The
semget()
system call returns the 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).
The values of the semaphores in a newly created set are indeterminate.
(POSIX.1-2001 is explicit on this point.)
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.
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.
RETURN VALUE
If successful, the return value will be the semaphore set identifier
(a non-negative integer), otherwise -1
is returned, with
errno
indicating the error.
ERRORS
On failure
errno
will be set to one of the following:
EACCES
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.
EEXIST
A semaphore set exists for
key
and
semflg
specified both
IPC_CREAT
and
IPC_EXCL.
EINVAL
nsems
is less than 0 or greater than the limit on the number
of semaphores per semaphore set
(SEMMSL),
or a semaphore set corresponding to
key
already exists, and
nsems
is larger than the number of semaphores in that set.
ENOENT
No semaphore set exists for
key
and
semflg
did not specify
IPC_CREAT.
ENOMEM
A semaphore set has to be created but the system does not have
enough memory for the new data structure.
ENOSPC
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.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
IPC_PRIVATE
isn't a flag field but a
key_t
type.
If this special value is used for
key,
the system call ignores everything but the least significant 9 bits of
semflg
and creates a new semaphore set (on success).
The following limits on semaphore set resources affect the
semget()
call:
SEMMNI
System wide maximum number of semaphore sets: policy dependent
(on Linux, this limit can be read and modified via the fourth field of
/proc/sys/kernel/sem).
SEMMSL
Maximum number of semaphores per semid: implementation dependent
(on Linux, this limit can be read and modified via the first field of
/proc/sys/kernel/sem).
SEMMNS
System wide maximum number of semaphores: policy dependent
(on Linux, this limit can be read and modified via the second field of
/proc/sys/kernel/sem).
Values greater than
SEMMSL * SEMMNI
makes it irrelevant.
BUGS
The name choice
IPC_PRIVATE
was perhaps unfortunate,
IPC_NEW
would more clearly show its function.
The semaphores in a set are not initialized by
semget().
In order to initialize the semaphores,
semctl(2)
must be used to perform a
SETVAL
or a
SETALL
operation on the semaphore set.
(Where multiple peers do not know who will be the first to
initialize the set, checking for a non-zero
sem_otime
in the associated data structure retrieved by a
semctl(2)
IPC_STAT
operation can be used to avoid races.)
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
