semget(2)                                                     semget(2)

  semget()

  NAME

    semget() - get or create a set of semaphores

  SYNOPSIS

    #include 

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

  DESCRIPTION

    The semget(2) function returns the semaphore identifier associated with
    key. If the following are true, semget(2) creates the semaphore
    identifier, its associated semid_ds data structure and the associated set
    of semaphores (the number of semaphores in the set is determined by
    nsems):
    *     The key is IPC_PRIVATE.
    *     The key doesn't already have a semaphore associated with it, and the
          value of (semflg & IPC_CREAT) is not zero.

    The semid_ds data structure has the following members:

    struct semid_ds {
       struct ipc_perm sem_perm      /* operation permission structure */
       unsigned short int sem_nsems  /* number of semaphores in set */
       time_t sem_otime              /* time of last semop() */
       time_t sem_ctime              /* time of last change by semctl() */
    }

    The new semid_ds data structure is initialized to:
    *     The IDs in the operation permissions structure are set to those of
          the effective IDs of the calling process. That is, the members
          sem_perm.cuid sem_perm.uid sem_perm.cgid and sem_perm.gid are set to
          the value of the calling process' effective user ID and effective
          group ID. (On INTERIX systems, these are the user ID and the group
          ID, respectively.)
    *     The low-order 9 bits of sem_perm.mode are set to the low-order 9
          bits of semflg.
    *     The value of sem_nsems is set to the value of nsems.
    *     The value of sem_otime is set to 0.
    *     The value of sem_ctime is set to the current time.

    Note:
        The data structure associated with each semaphore in the set is not
        initialized. Use the semctl(2) function with the command SETVAL or
        SETALL to initialize each semaphore.

    A semaphore contains the following members (the data structure is
    anonymous):
    unsigned short int     semval      semaphore value

    pid_t                  sempid      process ID of last operation

    unsigned short int     semncnt     number of processes waiting for
                                       semval to become greater than
                                       current value

    unsigned short int     semzcnt     number of processes waiting for
                                       semval to become 0

    The header file  includes .

  RETURN VALUE

    On success, semget(2) returns the semaphore identifier, a non-negative
    integer; on failure, it returns -1 and sets errno to indicate the error.

  ERRORS

    The semget(2) function can fail for these reasons:

    [EACCES]
        Although a semaphore identifier exists for key, its permissions are
        not compatible with the low-order 9 bits of semflg.

    [EEXIST]
        Although a semaphore identifier exists for key, the value of ((semflg
        & IPC_CREAT) && (semflg & IPC_EXCL)) is non-zero.

    [EINVAL]
        The value of nsems is outside the system boundaries (less than or
        equal to 0 or greater than the limit), or an identifier exists for key
        but the number of semaphores in the set associated with it is less
        than nsems and nsems isn't equal to 0.

    [ENOENT]
        No semaphore identifier exists for key, and (semflg & IPC_CREAT)
        equals 0.

    [ENOSPC]
        Creating the identifier would exceed the system-wide maximum number of
        semaphores allowed.

  SEE ALSO

    semctl(2)

    semop(2)

  USAGE NOTES

    The semget function is thread safe.

    The semget function is not async-signal safe.