man-pages/man7/sem_overview.7

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.TH SEM_OVERVIEW 7 2006-03-25 "Linux" "Linux Programmer's Manual"
.SH NAME
sem_overview \- Overview of POSIX semaphores
.SH 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
.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.
.SS Named semaphores
A named semaphore is identified by a name of the form
.IR /somename .
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).
.SS 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 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
.BR semget (2),
or a POSIX shared memory object built created using
.BR shm_open (3)).

Before being used, an unnamed semaphore must be initialised 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).
.SH LINUX SPECIFIC DETAILS
.SS 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.
.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 \-lrt
to link against the real-time library,
.IR librt .
.SS Accessing named semaphores via the file system
On Linux, named semaphores are created in a virtual file system,
normally mounted under
.IR /dev/shm ,
with names of the form
.IR \fBsem.\fPname .
.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_init (3),
.BR sem_getvalue (3),
.BR sem_open (3),
.BR sem_post (3),
.BR sem_unlink (3),
.BR sem_wait (3),
.BR pthreads (7)