NAME

semget − get set of semaphores

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semget (key, nsems, semflg)
key_t key;
int nsems, 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 will never be 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 may be requested by or’ing the following masks into semflg.

IPC_CREAT: Create a semaphore identifier, if one does not already exist for key. 

IPC_EXCL: If IPC_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 the glossary.

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);

ERRORS

Semget will fail if one or more of the following are 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 associated 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 maximum number of allowed semaphore identifiers system wide would be exceeded. 

[ENOSPC] A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system wide would be exceeded. 

[EEXIST] A semaphore identifier exists for key but ( (semflg & IPC_CREAT) && ( semflg & IPC_EXCL) ) is “true”. 

RETURN VALUE

Upon successful completion, a non-negative integer, namely a semaphore identifier, is returned.  Otherwise, a value of −1 is returned and errno is set to indicate the error. 

SEE ALSO

ipcrm(1), ipcs(1), semctl(2), semop(2), stdipc(3C). 

STANDARDS CONFORMANCE

semget: SVID2, XPG2, XPG3

Hewlett-Packard Company  —  HP-UX Release 7.0: Sept 1989