SEMGET
Section: Linux Programmer's Manual (2)
Updated: November 1, 1993
Index
Return to Main Contents
NAME
semget - get a 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 function returns the semaphore set identifier
associated to the value of the argument
key.
A new set of
nsems
semaphores is created if
key
has value
IPC_PRIVATE
or
key
isn't
IPC_PRIVATE,
no existing message queue is associated to
key,
and
IPC_CREAT
is asserted in
semflg
(i.e.
semflg&IPC_CREAT
isn't zero).
The presence in
semflg
of the fields
IPC_CREAT
and
IPC_EXCL
plays the same role, with respect to the existence
of the semaphore set, as the presence
of
O_CREAT
and
O_EXCL
in the mode argument of the
open(2)
system call: i.e. the
msgget
function fails if
semflg
asserts both
IPC_CREAT
and
IPC_EXCL
and a semaphore set already exists for
key.
Upon creation, the lower 9 bits of the argument
semflg
define the access permissions (for owner, group and others)
to the semaphore set in the same format, and with the same
meaning, as for the access permissions parameter in the
open(2)
or
creat(2)
system calls (though the execute permissions are not used by the system,
and write permissions, for a semaphore set, effectively means alter
permissions).
Furthermore, while creating,
the system call initializes the system semaphore set data structure
semid_ds
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 lowest order 9 bits of
sem_perm.mode
are set to the lowest order 9 bit 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 the system call isn't a create one.
Otherwise
nsems
must be greater than
0
and less or equal to the maximum number of semaphores per semid,
(SEMMSL).
If the semaphore set already exists, the access permissions are
verified, and a check is made to see if it is marked for destruction.
RETURN VALUE
If successful, the return value will be the semaphore set identifier
(a positive integer), otherwise
-1
with
errno
indicating the error.
ERRORS
For a failing return,
errno
will be set to one among the following values:
- EACCES
-
A semaphore set exists for
key,
but the calling process has no access permissions to the set.
- EEXIST
-
A semaphore set exists for
key
and
semflg
was asserting both
IPC_CREAT
and
IPC_EXCL.
- EIDRM
-
The semaphore set is marked as to be deleted.
- ENOENT
-
No semaphore set exists for
key
and
semflg
wasn't asserting
IPC_CREAT.
- ENOMEM
-
A semaphore set has to be created but the system has not 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.
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 lowest order 9 bits of
semflg
and creates a new semaphore set (on success).
The followings are limits on semaphore set resources affecting a
semget
call:
- SEMMNI
-
System wide maximum number of semaphore sets: policy dependent.
- SEMMSL
-
Maximum number of semaphores per semid: implementation dependent
(500 currently).
- SEMMNS
-
System wide maximum number of semaphores: policy dependent.
Values greater than
SEMMSL * SEMMNI
makes it irrelevant.
BUGS
Use of
IPC_PRIVATE
don't inhibits to other processes the access to the allocated
semaphore set.
As for the files, there is currently no intrinsic way for a process to ensure
exclusive access to a semaphore set.
Asserting both
IPC_CREAT
and
IPC_EXCL
in
semflg
only ensures (on success) that a new semaphore set will be created,
it doesn't imply exclusive access to the semaphore set.
The data structure associated with each semaphore in the set
isn't initialized by the system call.
In order to initialize those data structures, one has to execute a
subsequent call to
semctl(2)
to perform a
SETVAL
or a
SETALL
command on the semaphore set.
SEE ALSO
ftok(3),
ipc(5),
semctl(2),
semop(2).
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
This document was created by
man2html,
using the manual pages.
Time: 12:25:29 GMT, March 22, 2025