Section: Linux Programmer's Manual (2)
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semget - get a semaphore set identifier
int semget(key_t key,
system call returns the semaphore set identifier
associated with the argument
A new set of
semaphores is created if
has the value
or if no existing semaphore set is associated with
is specified in
and a semaphore set already exists for
(This is analogous to the effect of the combination
O_CREAT | O_EXCL
Upon creation, the least significant 9 bits of the argument
define the permissions (for owner, group and others)
for the semaphore set.
These bits have the same format, and the same
meaning, as the
(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,
initialises the semaphore values to 0,
a portable application cannot rely on this:
it should explicitly initialise the semaphores to the desired values.
When creating a new semaphore set,
initialises the set's associated data structure,
are set to the effective user ID of the calling process.
are set to the effective group ID of the calling process.
The least significant 9 bits of
are set to the least significant 9 bits of
is set to the value of
is set to 0.
is set to the current time.
can be 0
(a don't care)
when a semaphore set is not being created.
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
If successful, the return value will be the semaphore set identifier
(a nonnegative integer), otherwise -1
is returned, with
indicating the error.
will be set to one of the following:
A semaphore set exists for
but the calling process does not have permission to access the set,
and does not have the
A semaphore set exists for
is less than 0 or greater than the limit on the number
of semaphores per semaphore set
or a semaphore set corresponding to
already exists, and
is larger than the number of semaphores in that set.
No semaphore set exists for
did 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
or the system wide maximum number of semaphores
would be exceeded.
isn't a flag field but a
If this special value is used for
the system call ignores everything but the least significant 9 bits of
and creates a new semaphore set (on success).
The following limits on semaphore set resources affect the
System wide maximum number of semaphore sets: policy dependent
(on Linux, this limit can be read and modified via the fourth field of
Maximum number of semaphores per semid: implementation dependent
(on Linux, this limit can be read and modified via the first field of
System wide maximum number of semaphores: policy dependent
(on Linux, this limit can be read and modified via the second field of
Values greater than
SEMMSL * SEMMNI
makes it irrelevant.
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
The semaphores in a set are not initialised by
In order to initialise the semaphores,
must be used to perform a
operation on the semaphore set.
(Where multiple peers do not know who will be the first to
initialise the set, checking for a non-zero
in the associated data structure retrieved by a
operation can be used to avoid races.)
- RETURN VALUE
- CONFORMING TO
- SEE ALSO
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