NAME
semget —
get set of semaphores
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/sem.h>
int
semget(key_t
key, int nsems,
int semflg);
DESCRIPTION
Thesemget()
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. If both the IPC_CREAT
bit and the IPC_EXCL bit are set in
semflg, and key has a semaphore
set associated with it already, the operation will fail.
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_CREATandIPC_EXCLare set in semflg, and a semaphore set is already associated with key. - [
EINVAL] - nsems is less than 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_CREATis not set in semflg and no semaphore set associated with key was found.
SEE ALSO
STANDARDS
The semget system call conforms to
X/Open System Interfaces and Headers Issue 5
(“XSH5”).
HISTORY
Semaphores appeared in the first release of AT&T System V UNIX.