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 1.4 IPC system calls
 ====================
 
 This section provides an overview of the IPC system calls. See the
 specific sections on each type of resource for details.
 
    Each type of mechanism provides a "get", "ctl" and one or more "op"
 system calls that allow the user to create or procure the resource
 (get), define its behaviour or destroy it (ctl) and manipulate the
 resources (op).
 
 1.4.1 The "get" system calls
 ----------------------------
 
 The `get' call typically takes a KEY and returns a numeric ID that is
 used for further access.  The ID is an index into the resource table. A
 sequence number is maintained and incremented when a resource is
 destroyed so that access using an obsolete ID is likely to fail.
 
    The user also specifies the permissions and other behaviour
 charecteristics for the current access. The flags are or-ed with the
 permissions when invoking system calls as in:
      msgflg = IPC_CREAT | IPC_EXCL | 0666;
      id = msgget (key, msgflg);
 
    * `key' : IPC_PRIVATE => new instance of resource is initialized.
 
    * `flags' :
           IPC_CREAT : resource created for KEY if it does not exist.
 
           IPC_CREAT | IPC_EXCL : fail if resource exists for KEY.
 
    * returns : an identifier used for all further access to the
      resource.
 
    Note that IPC_PRIVATE is not a flag but a special `key' that ensures
 (when the call is successful) that a new resource is created.
 
    Use of IPC_PRIVATE does not make the resource inaccessible to other
 users. For this you must set the access permissions appropriately.
 
    There is currently no way for a process to ensure exclusive access
 to a resource. IPC_CREAT | IPC_EXCL only ensures (on success) that a new
 resource was initialized. It does not imply exclusive access.
 
 See Also :  msgget,  semget,  shmget.
 
 1.4.2 The "ctl" system calls
 ----------------------------
 
 Provides or alters the information stored in the structure that
 describes the resource indexed by ID.
 
      #include <sys/msg.h>
      struct msqid_ds buf;
      err = msgctl (id, IPC_STAT, &buf);
      if (err)
              !$#%*
      else
              printf ("creator uid = %d\n", buf.msg_perm.cuid);
              ....
 
 Commands supported by all `ctl' calls:
    * IPC_STAT : read info on resource  specified by id into user
      allocated buffer. The user must have read access to the resource.
 
    * IPC_SET : write info from buffer into resource data structure. The
      user must be owner creator or super-user.
 
    * IPC_RMID : remove resource. The user must be the owner, creator or
      super-user.
 
    The IPC_RMID command results in immediate removal of a message queue
 or semaphore array. Shared memory segments however, are only destroyed
 upon the last detach after IPC_RMID is executed.
 
    The `semctl' call provides a number of command options that allow
 the user to determine or set the values of the semaphores in an array.
 
 See Also:  msgctl,  semctl,  shmctl.
 
 1.4.3 The "op" system calls
 ---------------------------
 
 Used to send or receive messages, read or alter semaphore values,
 attach or detach shared memory segments.  The IPC_NOWAIT flag will
 cause the operation to fail with error EAGAIN if the process has to
 wait on the call.
 
 `flags' : IPC_NOWAIT  => return with error if a wait is required.
 
 See Also:  msgsnd, msgrcv, semop, shmat,
  shmdt.
 

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