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 (see intro(2)) are created for key if one of the following are true:

Key is equal to IPC_PRIVATE. 

Key does not already have a semaphore identifier associated with it, and (semflg & IPC_CREAT) is “true”. 

Upon creation, the data structure associated with the new semaphore identifier is initialized as follows:

Sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid are set equal to the effective user ID and effective group ID, respectively, 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. 

Semget will fail if one or more of the following are true:

Nsems is either less than or equal to zero or greater than the system imposed limit.  ­[EINVAL]

A semaphore identifier exists for key but operation permission (see intro(2)) as specified by the low-order 9 bits of semflg would not be granted.  ­[EACCES]

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.  ­[EINVAL]

A semaphore identifier does not exist for key and (semflg & IPC_CREAT) is “false”.  ­[ENOENT]

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.  ­[ENOSPC]

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

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

semctl(2), semop(2). 
 

Version 2.5  —  April 22, 1987