HP-UX Reference (11i v1 00/12) - 2 System Calls (vol 5)
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STANDARD Printed by: Nora Chuang [nchuang] STANDARD
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semget(2) semget(2)
NAME
semget - get set of semaphores
SYNOPSIS
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
DESCRIPTION
semget() returns the semaphore identifier associated with key.
A semaphore identifier and associated data structure and set containing nsems semaphores are created for
key if one of the following is true:
key is equal to IPC_PRIVATE . This call creates a new identifier, subject to available resources. The
identifier is never returned by another call to semget() until it has been released by a call to
semctl(). The identifier should be used among the calling process and its descendents; however, it
is not a requirement. The resource can be accessed by any process having the proper permissions.
key does not already have a semaphore identifier associated with it, and (semflg &
IPC_CREAT)is
‘‘true’’.
Specific behavior can be requested by
ORing the following masks into semflg.
IPC_CREAT: Create a semaphore identifier if one does not already exist for key.
IPC_EXCL:IfIPC_CREAT is specified and key already has a semaphore identifier associated with
it, return an error.
The low-order 9 bits of semflg are the semaphore operation permissions which are defined in glossary(9).
Upon creation, the data structure associated with the new semaphore identifier is initialized as follows:
In the operation-permission structure,
sem_perm.cuid and sem_perm.uid are set equal to the
effective-user-ID of the calling process, while
sem_perm.cgid and sem_perm.gid are set to
the effective-group-ID of the calling process.
The low-order 9 bits of
sem_perm.mode
are set equal to the low-order 9 bits of semflg.
sem_nsems is set equal to the value of nsems.
sem_otime is set equal to 0 and sem_ctime is set equal to the current time.
EXAMPLES
The following call to semget() returns a semid associated with the key returned by ftok("myfile",
’A’). If a semid associated with the key does not exist, a new semid, set of 4 semaphores, and associated
data structure will be created. If a semid for the key already exists, the semid is simply returned.
int semid;
mysemid = semget (ftok("myfile",’A’), 4, IPC_CREAT | 0600);
RETURN VALUE
Upon successful completion, a non-negative integer, namely a semaphore identifier, is returned. Other-
wise, a value of −1 is returned and errno is set to indicate the error.
ERRORS
semget() fails if one or more of the following is true:
[EINVAL] nsems is either less than or equal to zero or greater than the system-imposed limit.
[EACCES] A semaphore identifier exists for key, but operation permission as specified by the
low-order 9 bits of semflg would not be granted.
[EINVAL] A semaphore identifier exists for key, but the number of semaphores in the set associ-
ated with it is less than nsems, and nsems is not equal to zero.
[ENOENT] A semaphore identifier does not exist for key and (semflg & IPC_CREAT ) is ‘‘false’’.
[ENOSPC] A semaphore identifier is to be created, but the system-imposed limit on the max-
imum number of allowed semaphore identifiers system wide would be exceeded.
HP-UX Release 11i: December 2000 − 1 − Section 2−−289
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