View Source: semget(2) - Waikato Linux Users Group

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SEMGET
!!!SEMGET
NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
NOTES
BUGS
CONFORMING TO
SEE ALSO
----
!!NAME


semget - get a semaphore set identifier
!!SYNOPSIS


__# include
____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 semaphore set 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 __semget__ 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__ doesn't inhibit 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.
!!CONFORMING TO


SVr4, SVID. SVr4 documents additional error conditions
EINVAL, EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT.
!!SEE ALSO


ftok(3), ipc(5), semctl(2),
semop(2)
----
5 pages link to semget(2):
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Last edited on Tuesday, June 4, 2002 12:23:45 am by "perry"
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