man7/sem_overview.7

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  23 .TH SEM_OVERVIEW 7 2006-03-25 "Linux 2.6.16" "Linux Programmer's Manual"
  24 .SH NAME
  25 sem_overview \- Overview of POSIX semaphores
  26 .SH DESCRIPTION
  27 POSIX semaphores allow processes and threads to synchronise their actions.
  28
  29 A semaphore is an integer whose value is never allowed to fall below zero.
  30 Two operations can be performed on semaphores:
  31 increment the semaphore value by one
  32 .RB ( sem_post (3));
  33 and decrement the semaphore value by one
  34 .RB ( sem_wait (3)).
  35 If the value of a semaphore is currently zero, then a
  36 .BR sem_wait (3)
  37 operation will block until the value becomes greater than zero.
  38
  39 POSIX semaphores come in two forms: named semaphores and
  40 unnamed semaphores.
  41 .SS Named semaphores
  42 A named semaphore is identified by a name of the form
  43 .IR /somename .
  44 Two processes can operate on the same named semaphore by passing
  45 the same name to
  46 .BR sem_open (3).
  47
  48 The
  49 .BR sem_open (3)
  50 function creates a new named semaphore or opens an existing
  51 named semaphore.
  52 After the semaphore has been opened, it can be operated on using
  53 .BR sem_post (3)
  54 and
  55 .BR sem_wait (3).
  56 When a process has finished using the semaphore, it can use
  57 .BR sem_close (3)
  58 to close the semaphore.
  59 When all processes have finished using the semaphore,
  60 it can be removed from the system using
  61 .BR sem_unlink (3).
  62 .SS Unnamed semaphores (memory-based semaphores)
  63 An unnamed semaphore does not have a name.
  64 Instead the semaphore is placed in a region of memory that
  65 is shared between multiple threads (a
  66 .IR "thread-shared semaphore" )
  67 or processes (a
  68 .IR "process-shared semaphore" ).
  69 A thread-shared semaphore is placed in an area of memory shared
  70 between by the threads of a process, for example, a global variable.
  71 A process-shared semaphore must be placed in a shared memory region
  72 (e.g., a System V shared memory segment created using
  73 .BR semget (2),
  74 or a POSIX shared memory object built created using
  75 .BR shm_open (3)).
  76
  77 Before being used, an unnamed semaphore must be initialised using
  78 .BR sem_init (3).
  79 It can then be operated on using
  80 .BR sem_post (3)
  81 and
  82 .BR sem_wait (3).
  83 When the semaphore is no longer required,
  84 and before the memory in which it is located is deallocated,
  85 the semaphore should be destroyed using
  86 .BR sem_destroy (3).
  87 .SH LINUX SPECIFIC DETAILS
  88 .SS Versions
  89 Prior to kernel 2.6, Linux only supported unnamed,
  90 thread-shared semaphores.
  91 On a system with Linux 2.6 and a glibc that provides the NPTL
  92 threading implementation,
  93 a complete implementation of POSIX semaphores is provided.
  94 .SS Persistence
  95 POSIX named semaphores have kernel persistence:
  96 if not removed by
  97 .BR sem_unlink (3),
  98 a semaphore will exist until the system is shut down.
  99 .SS Linking
 100 Programs using the POSIX semaphores API must be compiled with
 101 .I cc \-lrt
 102 to link against the real-time library,
 103 .IR librt .
 104 .SS Accessing named semaphores via the file system
 105 On Linux, named semaphores are created in a virtual file system,
 106 normally mounted under
 107 .IR /dev/shm ,
 108 with names of the form
 109 .IR \fBsem.\fPname .
 110 .SH "CONFORMING TO"
 111 POSIX.1-2001.
 112 .SH NOTES
 113 System V semaphores
 114 .RB ( semget (2),
 115 .BR semop (2),
 116 etc.) are an older semaphore API.
 117 POSIX semaphores provide a simpler, and better designed interface than
 118 System V semaphores;
 119 on the other hand POSIX semaphores are less widely available
 120 (especially on older systems) than System V semaphores.
 121 .SH EXAMPLE
 122 An example of the use of various POSIX semaphore functions is shown in
 123 .BR sem_wait (3).
 124 .SH "SEE ALSO"
 125 .BR sem_close (3),
 126 .BR sem_destroy (3),
 127 .BR sem_init (3),
 128 .BR sem_getvalue (3),
 129 .BR sem_open (3),
 130 .BR sem_post (3),
 131 .BR sem_unlink (3),
 132 .BR sem_wait (3),
 133 .BR pthreads (7)