semget — get a System V semaphore set identifier
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>
int
semget( |
key_t key, |
| int nsems, | |
int semflg); |
The semget() system call
returns the System V semaphore set identifier associated with
the argument key. A
new set of nsems
semaphores is created if key has the value IPC_PRIVATE or if no existing semaphore set
is associated with key and IPC_CREAT is specified in semflg.
If semflg
specifies both IPC_CREAT and
IPC_EXCL and a semaphore set
already exists for key, then semget() fails with errno set to EEXIST. (This is analogous to the effect
of the combination O_CREAT |
O_EXCL for open(2).)
Upon creation, the least significant 9 bits of the
argument 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 mode argument of open(2) (though the execute
permissions are not meaningful for semaphores, and write
permissions mean permission to alter semaphore values).
When creating a new semaphore set, semget() initializes the set's associated
data structure, semid_ds (see semctl(2)), as follows:
sem_perm.cuid and
sem_perm.uid
are set to the effective user ID of the calling
process.
sem_perm.cgid and
sem_perm.gid
are set to the effective group ID of the calling
process.
The least significant 9 bits of sem_perm.mode are set
to the least significant 9 bits of semflg.
sem_nsems
is set to the value of nsems.
sem_otime
is set to 0.
sem_ctime
is set to the current time.
The argument nsems
can be 0 (a don't care) when a semaphore set is not being
created. Otherwise, nsems must be greater than 0
and less than or equal to the maximum number of semaphores
per semaphore set (SEMMSL).
If the semaphore set already exists, the permissions are verified.
If successful, the return value will be the semaphore set
identifier (a nonnegative integer), otherwise, −1 is
returned, with errno indicating
the error.
On failure, errno will be set
to one of the following:
A semaphore set exists for key, but the calling
process does not have permission to access the set, and
does not have the CAP_IPC_OWNER capability.
IPC_CREAT and
IPC_EXCL were specified
in semflg, but
a semaphore set already exists for key.
nsems is
less than 0 or greater than the limit on the number of
semaphores per semaphore set (SEMMSL).
A semaphore set corresponding to key already exists, but
nsems is larger
than the number of semaphores in that set.
No semaphore set exists for key and semflg did not specify
IPC_CREAT.
A semaphore set has to be created but the system does not have enough memory for the new data structure.
A semaphore set has to be created but the system
limit for the maximum number of semaphore sets
(SEMMNI), or the system
wide maximum number of semaphores (SEMMNS), would be exceeded.
The inclusion of <sys/types.h> and <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.
IPC_PRIVATE isn't a flag
field but a key_t type. If this
special value is used for key, the system call ignores
all but the least significant 9 bits of semflg and creates a new
semaphore set (on success).
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.
Initialization can be done using semctl(2) SETVAL or SETALL operation. Where multiple peers do
not know who will be the first to initialize the set,
checking for a nonzero sem_otime in the associated
data structure retrieved by a semctl(2) IPC_STAT operation can be used to avoid
races.
The following limits on semaphore set resources affect
the semget() call:
SEMMNISystem-wide limit on the number of semaphore sets.
On Linux systems before version 3.19, the default
value for this limit was 128. Since Linux 3.19, the
default value is 32,000. On Linux, this limit can be
read and modified via the fourth field of
/proc/sys/kernel/sem.
SEMMSLMaximum number of semaphores per semaphore ID. On
Linux systems before version 3.19, the default value
for this limit was 250. Since Linux 3.19, the default
value is 32,000. On Linux, this limit can be read and
modified via the first field of /proc/sys/kernel/sem.
SEMMNSSystem-wide limit on the number of semaphores:
policy dependent (on Linux, this limit can be read
and modified via the second field of /proc/sys/kernel/sem). Note that
the number of semaphores system-wide is also limited
by the product of SEMMSL and SEMMNI.
The name choice IPC_PRIVATE
was perhaps unfortunate, IPC_NEW would more clearly show its
function.
This page is part of release 4.00 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
http://www.kernel.org/doc/man−pages/.
|
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esrthyrsus.com> Modified 1 Jan 2002, Martin Schulze <joeyinfodrom.org> Modified 4 Jan 2002, Michael Kerrisk <mtk.manpagesgmail.com> Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com> Language and formatting clean-ups Added notes on /proc files Rewrote BUGS note about semget()'s failure to initialize semaphore values |