SEMGET(2) | System Calls Manual | SEMGET(2)

# NAME

`semget` —
get set of semaphores

# LIBRARY

Standard C Library (libc, -lc)

# 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. 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_CREAT` and `IPC_EXCL` are 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_CREAT` is not set in semflg and no semaphore set associated with key was found.

# SEE ALSO

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

# 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.

May 13, 2004 | NetBSD 9.5