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-.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
-.\" and Copyright (C) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com>
-.\" Modified 1 Jan 2002, Martin Schulze <joey@infodrom.org>
-.\" Modified 4 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
-.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
-.\" Added notes on capability requirements
-.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
-.\" Language and formatting clean-ups
-.\" Added notes on /proc files
-.\" Rewrote BUGS note about semget()'s failure to initialize
-.\" semaphore values
-.\"
-.TH semget 2 2023-10-31 "Linux man-pages 6.7"
-.SH NAME
-semget \- get a System V semaphore set identifier
-.SH LIBRARY
-Standard C library
-.RI ( libc ", " \-lc )
-.SH SYNOPSIS
-.nf
-.B #include <sys/sem.h>
-.fi
-.P
-.BI "int semget(key_t " key ,
-.BI "int " nsems ,
-.BI "int " semflg );
-.SH DESCRIPTION
-The
-.BR semget ()
-system call returns the System\ V semaphore set identifier
-associated with the argument
-.IR key .
-It may be used either to obtain the identifier of a previously created
-semaphore set (when
-.I semflg
-is zero and
-.I key
-does not have the value
-.BR IPC_PRIVATE ),
-or to create a new set.
-.P
-A new set of
-.I nsems
-semaphores is created if
-.I key
-has the value
-.B IPC_PRIVATE
-or if no existing semaphore set is associated with
-.I key
-and
-.B IPC_CREAT
-is specified in
-.IR semflg .
-.P
-If
-.I semflg
-specifies both
-.B IPC_CREAT
-and
-.B IPC_EXCL
-and a semaphore set already exists for
-.IR key ,
-then
-.BR semget ()
-fails with
-.I errno
-set to
-.BR EEXIST .
-(This is analogous to the effect of the combination
-.B O_CREAT | O_EXCL
-for
-.BR open (2).)
-.P
-Upon creation, the least significant 9 bits of the argument
-.I semflg
-define the permissions (for owner, group, and others)
-for the semaphore set.
-These bits have the same format, and the same
-meaning, as the
-.I mode
-argument of
-.BR open (2)
-(though the execute permissions are
-not meaningful for semaphores, and write permissions mean permission
-to alter semaphore values).
-.P
-When creating a new semaphore set,
-.BR semget ()
-initializes the set's associated data structure,
-.I semid_ds
-(see
-.BR semctl (2)),
-as follows:
-.IP \[bu] 3
-.I sem_perm.cuid
-and
-.I sem_perm.uid
-are set to the effective user ID of the calling process.
-.IP \[bu]
-.I sem_perm.cgid
-and
-.I sem_perm.gid
-are set to the effective group ID of the calling process.
-.IP \[bu]
-The least significant 9 bits of
-.I sem_perm.mode
-are set to the least significant 9 bits of
-.IR semflg .
-.IP \[bu]
-.I sem_nsems
-is set to the value of
-.IR nsems .
-.IP \[bu]
-.I sem_otime
-is set to 0.
-.IP \[bu]
-.I sem_ctime
-is set to the current time.
-.P
-The argument
-.I nsems
-can be 0
-(a don't care)
-when a semaphore set is not being created.
-Otherwise,
-.I nsems
-must be greater than 0
-and less than or equal to the maximum number of semaphores per semaphore set
-.RB ( SEMMSL ).
-.P
-If the semaphore set already exists, the permissions are
-verified.
-.\" and a check is made to see if it is marked for destruction.
-.SH RETURN VALUE
-On success,
-.BR semget ()
-returns the semaphore set identifier (a nonnegative integer).
-On failure, \-1 is returned, and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-.TP
-.B EACCES
-A semaphore set exists for
-.IR key ,
-but the calling process does not have permission to access the set,
-and does not have the
-.B CAP_IPC_OWNER
-capability in the user namespace that governs its IPC namespace.
-.TP
-.B EEXIST
-.B IPC_CREAT
-and
-.B IPC_EXCL
-were specified in
-.IR semflg ,
-but a semaphore set already exists for
-.IR key .
-.\" .TP
-.\" .B EIDRM
-.\" The semaphore set is marked to be deleted.
-.TP
-.B EINVAL
-.I nsems
-is less than 0 or greater than the limit on the number
-of semaphores per semaphore set
-.RB ( SEMMSL ).
-.TP
-.B EINVAL
-A semaphore set corresponding to
-.I key
-already exists, but
-.I nsems
-is larger than the number of semaphores in that set.
-.TP
-.B ENOENT
-No semaphore set exists for
-.I key
-and
-.I semflg
-did not specify
-.BR IPC_CREAT .
-.TP
-.B ENOMEM
-A semaphore set has to be created but the system does not have
-enough memory for the new data structure.
-.TP
-.B ENOSPC
-A semaphore set has to be created but the system limit for the maximum
-number of semaphore sets
-.RB ( SEMMNI ),
-or the system wide maximum number of semaphores
-.RB ( SEMMNS ),
-would be exceeded.
-.SH STANDARDS
-POSIX.1-2008.
-.SH HISTORY
-SVr4, POSIX.1-2001.
-.\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
-.\" ERANGE, EFAULT.
-.SH NOTES
-.B IPC_PRIVATE
-isn't a flag field but a
-.I key_t
-type.
-If this special value is used for
-.IR key ,
-the system call ignores all but the least significant 9 bits of
-.I semflg
-and creates a new semaphore set (on success).
-.\"
-.SS Semaphore initialization
-The values of the semaphores in a newly created set are indeterminate.
-(POSIX.1-2001 and POSIX.1-2008 are explicit on this point,
-although POSIX.1-2008 notes that a future version of the standard
-may require an implementation to initialize the semaphores to 0.)
-Although Linux, like many other implementations,
-initializes the semaphore values to 0,
-a portable application cannot rely on this:
-it should explicitly initialize the semaphores to the desired values.
-.\" In truth, every one of the many implementations that I've tested sets
-.\" the values to zero, but I suppose there is/was some obscure
-.\" implementation out there that does not.
-.P
-Initialization can be done using
-.BR semctl (2)
-.B SETVAL
-or
-.B SETALL
-operation.
-Where multiple peers do not know who will be the first to
-initialize the set, checking for a nonzero
-.I sem_otime
-in the associated data structure retrieved by a
-.BR semctl (2)
-.B IPC_STAT
-operation can be used to avoid races.
-.\"
-.SS Semaphore limits
-The following limits on semaphore set resources affect the
-.BR semget ()
-call:
-.TP
-.B SEMMNI
-System-wide limit on the number of semaphore sets.
-Before Linux 3.19,
-the default value for this limit was 128.
-Since Linux 3.19,
-.\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
-the default value is 32,000.
-On Linux, this limit can be read and modified via the fourth field of
-.IR /proc/sys/kernel/sem .
-.\" This /proc file is not available in Linux 2.2 and earlier -- MTK
-.TP
-.B SEMMSL
-Maximum number of semaphores per semaphore ID.
-Before Linux 3.19,
-the default value for this limit was 250.
-Since Linux 3.19,
-.\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
-the default value is 32,000.
-On Linux, this limit can be read and modified via the first field of
-.IR /proc/sys/kernel/sem .
-.TP
-.B SEMMNS
-System-wide limit on the number of semaphores: policy dependent
-(on Linux, this limit can be read and modified via the second field of
-.IR /proc/sys/kernel/sem ).
-Note that the number of semaphores system-wide
-is also limited by the product of
-.B SEMMSL
-and
-.BR SEMMNI .
-.SH BUGS
-The name choice
-.B IPC_PRIVATE
-was perhaps unfortunate,
-.B IPC_NEW
-would more clearly show its function.
-.SH EXAMPLES
-The program shown below uses
-.BR semget ()
-to create a new semaphore set or retrieve the ID of an existing set.
-It generates the
-.I key
-for
-.BR semget ()
-using
-.BR ftok (3).
-The first two command-line arguments are used as the
-.I pathname
-and
-.I proj_id
-arguments for
-.BR ftok (3).
-The third command-line argument is an integer that specifies the
-.I nsems
-argument for
-.BR semget ().
-Command-line options can be used to specify the
-.B IPC_CREAT
-.RI ( \-c )
-and
-.B IPC_EXCL
-.RI ( \-x )
-flags for the call to
-.BR semget ().
-The usage of this program is demonstrated below.
-.P
-We first create two files that will be used to generate keys using
-.BR ftok (3),
-create two semaphore sets using those files, and then list the sets using
-.BR ipcs (1):
-.P
-.in +4n
-.EX
-$ \fBtouch mykey mykey2\fP
-$ \fB./t_semget \-c mykey p 1\fP
-ID = 9
-$ \fB./t_semget \-c mykey2 p 2\fP
-ID = 10
-$ \fBipcs \-s\fP
-\&
-\-\-\-\-\-\- Semaphore Arrays \-\-\-\-\-\-\-\-
-key semid owner perms nsems
-0x7004136d 9 mtk 600 1
-0x70041368 10 mtk 600 2
-.EE
-.in
-.P
-Next, we demonstrate that when
-.BR semctl (2)
-is given the same
-.I key
-(as generated by the same arguments to
-.BR ftok (3)),
-it returns the ID of the already existing semaphore set:
-.P
-.in +4n
-.EX
-$ \fB./t_semget \-c mykey p 1\fP
-ID = 9
-.EE
-.in
-.P
-Finally, we demonstrate the kind of collision that can occur when
-.BR ftok (3)
-is given different
-.I pathname
-arguments that have the same inode number:
-.P
-.in +4n
-.EX
-$ \fBln mykey link\fP
-$ \fBls \-i1 link mykey\fP
-2233197 link
-2233197 mykey
-$ \fB./t_semget link p 1\fP # Generates same key as \[aq]mykey\[aq]
-ID = 9
-.EE
-.in
-.SS Program source
-\&
-.\" SRC BEGIN (t_semget.c)
-.EX
-/* t_semget.c
-\&
- Licensed under GNU General Public License v2 or later.
-*/
-#include <stdio.h>
-#include <stdlib.h>
-#include <sys/ipc.h>
-#include <sys/sem.h>
-#include <unistd.h>
-\&
-static void
-usage(const char *pname)
-{
- fprintf(stderr, "Usage: %s [\-cx] pathname proj\-id num\-sems\en",
- pname);
- fprintf(stderr, " \-c Use IPC_CREAT flag\en");
- fprintf(stderr, " \-x Use IPC_EXCL flag\en");
- exit(EXIT_FAILURE);
-}
-\&
-int
-main(int argc, char *argv[])
-{
- int semid, nsems, flags, opt;
- key_t key;
-\&
- flags = 0;
- while ((opt = getopt(argc, argv, "cx")) != \-1) {
- switch (opt) {
- case \[aq]c\[aq]: flags |= IPC_CREAT; break;
- case \[aq]x\[aq]: flags |= IPC_EXCL; break;
- default: usage(argv[0]);
- }
- }
-\&
- if (argc != optind + 3)
- usage(argv[0]);
-\&
- key = ftok(argv[optind], argv[optind + 1][0]);
- if (key == \-1) {
- perror("ftok");
- exit(EXIT_FAILURE);
- }
-\&
- nsems = atoi(argv[optind + 2]);
-\&
- semid = semget(key, nsems, flags | 0600);
- if (semid == \-1) {
- perror("semget");
- exit(EXIT_FAILURE);
- }
-\&
- printf("ID = %d\en", semid);
-\&
- exit(EXIT_SUCCESS);
-}
-.EE
-.\" SRC END
-.SH SEE ALSO
-.BR semctl (2),
-.BR semop (2),
-.BR ftok (3),
-.BR capabilities (7),
-.BR sem_overview (7),
-.BR sysvipc (7)
generated by cgit v1.2.3 (git 2.39.1) at 2024-11-01 19:24:34 +0000