SEMGET(2) Linux Programmer's Manual SEMGET(2)
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 associ
ated 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. sem
flg&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 oth
ers) 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 permis
sions, for a semaphore set, effectively means alter per
missions).
Furthermore, while creating, the system call initializes
the system semaphore set data structure semid_ds as fol
lows:
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
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SEMGET(2) Linux Programmer's Manual SEMGET(2)
than 0 and less or equal to the maximum number of
semaphores per semid, (SEMMSL).
If the semaphore set already exists, the access permis
sions 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 sys
tem has not enough memory for the new data
structure.
ENOSPC A semaphore set has to be created but the sys
tem limit for the maximum number of semaphore
sets (SEMMNI), or the system wide maximum num
ber 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 cre
ates 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: imple
mentation dependent (500 currently).
SEMMNS System wide maximum number of semaphores: pol
icy dependent. Values greater than SEMMSL *
SEMMNI makes it irrelevant.
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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 cre
ated, 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).
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