Description

   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.