mirror of https://github.com/mkerrisk/man-pages
288 lines
7.3 KiB
Groff
288 lines
7.3 KiB
Groff
.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 1 Jan 2002, Martin Schulze <joey@infodrom.org>
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.\" Modified 4 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Added notes on capability requirements
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.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Language and formatting clean-ups
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.\" Added notes on /proc files
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.\" Rewrote BUGS note about semget()'s failure to initialize
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.\" semaphore values
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.\"
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.TH SEMGET 2 2004-05-27 "Linux" "Linux Programmer's Manual"
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.SH NAME
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semget \- get a semaphore set identifier
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.SH SYNOPSIS
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.nf
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.B
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#include <sys/types.h>
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.B
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#include <sys/ipc.h>
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.B
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#include <sys/sem.h>
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.fi
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.sp
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.BI "int semget(key_t " key ,
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.BI "int " nsems ,
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.BI "int " semflg );
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.SH DESCRIPTION
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The
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.BR semget ()
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system call returns the semaphore set identifier
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associated with the argument
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.IR key .
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A new set of
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.I nsems
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semaphores is created if
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.I key
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has the value
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.B IPC_PRIVATE
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or if no existing semaphore set is associated with
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.I key
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and
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.B IPC_CREAT
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is specified in
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.IR semflg .
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.PP
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If
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.I semflg
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specifies both
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.B IPC_CREAT
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and
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.B IPC_EXCL
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and a semaphore set already exists for
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.IR key ,
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then
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.BR semget ()
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fails with
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.I errno
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set to
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.BR EEXIST .
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(This is analogous to the effect of the combination
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.B O_CREAT | O_EXCL
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for
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.BR open (2).)
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.PP
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Upon creation, the least significant 9 bits of the argument
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.I semflg
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define the permissions (for owner, group and others)
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for the semaphore set.
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These bits have the same format, and the same
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meaning, as the
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.I mode
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argument of
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.BR open (2)
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(though the execute permissions are
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not meaningful for semaphores, and write permissions mean permission
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to alter semaphore values).
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.PP
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The values of the semaphores in a newly created set are indeterminate.
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(POSIX.1-2001 is explicit on this point.)
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Although Linux, like many other implementations,
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initializes the semaphore values to 0,
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a portable application cannot rely on this:
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it should explicitly initialize the semaphores to the desired values.
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.\" In truth, every one of the many implementations that I've tested sets
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.\" the values to zero, but I suppose there is/was some obscure
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.\" implementation out there that does not.
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.PP
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When creating a new semaphore set,
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.BR semget ()
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initializes the set's associated data structure,
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.I semid_ds
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(see
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.BR semctl (2)),
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as follows:
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.IP
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.I sem_perm.cuid
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and
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.I sem_perm.uid
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are set to the effective user ID of the calling process.
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.IP
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.I sem_perm.cgid
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and
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.I sem_perm.gid
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are set to the effective group ID of the calling process.
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.IP
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The least significant 9 bits of
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.I sem_perm.mode
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are set to the least significant 9 bits of
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.IR semflg .
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.IP
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.I sem_nsems
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is set to the value of
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.IR nsems .
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.IP
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.I sem_otime
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is set to 0.
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.IP
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.I sem_ctime
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is set to the current time.
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.PP
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The argument
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.I nsems
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can be 0
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(a don't care)
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when a semaphore set is not being created.
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Otherwise
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.I nsems
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must be greater than 0
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and less than or equal to the maximum number of semaphores per semaphore set
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.RB ( SEMMSL ).
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.PP
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If the semaphore set already exists, the permissions are
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verified.
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.\" and a check is made to see if it is marked for destruction.
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.SH "RETURN VALUE"
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If successful, the return value will be the semaphore set identifier
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(a non-negative integer), otherwise \-1
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is returned, with
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.I errno
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indicating the error.
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.SH ERRORS
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On failure
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.I errno
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will be set to one of the following:
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.TP 11
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.B EACCES
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A semaphore set exists for
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.IR key ,
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but the calling process does not have permission to access the set,
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and does not have the
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.BR CAP_IPC_OWNER
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capability.
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.TP
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.B EEXIST
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A semaphore set exists for
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.I key
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and
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.I semflg
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specified both
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.B IPC_CREAT
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and
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.BR IPC_EXCL .
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.\" .TP
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.\" .B EIDRM
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.\" The semaphore set is marked to be deleted.
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.TP
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.B EINVAL
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.IR nsems
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is less than 0 or greater than the limit on the number
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of semaphores per semaphore set
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.RB ( SEMMSL ),
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or a semaphore set corresponding to
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.I key
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already exists, and
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.I nsems
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is larger than the number of semaphores in that set.
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.TP
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.B ENOENT
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No semaphore set exists for
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.I key
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and
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.I semflg
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did not specify
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.BR IPC_CREAT .
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.TP
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.B ENOMEM
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A semaphore set has to be created but the system does not have
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enough memory for the new data structure.
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.TP
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.B ENOSPC
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A semaphore set has to be created but the system limit for the maximum
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number of semaphore sets
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.RB ( SEMMNI ),
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or the system wide maximum number of semaphores
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.RB ( SEMMNS ),
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would be exceeded.
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.SH "CONFORMING TO"
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SVr4, POSIX.1-2001.
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.\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
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.\" ERANGE, EFAULT.
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.SH NOTES
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.B IPC_PRIVATE
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isn't a flag field but a
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.I key_t
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type.
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If this special value is used for
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.IR key ,
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the system call ignores everything but the least significant 9 bits of
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.I semflg
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and creates a new semaphore set (on success).
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.PP
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The following limits on semaphore set resources affect the
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.BR semget ()
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call:
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.TP 11
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.B SEMMNI
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System wide maximum number of semaphore sets: policy dependent
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(on Linux, this limit can be read and modified via the fourth field of
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.IR /proc/sys/kernel/sem ).
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.\" This /proc file is not available in Linux 2.2 and earlier -- MTK
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.TP
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.B SEMMSL
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Maximum number of semaphores per semid: implementation dependent
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(on Linux, this limit can be read and modified via the first field of
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.IR /proc/sys/kernel/sem ).
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.TP
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.B SEMMNS
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System wide maximum number of semaphores: policy dependent
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(on Linux, this limit can be read and modified via the second field of
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.IR /proc/sys/kernel/sem ).
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Values greater than
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.B SEMMSL * SEMMNI
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makes it irrelevant.
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.SH BUGS
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The name choice
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.B IPC_PRIVATE
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was perhaps unfortunate, IPC_NEW
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would more clearly show its function.
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.LP
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The semaphores in a set are not initialized by
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.BR semget ().
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.\" In fact they are initialized to zero on Linux, but POSIX.1-2001
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.\" does not specify this, and we can't portably rely on it.
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In order to initialize the semaphores,
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.BR semctl (2)
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must be used to perform a
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.B SETVAL
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or a
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.B SETALL
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operation on the semaphore set.
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(Where multiple peers do not know who will be the first to
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initialize the set, checking for a non-zero
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.I sem_otime
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in the associated data structure retrieved by a
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.BR semctl (2)
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.B IPC_STAT
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operation can be used to avoid races.)
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.SH "SEE ALSO"
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.BR semctl (2),
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.BR semop (2),
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.BR ftok (3),
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.BR capabilities (7),
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.BR sem_overview (7),
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.BR svipc (7)
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