1sem_overview(7) Miscellaneous Information Manual sem_overview(7)
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6 sem_overview - overview of POSIX semaphores
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9 POSIX semaphores allow processes and threads to synchronize their ac‐
10 tions.
11 A semaphore is an integer whose value is never allowed to fall below
13 zero. Two operations can be performed on semaphores: increment the
14 semaphore value by one (sem_post(3)); and decrement the semaphore value
15 by one (sem_wait(3)). If the value of a semaphore is currently zero,
16 then a sem_wait(3) operation will block until the value becomes greater
17 than zero.
18 POSIX semaphores come in two forms: named semaphores and unnamed sema‐
20 phores.
21 Named semaphores
23 A named semaphore is identified by a name of the form /somename;
24 that is, a null-terminated string of up to NAME_MAX-4 (i.e.,
25 251) characters consisting of an initial slash, followed by one
26 or more characters, none of which are slashes. Two processes
27 can operate on the same named semaphore by passing the same name
28 to sem_open(3).
29 The sem_open(3) function creates a new named semaphore or opens
31 an existing named semaphore. After the semaphore has been
32 opened, it can be operated on using sem_post(3) and sem_wait(3).
33 When a process has finished using the semaphore, it can use
34 sem_close(3) to close the semaphore. When all processes have
35 finished using the semaphore, it can be removed from the system
36 using sem_unlink(3).
37 Unnamed semaphores (memory-based semaphores)
39 An unnamed semaphore does not have a name. Instead the sema‐
40 phore is placed in a region of memory that is shared between
41 multiple threads (a thread-shared semaphore) or processes (a
42 process-shared semaphore). A thread-shared semaphore is placed
43 in an area of memory shared between the threads of a process,
44 for example, a global variable. A process-shared semaphore must
45 be placed in a shared memory region (e.g., a System V shared
46 memory segment created using shmget(2), or a POSIX shared memory
47 object built created using shm_open(3)).
48 Before being used, an unnamed semaphore must be initialized us‐
50 ing sem_init(3). It can then be operated on using sem_post(3)
51 and sem_wait(3). When the semaphore is no longer required, and
52 before the memory in which it is located is deallocated, the
53 semaphore should be destroyed using sem_destroy(3).
54 The remainder of this section describes some specific details of the
56 Linux implementation of POSIX semaphores.
57 Versions
59 Before Linux 2.6, Linux supported only unnamed, thread-shared sema‐
60 phores. On a system with Linux 2.6 and a glibc that provides the NPTL
61 threading implementation, a complete implementation of POSIX semaphores
62 is provided.
63 Persistence
65 POSIX named semaphores have kernel persistence: if not removed by
66 sem_unlink(3), a semaphore will exist until the system is shut down.
67 Linking
69 Programs using the POSIX semaphores API must be compiled with cc
70 -pthread to link against the real-time library, librt.
71 Accessing named semaphores via the filesystem
73 On Linux, named semaphores are created in a virtual filesystem, nor‐
74 mally mounted under /dev/shm, with names of the form sem.somename.
75 (This is the reason that semaphore names are limited to NAME_MAX-4
76 rather than NAME_MAX characters.)
77 Since Linux 2.6.19, ACLs can be placed on files under this directory,
79 to control object permissions on a per-user and per-group basis.
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82 System V semaphores (semget(2), semop(2), etc.) are an older semaphore
83 API. POSIX semaphores provide a simpler, and better designed interface
84 than System V semaphores; on the other hand POSIX semaphores are less
85 widely available (especially on older systems) than System V sema‐
86 phores.
87
89 An example of the use of various POSIX semaphore functions is shown in
90 sem_wait(3).
91
93 sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3),
94 sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7),
95 shm_overview(7)
96Linux man-pages 6.04 2022-12-04 sem_overview(7)