mirror of https://github.com/mkerrisk/man-pages
585 lines
14 KiB
Groff
585 lines
14 KiB
Groff
.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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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.\" manual page may be incorrect or out-of-date.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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.\" %%%LICENSE_END
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.\"
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.\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aeb@cwi.nl>
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.\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aeb@cwi.nl>
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.\" Modified 20 Dec 2001, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified 21 Dec 2001, aeb
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.\" Modified 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Added notes on CAP_IPC_OWNER requirement
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.\" Modified 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
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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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.\" Rewrote semun text
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.\" Added semid_ds and ipc_perm structure definitions
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.\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions.
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.\"
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.TH SEMCTL 2 2017-09-15 "Linux" "Linux Programmer's Manual"
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.SH NAME
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semctl \- System V semaphore control operations
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.B #include <sys/ipc.h>
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.B #include <sys/sem.h>
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.PP
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.BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"
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.fi
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.SH DESCRIPTION
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.BR semctl ()
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performs the control operation specified by
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.I cmd
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on the System\ V semaphore set identified by
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.IR semid ,
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or on the
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.IR semnum -th
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semaphore of that set.
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(The semaphores in a set are numbered starting at 0.)
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.PP
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This function has three or four arguments, depending on
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.IR cmd .
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When there are four, the fourth has the type
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.IR "union semun" .
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The \fIcalling program\fP must define this union as follows:
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.PP
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.in +4n
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.EX
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union semun {
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int val; /* Value for SETVAL */
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struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
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unsigned short *array; /* Array for GETALL, SETALL */
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struct seminfo *__buf; /* Buffer for IPC_INFO
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(Linux-specific) */
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};
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.EE
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.in
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.PP
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The
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.I semid_ds
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data structure is defined in \fI<sys/sem.h>\fP as follows:
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.PP
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.in +4n
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.EX
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struct semid_ds {
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struct ipc_perm sem_perm; /* Ownership and permissions */
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time_t sem_otime; /* Last semop time */
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time_t sem_ctime; /* Last change time */
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unsigned long sem_nsems; /* No. of semaphores in set */
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};
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.EE
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.in
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.PP
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The
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.I ipc_perm
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structure is defined as follows
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(the highlighted fields are settable using
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.BR IPC_SET ):
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.PP
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.in +4n
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.EX
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struct ipc_perm {
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key_t __key; /* Key supplied to semget(2) */
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uid_t \fBuid\fP; /* Effective UID of owner */
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gid_t \fBgid\fP; /* Effective GID of owner */
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uid_t cuid; /* Effective UID of creator */
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gid_t cgid; /* Effective GID of creator */
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unsigned short \fBmode\fP; /* Permissions */
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unsigned short __seq; /* Sequence number */
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};
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.EE
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.in
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.PP
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Valid values for
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.I cmd
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are:
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.TP 10
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.B IPC_STAT
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Copy information from the kernel data structure associated with
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.I semid
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into the
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.I semid_ds
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structure pointed to by
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.IR arg.buf .
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The argument
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.I semnum
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is ignored.
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The calling process must have read permission on the semaphore set.
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.TP
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.B IPC_SET
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Write the values of some members of the
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.I semid_ds
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structure pointed to by
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.I arg.buf
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to the kernel data structure associated with this semaphore set,
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updating also its
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.I sem_ctime
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member.
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The following members of the structure are updated:
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.IR sem_perm.uid ,
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.IR sem_perm.gid ,
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and (the least significant 9 bits of)
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.IR sem_perm.mode .
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The effective UID of the calling process must match the owner
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.RI ( sem_perm.uid )
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or creator
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.RI ( sem_perm.cuid )
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of the semaphore set, or the caller must be privileged.
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The argument
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.I semnum
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is ignored.
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.TP
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.B IPC_RMID
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Immediately remove the semaphore set,
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awakening all processes blocked in
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.BR semop (2)
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calls on the set (with an error return and
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.I errno
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set to
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.BR EIDRM ).
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The effective user ID of the calling process must
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match the creator or owner of the semaphore set,
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or the caller must be privileged.
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The argument
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.I semnum
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is ignored.
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.TP
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.BR IPC_INFO " (Linux-specific)"
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Return information about system-wide semaphore limits and
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parameters in the structure pointed to by
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.IR arg.__buf .
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This structure is of type
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.IR seminfo ,
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defined in
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.I <sys/sem.h>
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if the
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.B _GNU_SOURCE
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feature test macro is defined:
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.IP
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.in +4n
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.EX
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struct seminfo {
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int semmap; /* Number of entries in semaphore
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map; unused within kernel */
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int semmni; /* Maximum number of semaphore sets */
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int semmns; /* Maximum number of semaphores in all
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semaphore sets */
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int semmnu; /* System-wide maximum number of undo
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structures; unused within kernel */
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int semmsl; /* Maximum number of semaphores in a
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set */
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int semopm; /* Maximum number of operations for
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semop(2) */
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int semume; /* Maximum number of undo entries per
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process; unused within kernel */
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int semusz; /* Size of struct sem_undo */
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int semvmx; /* Maximum semaphore value */
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int semaem; /* Max. value that can be recorded for
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semaphore adjustment (SEM_UNDO) */
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};
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.EE
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.in
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.IP
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The
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.IR semmsl ,
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.IR semmns ,
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.IR semopm ,
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and
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.I semmni
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settings can be changed via
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.IR /proc/sys/kernel/sem ;
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see
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.BR proc (5)
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for details.
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.TP
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.BR SEM_INFO " (Linux-specific)"
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Return a
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.I seminfo
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structure containing the same information as for
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.BR IPC_INFO ,
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except that the following fields are returned with information
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about system resources consumed by semaphores: the
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.I semusz
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field returns the number of semaphore sets that currently exist
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on the system; and the
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.I semaem
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field returns the total number of semaphores in all semaphore sets
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on the system.
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.TP
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.BR SEM_STAT " (Linux-specific)"
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Return a
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.I semid_ds
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structure as for
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.BR IPC_STAT .
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However, the
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.I semid
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argument is not a semaphore identifier, but instead an index into
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the kernel's internal array that maintains information about
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all semaphore sets on the system.
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.TP
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.B GETALL
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Return
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.B semval
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(i.e., the current value)
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for all semaphores of the set into
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.IR arg.array .
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The argument
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.I semnum
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is ignored.
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The calling process must have read permission on the semaphore set.
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.TP
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.B GETNCNT
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Return the value of
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.B semncnt
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for the
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.IR semnum \-th
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semaphore of the set
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(i.e., the number of processes waiting for an increase of
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.B semval
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for the
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.IR semnum \-th
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semaphore of the set).
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The calling process must have read permission on the semaphore set.
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.TP
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.B GETPID
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Return the value of
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.B sempid
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for the
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.IR semnum \-th
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semaphore of the set.
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This is the PID of the process that last performed an operation on
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that semaphore (but see NOTES).
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The calling process must have read permission on the semaphore set.
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.TP
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.B GETVAL
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Return the value of
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.B semval
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for the
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.IR semnum \-th
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semaphore of the set.
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The calling process must have read permission on the semaphore set.
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.TP
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.B GETZCNT
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Return the value of
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.B semzcnt
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for the
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.IR semnum \-th
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semaphore of the set
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(i.e., the number of processes waiting for
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.B semval
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of the
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.IR semnum \-th
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semaphore of the set to become 0).
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The calling process must have read permission on the semaphore set.
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.TP
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.B SETALL
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Set
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.B semval
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for all semaphores of the set using
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.IR arg.array ,
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updating also the
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.I sem_ctime
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member of the
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.I semid_ds
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structure associated with the set.
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Undo entries (see
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.BR semop (2))
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are cleared for altered semaphores in all processes.
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If the changes to semaphore values would permit blocked
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.BR semop (2)
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calls in other processes to proceed, then those processes are woken up.
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The argument
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.I semnum
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is ignored.
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The calling process must have alter (write) permission on
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the semaphore set.
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.TP
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.B SETVAL
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Set the value of
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.B semval
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to
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.I arg.val
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for the
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.IR semnum \-th
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semaphore of the set, updating also the
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.I sem_ctime
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member of the
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.I semid_ds
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structure associated with the set.
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Undo entries are cleared for altered semaphores in all processes.
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If the changes to semaphore values would permit blocked
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.BR semop (2)
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calls in other processes to proceed, then those processes are woken up.
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The calling process must have alter permission on the semaphore set.
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.SH RETURN VALUE
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On failure,
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.BR semctl ()
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returns \-1
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with
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.I errno
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indicating the error.
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.PP
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Otherwise, the system call returns a nonnegative value depending on
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.I cmd
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as follows:
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.TP 10
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.B GETNCNT
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the value of
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.BR semncnt .
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.TP
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.B GETPID
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the value of
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.BR sempid .
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.TP
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.B GETVAL
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the value of
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.BR semval .
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.TP
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.B GETZCNT
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the value of
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.BR semzcnt .
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.TP
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.B IPC_INFO
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the index of the highest used entry in the
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kernel's internal array recording information about all
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semaphore sets.
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(This information can be used with repeated
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.B SEM_STAT
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operations to obtain information about all semaphore sets on the system.)
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.TP
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.B SEM_INFO
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as for
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.BR IPC_INFO .
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.TP
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.B SEM_STAT
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the identifier of the semaphore set whose index was given in
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.IR semid .
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.PP
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All other
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.I cmd
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values return 0 on success.
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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
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.B EACCES
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The argument
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.I cmd
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has one of the values
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.BR GETALL ,
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.BR GETPID ,
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.BR GETVAL ,
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.BR GETNCNT ,
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.BR GETZCNT ,
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.BR IPC_STAT ,
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.BR SEM_STAT ,
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.BR SETALL ,
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or
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.B SETVAL
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and the calling process does not have the required
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permissions on the semaphore set and does not have the
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.B CAP_IPC_OWNER
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capability in the user namespace that governs its IPC namespace.
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.TP
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.B EFAULT
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The address pointed to by
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.I arg.buf
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or
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.I arg.array
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isn't accessible.
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.TP
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.B EIDRM
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The semaphore set was removed.
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.TP
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.B EINVAL
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Invalid value for
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.I cmd
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or
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.IR semid .
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Or: for a
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.B SEM_STAT
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operation, the index value specified in
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.I semid
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referred to an array slot that is currently unused.
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.TP
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.B EPERM
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The argument
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.I cmd
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has the value
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.B IPC_SET
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or
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.B IPC_RMID
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but the effective user ID of the calling process is not the creator
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(as found in
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.IR sem_perm.cuid )
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or the owner
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(as found in
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.IR sem_perm.uid )
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of the semaphore set,
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and the process does not have the
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.B CAP_SYS_ADMIN
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capability.
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.TP
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.B ERANGE
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The argument
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.I cmd
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has the value
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.B SETALL
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or
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.B SETVAL
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and the value to which
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.B semval
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is to be set (for some semaphore of the set) is less than 0
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or greater than the implementation limit
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.BR SEMVMX .
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008, SVr4.
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.\" SVr4 documents more error conditions EINVAL and EOVERFLOW.
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.PP
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POSIX.1 specifies the
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.\" POSIX.1-2001, POSIX.1-2008
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.I sem_nsems
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field of the
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.I semid_ds
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structure as having the type
|
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.IR "unsigned\ short" ,
|
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and the field is so defined on most other systems.
|
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It was also so defined on Linux 2.2 and earlier,
|
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but, since Linux 2.4, the field has the type
|
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.IR "unsigned\ long" .
|
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.SH NOTES
|
|
The inclusion of
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.I <sys/types.h>
|
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and
|
|
.I <sys/ipc.h>
|
|
isn't required on Linux or by any version of POSIX.
|
|
However,
|
|
some old implementations required the inclusion of these header files,
|
|
and the SVID also documented their inclusion.
|
|
Applications intended to be portable to such old systems may need
|
|
to include these header files.
|
|
.\" Like Linux, the FreeBSD man pages still document
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.\" the inclusion of these header files.
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.PP
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The
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.BR IPC_INFO ,
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.B SEM_STAT
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and
|
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.B SEM_INFO
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operations are used by the
|
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.BR ipcs (1)
|
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program to provide information on allocated resources.
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In the future these may modified or moved to a
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.I /proc
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filesystem interface.
|
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.PP
|
|
Various fields in a \fIstruct semid_ds\fP were typed as
|
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.I short
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under Linux 2.2
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and have become
|
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.I long
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under Linux 2.4.
|
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To take advantage of this,
|
|
a recompilation under glibc-2.1.91 or later should suffice.
|
|
(The kernel distinguishes old and new calls by an
|
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.B IPC_64
|
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flag in
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.IR cmd .)
|
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.PP
|
|
In some earlier versions of glibc, the
|
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.I semun
|
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union was defined in \fI<sys/sem.h>\fP, but POSIX.1 requires
|
|
.\" POSIX.1-2001, POSIX.1-2008
|
|
that the caller define this union.
|
|
On versions of glibc where this union is \fInot\fP defined,
|
|
the macro
|
|
.B _SEM_SEMUN_UNDEFINED
|
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is defined in \fI<sys/sem.h>\fP.
|
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.PP
|
|
The following system limit on semaphore sets affects a
|
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.BR semctl ()
|
|
call:
|
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.TP
|
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.B SEMVMX
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Maximum value for
|
|
.BR semval :
|
|
implementation dependent (32767).
|
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.PP
|
|
For greater portability, it is best to always call
|
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.BR semctl ()
|
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with four arguments.
|
|
.\"
|
|
.SS The sempid value
|
|
POSIX.1 defines
|
|
.I sempid
|
|
as the "process ID of [the] last operation" on a semaphore,
|
|
and explicitly notes that this value is set by a successful
|
|
.BR semop (2)
|
|
call, with the implication that no other interface affects the
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.I sempid
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value.
|
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.PP
|
|
While some implementations conform to the behavior specified in POSIX.1,
|
|
others do not.
|
|
(The fault here probably lies with POSIX.1 inasmuch as it likely failed
|
|
to capture the full range of existing implementation behaviors.)
|
|
Various other implementations
|
|
.\" At least OpenSolaris (and, one supposes, older Solaris) and Darwin
|
|
also update
|
|
.I sempid
|
|
for the other operations that update the value of a semaphore: the
|
|
.B SETVAL
|
|
and
|
|
.B SETALL
|
|
operations, as well as the semaphore adjustments performed
|
|
on process termination as a consequence of the use of the
|
|
.B SEM_UNDO
|
|
flag (see
|
|
.BR semop (2)).
|
|
.PP
|
|
Linux also updates
|
|
.I sempid
|
|
for
|
|
.BR SETVAL
|
|
operations and semaphore adjustments.
|
|
However, somewhat inconsistently, up to and including 4.5,
|
|
Linux did not update
|
|
.I sempid
|
|
for
|
|
.BR SETALL
|
|
operations.
|
|
This was rectified
|
|
.\" commit a5f4db877177d2a3d7ae62a7bac3a5a27e083d7f
|
|
in Linux 4.6.
|
|
.SH SEE ALSO
|
|
.BR ipc (2),
|
|
.BR semget (2),
|
|
.BR semop (2),
|
|
.BR capabilities (7),
|
|
.BR sem_overview (7),
|
|
.BR svipc (7)
|