Section (2) semget
semget — get a System V semaphore set identifier
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>
semget() system call
returns the System V semaphore set identifier associated with
may be used either to obtain the identifier of a previously
created semaphore set (when
semflg is zero and
key does not have the value
IPC_PRIVATE), or to create a
A new set of
semaphores is created if
key has the value
IPC_PRIVATE or if no existing semaphore set
is associated with
IPC_CREAT is specified in
IPC_EXCL and a semaphore set
already exists for
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
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_zsingle_quotesz_s associated
semid_ds (see semctl(2)), as follows:
sem_perm.uidare set to the effective user ID of the calling process.
sem_perm.gidare set to the effective group ID of the calling process.
The least significant 9 bits of
sem_perm.modeare set to the least significant 9 bits of
sem_nsemsis set to the value of
sem_otimeis set to 0.
sem_ctimeis set to the current time.
can be 0 (a don_zsingle_quotesz_t care) when a semaphore set is not being
nsems must be greater than 0
and less than or equal to the maximum number of semaphores
per semaphore set (
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
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_OWNERcapability in the user namespace that governs its IPC namespace.
IPC_EXCLwere specified in
semflg, but a semaphore set already exists for
nsemsis less than 0 or greater than the limit on the number of semaphores per semaphore set (
A semaphore set corresponding to
keyalready exists, but
nsemsis larger than the number of semaphores in that set.
No semaphore set exists for
semflgdid not specify
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
isn_zsingle_quotesz_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_zsingle_quotesz_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)
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
The following limits on semaphore set resources affect
System-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
Maximum 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
System-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
The name choice
was perhaps unfortunate,
IPC_NEW would more clearly show its
This page is part of release 5.04 of the Linux
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and the latest version of this page, can be found at
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it)
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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()_zsingle_quotesz_s failure to initialize