man3/sem_init.3

   1 .\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .TH SEM_INIT 3 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
   6 .SH NAME
   7 sem_init \- initialize an unnamed semaphore
   8 .SH LIBRARY
   9 POSIX threads library
  10 .RI ( libpthread ", " \-lpthread )
  11 .SH SYNOPSIS
  12 .nf
  13 .B #include <semaphore.h>
  14 .PP
  15 .BI "int sem_init(sem_t *" sem ", int " pshared ", unsigned int " value );
  16 .fi
  17 .SH DESCRIPTION
  18 .BR sem_init ()
  19 initializes the unnamed semaphore at the address pointed to by
  20 .IR sem .
  21 The
  22 .I value
  23 argument specifies the initial value for the semaphore.
  24 .PP
  25 The
  26 .I pshared
  27 argument indicates whether this semaphore is to be shared
  28 between the threads of a process, or between processes.
  29 .PP
  30 If
  31 .I pshared
  32 has the value 0,
  33 then the semaphore is shared between the threads of a process,
  34 and should be located at some address that is visible to all threads
  35 (e.g., a global variable, or a variable allocated dynamically on
  36 the heap).
  37 .PP
  38 If
  39 .I pshared
  40 is nonzero, then the semaphore is shared between processes,
  41 and should be located in a region of shared memory (see
  42 .BR shm_open (3),
  43 .BR mmap (2),
  44 and
  45 .BR shmget (2)).
  46 (Since a child created by
  47 .BR fork (2)
  48 inherits its parent's memory mappings, it can also access the semaphore.)
  49 Any process that can access the shared memory region
  50 can operate on the semaphore using
  51 .BR sem_post (3),
  52 .BR sem_wait (3),
  53 and so on.
  54 .PP
  55 Initializing a semaphore that has already been initialized
  56 results in undefined behavior.
  57 .SH RETURN VALUE
  58 .BR sem_init ()
  59 returns 0 on success;
  60 on error, \-1 is returned, and
  61 .I errno
  62 is set to indicate the error.
  63 .SH ERRORS
  64 .TP
  65 .B EINVAL
  66 .I value
  67 exceeds
  68 .BR SEM_VALUE_MAX .
  69 .TP
  70 .B ENOSYS
  71 .I pshared
  72 is nonzero,
  73 but the system does not support process-shared semaphores (see
  74 .BR sem_overview (7)).
  75 .SH ATTRIBUTES
  76 For an explanation of the terms used in this section, see
  77 .BR attributes (7).
  78 .ad l
  79 .nh
  80 .TS
  81 allbox;
  82 lbx lb lb
  83 l l l.
  84 Interface       Attribute       Value
  85 T{
  86 .BR sem_init ()
  87 T}      Thread safety   MT-Safe
  88 .TE
  89 .hy
  90 .ad
  91 .sp 1
  92 .SH STANDARDS
  93 POSIX.1-2001.
  94 .SH NOTES
  95 Bizarrely, POSIX.1-2001 does not specify the value that should
  96 be returned by a successful call to
  97 .BR sem_init ().
  98 POSIX.1-2008 rectifies this, specifying the zero return on success.
  99 .SH EXAMPLES
 100 See
 101 .BR shm_open (3)
 102 and
 103 .BR sem_wait (3).
 104 .SH SEE ALSO
 105 .BR sem_destroy (3),
 106 .BR sem_post (3),
 107 .BR sem_wait (3),
 108 .BR sem_overview (7)