Copyright (C) 1993, Digital Equipment Corporation. 
 All rights reserved. 
 See the file COPYRIGHT for a full description. 
 Last modified on Thu Jul 15 12:07:50 PDT 1993 by mcjones 

A Pipe.T, or pipe, is a file handle that provides access to one endpoint of a unidirectional channel that is typically used to communicate between a parent and a child process or two sibling processes. (See Process.Create.)


IMPORT File, OSError;

TYPE T <: File.T;

VAR (*CONST*) FileType: File.Type;
Equal to {\tt Atom.FromText("Pipe").}

PROCEDURE Open(VAR (*OUT*) hr, hw: T) RAISES {OSError.E};
Create a new channel allowing bytes written to hw to be read from hr.

END Pipe.
Like every File.T, a pipe h has the components

      type(h)      an atom, equal to FileType
      readable(h)  a boolean
      writable(h)  a boolean
Exactly one of readable(h) and writable(h) is true (until the pipe is closed).

A pipe h also has the component

      channel(h)   a channel
If there are pipes hw and hr with channel(hw) = channel(hr), writable(hw), and readable(hr), then a process holding hw can send information to a process holding hr.

A channel c has the components

      seq(c)  a sequence of bytes
      w(c)    a non-negative integer, the index of the next byte to write
      r(c)    a non-negative integer, the index of the next byte to read
      nw(c)   a non-negative integer, the number of pipes writing c
      nr(c)   a non-negative integer, the number of pipes reading c
It is possible (but not very useful) for a channel to have values of nw(c) or nr(c) other than zero or one (see Process.Create).

Open creates a channel c with

      w(c) = r(c) = 0
      nw(c) = nr(c) = 1
and two pipes hr and hw with

      type(hr) = type(hw) = FileType
      readable(hr) = writable(hw) = TRUE
      writable(hr) = readable(hw) = FALSE
      channel(hr) = channel(hw) = c

The meaning of the call, mayBlock)
is given by the specification of together with these definitions, where c = channel(h):

      src(h)    = seq(c)
      srcCur(h) = r(c)
      srcEof(h) = (nw(c) = 0)
Note that end-of-file is not reported until after the last pipe that can write on the channel is closed; subsequent reads are legal but always report end-of-file.

The meaning of the call

is given by the specification of File.T.write together with these definitions, where c = channel(h):

      snk(h)    = seq(c)
      snkCur(h) = w(c)
In some implementations, a channel has a bounded buffer, so write may have to block. If nr(channel(h)) = 0, that is, no pipe can read h's channel, write raises OSError.E.

The call

assigns FileType to stat.type. Its effect on stat.modificationTime and stat.size is undefined.

The call

is equivalent to

      IF readable(h) THEN
      readable(h) := FALSE;
      writable(h) := FALSE
The channel connecting a pair of pipes is necessarily monitored, since the purpose of the channel is to allow asynchronous communication via the pipes. Nevertheless, an individual pipe should be treated as unmonitored, thus avoiding the question of the unit of atomicity for reads and writes.

Pipe's implementation is in: