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: 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 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.  Otherwise, 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 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. 

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

SEE ALSO

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

STANDARDS CONFORMANCE

semget(): SVID2, XPG2, XPG3, XPG4

Hewlett-Packard Company  —  HP-UX Release 9.0: August 1992