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semget
Returns a semaphore identifier.
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semget (key_t key, int nsems, int semflg);
Parameters
key Either a user-defined key value to rendezvous with the
semaphore set, or IPC_PRIVATE. If IPC_PRIVATE is specified, a
new semaphore set is created, but other processes cannot
rendezvous by key. Refer to the description of ftok() for
details about obtaining user-defined key values
nsems The number of semaphores in the set. The maximum number of
semaphores per set is 4096.
semflg Valid flags for this function are:
IPC_CREAT If a semaphore set is not already associated with
key, a new semaphore identifier is allocated and a
semaphore set and data structure are associated
with it. If a semaphore identifier is already
associated with key, and IPC_EXCL is not specified,
semget() returns the semaphore identifier currently
associated with key.
IPC_EXCL If specified with IPC_CREAT, semget() returns an
error if a semaphore identifier is currently
associated with key.
MODE The lower nine bits of semflg contain the access
permission bits (similar to the nine-bit mask found
in file entries). They define the access rights
for the owner, the group, and other users on the
system.
Return Values
>=0 Success. A semaphore identifier is returned.
-1 An error occurred, and errno is set to indicate the error
condition.
Description
The semget() function returns a semaphore identifier associated with the
value passed in key. A semaphore identifier and the associated data
structure and semaphore set containing nsems semaphores are created for
key if one of the following conditions is true:
* The value passed in key is equal to IPC_PRIVATE.
* The value passed in key does not already have a semaphore
identifier associated with it, and the semflg specifies IPC_CREAT.
The data structure associated with the new semaphore identifier is
initialized to the following values:
sem_perm.cuid Effective user ID of the calling process (creator user
ID)
sem_perm.uid Effective user ID of the calling process (owner user ID)
sem_perm.cgid Effective group ID of the calling process (creator group
ID)
sem_perm.gid Effective group ID of the calling process (owner group
ID)
sem_perm.mode Low-order 9 bits are set equal to the low-order 9 bits of
semflg
sem_nsems The value of nsems
sem_otime 0
sem_ctime Current time
Implementation Considerations
The maximum number of semaphores per set is 4096.
An MPE/iX system manager can use the MPE/iX SVIPC utility to
interactively configure:
* The maximum number of semaphores in a semaphore set
* The maximum value for a semaphore
* The maximum semaphore operation value
* The maximum semaphore adjust value
* The maximum number of semaphore sets allowed system wide
* [REV BEG]
The maximum number of semaphore operations allowed per semop()
call[REV END]
Refer to the section "Managing SVID IPC Services" for more information.
Errors
If an error occurs, errno is set to one of the following values.
EACCES CAUSE A semaphore identifier exists for key and the calling
process does not have permission (specified by the
low-order 9 bits of semflg).
ACTION Ensure the calling process has access permissions
required to access the existing semaphore identifier.
EEXIST CAUSE A semaphore identifier exists for key and semflg
specifies both IPC_CREAT and IPC_EXCL.
ACTION To access the existing identifier for key, retry the
operation without the IPC_EXCL option. To create a
new semaphore set with IPC_EXCL, a unique key must be
specified.
EINVAL CAUSE nsems is either less 1 or greater than the
system-defined limit, or a semaphore identifier
exists for key and the number of semaphores in the
set is less than nsems and nsems is not equal to 0.
ACTION If accessing an existing semaphore set, make sure the
nsems value does not exceed the number of semaphores
in the existing set. If creating a new semaphore
set, make sure nsems does not exceed the
system-defined limit.
ENOENT CAUSE A semaphore identifier does not exist for key and
semflg does not specify IPC_CREAT.
ACTION If attempting to access an existing semaphore set,
make sure the right key value was specified. If a
new semaphore set should be created for that key if
none exists, then make sure IPC_CREAT is specified.
ENOSPC CAUSE The number of semaphore identifiers would exceed the
system-defined limit.
ACTION None. The operation can be retried if another
semaphore identifier is removed from the system.
Table 2-0. (cont.)
ESYSERR CAUSE An operating system error occurred that does not map
directly to any of the above errors.
ACTION Examine the MPE/iX process error stack for the type
of system error.
See Also
semctl(), semop(), SVID2 (Section 12)