man7/sem_overview.7

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  26 .TH SEM_OVERVIEW 7 2008-06-15 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 sem_overview \- Overview of POSIX semaphores
  29 .SH DESCRIPTION
  30 POSIX semaphores allow processes and threads to synchronize their actions.
  31
  32 A semaphore is an integer whose value is never allowed to fall below zero.
  33 Two operations can be performed on semaphores:
  34 increment the semaphore value by one
  35 .RB ( sem_post (3));
  36 and decrement the semaphore value by one
  37 .RB ( sem_wait (3)).
  38 If the value of a semaphore is currently zero, then a
  39 .BR sem_wait (3)
  40 operation will block until the value becomes greater than zero.
  41
  42 POSIX semaphores come in two forms: named semaphores and
  43 unnamed semaphores.
  44 .TP
  45 .B Named semaphores
  46 A named semaphore is identified by a name of the form
  47 .IR /somename .
  48 Two processes can operate on the same named semaphore by passing
  49 the same name to
  50 .BR sem_open (3).
  51
  52 The
  53 .BR sem_open (3)
  54 function creates a new named semaphore or opens an existing
  55 named semaphore.
  56 After the semaphore has been opened, it can be operated on using
  57 .BR sem_post (3)
  58 and
  59 .BR sem_wait (3).
  60 When a process has finished using the semaphore, it can use
  61 .BR sem_close (3)
  62 to close the semaphore.
  63 When all processes have finished using the semaphore,
  64 it can be removed from the system using
  65 .BR sem_unlink (3).
  66 .TP
  67 .B Unnamed semaphores (memory-based semaphores)
  68 An unnamed semaphore does not have a name.
  69 Instead the semaphore is placed in a region of memory that
  70 is shared between multiple threads (a
  71 .IR "thread-shared semaphore" )
  72 or processes (a
  73 .IR "process-shared semaphore" ).
  74 A thread-shared semaphore is placed in an area of memory shared
  75 between by the threads of a process, for example, a global variable.
  76 A process-shared semaphore must be placed in a shared memory region
  77 (e.g., a System V shared memory segment created using
  78 .BR semget (2),
  79 or a POSIX shared memory object built created using
  80 .BR shm_open (3)).
  81
  82 Before being used, an unnamed semaphore must be initialized using
  83 .BR sem_init (3).
  84 It can then be operated on using
  85 .BR sem_post (3)
  86 and
  87 .BR sem_wait (3).
  88 When the semaphore is no longer required,
  89 and before the memory in which it is located is deallocated,
  90 the semaphore should be destroyed using
  91 .BR sem_destroy (3).
  92 .PP
  93 The remainder of this section describes some specific details
  94 of the Linux implementation of POSIX semaphores.
  95 .SS Versions
  96 Prior to kernel 2.6, Linux only supported unnamed,
  97 thread-shared semaphores.
  98 On a system with Linux 2.6 and a glibc that provides the NPTL
  99 threading implementation,
 100 a complete implementation of POSIX semaphores is provided.
 101 .SS Persistence
 102 POSIX named semaphores have kernel persistence:
 103 if not removed by
 104 .BR sem_unlink (3),
 105 a semaphore will exist until the system is shut down.
 106 .SS Linking
 107 Programs using the POSIX semaphores API must be compiled with
 108 .I cc \-lrt
 109 to link against the real-time library,
 110 .IR librt .
 111 .SS Accessing named semaphores via the file system
 112 On Linux, named semaphores are created in a virtual file system,
 113 normally mounted under
 114 .IR /dev/shm ,
 115 with names of the form
 116 .IR \fBsem.\fPname .
 117
 118 Since Linux 2.6.19, ACLs can be placed on files under this directory,
 119 to control object permissions on a per-user and per-group basis.
 120 .SH "CONFORMING TO"
 121 POSIX.1-2001.
 122 .SH NOTES
 123 System V semaphores
 124 .RB ( semget (2),
 125 .BR semop (2),
 126 etc.) are an older semaphore API.
 127 POSIX semaphores provide a simpler, and better designed interface than
 128 System V semaphores;
 129 on the other hand POSIX semaphores are less widely available
 130 (especially on older systems) than System V semaphores.
 131 .SH EXAMPLE
 132 An example of the use of various POSIX semaphore functions is shown in
 133 .BR sem_wait (3).
 134 .SH "SEE ALSO"
 135 .BR sem_close (3),
 136 .BR sem_destroy (3),
 137 .BR sem_getvalue (3),
 138 .BR sem_init (3),
 139 .BR sem_open (3),
 140 .BR sem_post (3),
 141 .BR sem_unlink (3),
 142 .BR sem_wait (3),
 143 .BR pthreads (7)