getvalue(3),
sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7)Linux 2.6.16 2006-03-25 SEM_OVERVIEW(7)