May 13, 2004 SEMGET 2

### NAME

semget - get set of semaphores

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```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`.