Section: Linux Programmer's Manual (2)
Return to Main Contents
semget - get a System V semaphore set identifier
int semget(key_t key,
system call returns the System V 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).
When creating a new semaphore set,
initializes 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
were specified in
but a semaphore set already exists for
is less than 0 or greater than the limit on the number
of semaphores per semaphore set
A semaphore set corresponding to
already exists, but
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.
The inclusion of
isn't required on Linux or by any version of POSIX.
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.
isn't a flag field but a
If this special value is used for
the system call ignores all but the least significant 9 bits of
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
Where multiple peers do not know who will be the first to
initialize the set, checking for a nonzero
in the associated data structure retrieved by a
operation can be used to avoid races.
The following limits on semaphore set resources affect the
System-wide limit on the 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 semaphore ID: implementation dependent
(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
Note that number of semaphores system-wide is also limited by the product of
The name choice
was perhaps unfortunate,
would more clearly show its function.
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- RETURN VALUE
- CONFORMING TO
- Semaphore initialization
- Semaphore limits
- SEE ALSO