NAME

semget — get semaphore set

SYNOPSIS

#include <sys/sem.h>

int
semget(key_t key, int nsems, int semflg);

DESCRIPTION

The semget() system call returns the semaphore identifier associated with key.

A new set containing nsems semaphores is created if either key is equal to IPC_PRIVATE, or key does not have a semaphore set associated with it and the IPC_CREAT bit is set in semflg.

The access modes of the created semaphores is specified in semflg as a bitwise OR of zero or more of the following values:

SEM_A        alter permission for owner
SEM_R        read permission for owner

SEM_A >> 3   alter permission for group
SEM_R >> 3   read permission for group

SEM_A >> 6   alter permission for other
SEM_R >> 6   read permission for other

If a new set of semaphores is created, the data structure associated with it (the semid_ds structure, see semctl(2)) is initialized as follows:

• sem_perm.cuid and sem_perm.uid are set to the effective UID of the calling process.
• sem_perm.gid and sem_perm.cgid are set to the effective GID of the calling process.
• sem_perm.mode is set to the lower 9 bits of semflg.
• sem_nsems is set to the value of nsems.
• sem_ctime is set to the current time.
• sem_otime is set to 0.

RETURN VALUES

semget() returns a non-negative semaphore identifier if successful. Otherwise, -1 is returned and errno is set to reflect the error.

ERRORS

[EACCES]

The caller has no permission to access a semaphore set already associated with key.

[EEXIST]

Both IPC_CREAT and IPC_EXCL are set in semflg, and a semaphore set is already associated with key.

[EINVAL]

nsems is less than or equal to 0 or greater than the system limit for the number in a semaphore set.

A semaphore set associated with key exists, but has fewer semaphores than the number specified in nsems.

[ENOSPC]

A new set of semaphores could not be created because the system limit for the number of semaphores or the number of semaphore sets has been reached.

[ENOENT]

IPC_CREAT was not set in semflg and no semaphore set associated with key was found.

SEE ALSO

ipcrm(1), ipcs(1), semctl(2), semop(2), ftok(3)