SEMGET
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index
Return to Main Contents
NAME
semget - get a System V semaphore set identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semget(key_t key,
int nsems,
int semflg);
DESCRIPTION
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.
RETURN VALUE
If successful, the return value will be the semaphore set identifier
(a nonnegative 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 in the user namespace that governs its IPC namespace.
- EEXIST
-
IPC_CREAT
and
IPC_EXCL
were specified in
semflg,
but a semaphore set already exists for
key.
- EINVAL
-
nsems
is less than 0 or greater than the limit on the number
of semaphores per semaphore set
(SEMMSL).
- EINVAL
-
A semaphore set corresponding to
key
already exists, but
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
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).
Semaphore initialization
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.
Semaphore limits
The following limits on semaphore set resources affect the
semget()
call:
- SEMMNI
-
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
/proc/sys/kernel/sem.
- SEMMSL
-
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
/proc/sys/kernel/sem.
- SEMMNS
-
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
SEMMSL
and
SEMMNI.
BUGS
The name choice
IPC_PRIVATE
was perhaps unfortunate,
IPC_NEW
would more clearly show its function.
SEE ALSO
semctl(2),
semop(2),
ftok(3),
capabilities(7),
sem_overview(7),
svipc(7)
COLOPHON
This page is part of release 4.13 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
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- Semaphore initialization
-
- Semaphore limits
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
|
|
- Running on 
-
:
: 