The semctl function provides a variety of semaphore control
    operations as specified by cmd. The fourth argument is optional
    and depends upon the operation requested. If required, it is of
    type union semun, which is explicitly declared as:

    union semun {
        int val;
        struct semid_ds *buf;
        unsigned short  *array;
    } arg;

    The following semaphore control operations as specified by cmd
    are executed with respect to the semaphore specified by semid and
    semnum. The level of permission required for each operation is
    shown with each command. The symbolic names for the values of cmd
    are defined in the <sem.h> header:

    o  GETVAL

       Returns the value of semval. Requires read permission.

    o  SETVAL

       Sets the value of semval to arg.val, where arg is the value
       of the fourth argument to semctl. When this command is
       successfully executed, the semadj value corresponding to the
       specified semaphore in all processes is cleared. Requires
       alter permission.

    o  GETPID

       Returns the value of sempid; requires read permission.

    o  GETNCNT

       Returns the value of semncnt; requires read permission.

    o  GETZCNT

       Returns the value of semzcnt; requires read permission.

    The following values of cmd operate on each semval in the set of
    semaphores:

    o  GETALL

       Returns the value of semval for each semaphore in the
       semaphore set and places it into the array pointed to by
       arg.array, where arg is the fourth argument to semctl;
       requires read permission.

    o  SETALL

       Sets the value of semval for each semaphore in the semaphore
       set according to the array pointed to by arg.array, where
       arg is the fourth argument to semctl. When this command is
       successfully executed, the semadj values corresponding to each
       specified semaphore in all processes are cleared. Requires
       alter permission.

    The following values of cmd are also available:

    o  IPC_STAT

       Places the current value of each member of the semid_ds data
       structure associated with semid into the structure pointed to
       by arg.buf, where arg is the fourth argument to semctl. The
       contents of this structure are defined in <sem.h>. Requires
       read permission.

    o  IPC_SET

       Sets the value of the following members of the semid_ds data
       structure associated with semid to the corresponding value
       found in the structure pointed to by arg.buf, where arg is the
       fourth argument to semctl:

       sem_perm.uid
       sem_perm.gid
       sem_perm.mode

       The mode bits specified in The Open Group Base Specifications
       IPC General Description section are copied into the
       corresponding bits of the sem_perm.mode associated with semid.
       The stored values of any other bits are unspecified.

       This command can only be executed by a process that has an
       effective user ID equal to either that of a process with
       appropriate privileges or to the value of sem_perm.cuid or
       sem_perm.uid in the semid_ds data structure associated with
       semid.

    o  IPC_RMID

       Removes the semaphore identifier specified by semid from
       the system and destroys the set of semaphores and semid_ds
       data structure associated with it. This command can only be
       executed by a process that has an effective user ID equal to
       either that of a process with appropriate privileges or to the
       value of sem_perm.cuid or sem_perm.uid in the semid_ds data
       structure associated with semid.