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

# 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)

October 23, 2021 | Debian