Copyright (C) 1993, Digital Equipment Corporation. 
 All rights reserved. 
 See the file COPYRIGHT for a full description. 
 Last modified on Fri Jul 15 13:45:59 PDT 1994 by mcjones 

A File.T, or {\em file handle}, is a source and/or sink of bytes. File handles provide an operating-system independent way to perform raw I/O. For buffered I/O, use the FileRd and FileWr interfaces instead. A file handle is created using OpenFile or OpenFileReadonly in the FS interface. \index{file!handle} \index{unbuffered file I/O} \index{file!unbuffered I/O} \index{I/O!unbuffered}


IMPORT Atom, OSError, Time;

  T <: Public;
    read(VAR (*OUT*) b: ARRAY OF Byte;
      mayBlock: BOOLEAN := TRUE): INTEGER RAISES {OSError.E};
    write(READONLY b: ARRAY OF Byte) RAISES {OSError.E};
    status(): Status RAISES {OSError.E};
    close() RAISES {OSError.E}
  Byte = BITS 8 FOR [0 .. 255];
  Status = RECORD
    type: Type;
    modificationTime: Time.T;
    size: LONGCARD;
  Type = Atom.T;

END File.
Formally, a file handle h has the components:

      type(h)      an atom, the type of file
      readable(h)  a boolean
      writable(h)  a boolean
      src(h)       (a REF to) a sequence of bytes
      srcCur(h)    an integer in the range [0..len(src(h))]
      srcEof(h)    a boolean
      snk(h)       (a REF to) a sequence of bytes
      snkCur(h)    an integer in the range [0..len(snk(h))]
The src... components are meaningful only if readable(h). The sequence src(h) is zero-based: src(h)[i] is valid for i from 0 to len(src(h))-1. For some subtypes of File.T, the sequence src(h) can grow without bound.

The snk... components are meaningful only if writable(h). The sequence snk(h) is zero based: snk(h)[i] is valid for i from 0 to len(snk(h))-1.

For full details on the semantics of a file handle, consult the interface defining the particular subtype, for example, Pipe.T, Terminal.T, or RegularFile.T. In the case where no exceptions are raised, the methods of the subtypes of File.T obey the following specifications:

The call, mayBlock)
is equivalent to

      IF NOT readable(h) OR NUMBER(b) = 0 THEN
        Cause checked runtime error
      IF srcCur(h) = len(src(h)) AND NOT srcEof(h) THEN
        IF NOT mayBlock THEN RETURN -1 END;
        Block until srcCur(h) < len(src(h)) OR srcEof(h)
      IF srcCur(h) = len(src(h)) THEN RETURN 0 END;
      Choose k such that:
        1 <= k <= MIN(NUMBER(b), len(src(h))-srcCur(h));
      FOR i := 0 TO k-1 DO
        b[i] := src(h)[srcCur(h)];
      RETURN k
\index{non-blocking read}

A result of zero always means end of file. The meaning of a subsequent read after end of file has been reached is undefined for a File.T but may be defined for a particular subtype.

The call

is equivalent to

      IF NOT writable(h) THEN Cause checked runtime error END;
      FOR i := 0 TO NUMBER(b)-1 DO
        IF snkCur(h) = len(snk(h)) THEN
          Extend snk(h) by one byte
        snk(h)[snkCur(h)] := b[i]
The read and write methods are not alertable because it isn't possible to alert a thread blocked in a Win32 ReadFile or WriteFile system call.

The call

returns a result whose type field contains type(h). See the documentation for each subtype of File.T for more details, including the values of the modificationTime and size fields of the result, if any.

The call

is equivalent to

      readable(h) := FALSE;
      writable(h) := FALSE
Additionally, it releases any subtype-specific resources used by h. Every file handle should be closed.

Clients should assume that file handles are unmonitored and should avoid concurrent accesses to a file handle from multiple threads. A particular subtype of File.T may provide a stronger specification with respect to atomicity.