SEMGET — C Library Procedures

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.

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() 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 (see intro(2)) 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) and (semflg & IPC_EXCL)) is “true”.

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