{ex4.source  00/02/10  nm -e ex4p.debugboo}

$pagewidth 120, lines 44$

$standard_level 'os_features'$
$type_coercion 'representation'$
$tables on, statement_number on, code_offsets on$
$list_code on$
$optimize off$

{===============================================================

Disclaimer:

   This program has several sloppy coding practices that can
   (and have) led to problems.  They were put in to help
   demonstrate program aborts ... I don't *really* code that way!

---------------------------------------------------------------

Note:

   This program uses PRINT in an effort to avoid using the
   Pascal/XL I/O package (e.g., writeln).  This shortens the
   amount of code emitted by the compiler, which makes it easier
   to use as a tool for teaching about Debug.

---------------------------------------------------------------

Program description:

   Open a file, read first record, display it.

---------------------------------------------------------------

Exercise 4.1:

   What filename is the program trying to open?

===============================================================}

program ex4;

const
   ccg            = 0;
   ccl            = 1;
   cce            = 2;

type
   pac80          = packed array [1..80] of char;

var
   buf            : string [80];
   c              : char;
   fid            : shortint;
   filename       : pac80;
   inx1           : integer;
   inx2           : integer;
   ktr            : integer;
   len            : integer;
   seed           : integer;  {used to generate "random" number}

Procedure fclose;                      intrinsic;
Function  fopen   : shortint;          intrinsic;
Function  fread   : shortint;          intrinsic;
Procedure print;                       intrinsic;
Procedure terminate;                   intrinsic;

{*************************************************}

begin

filename := 'abcdefghijklmnopqrstuvwxyz';

seed := 12345;
for ktr := 1 to 20 do
   begin

      {Calculate two "random" indices (inx1 and inx2)...}

   seed := (seed div 10) * (seed div 7);
   inx1 := (seed mod 26) + 1;
   inx2 := (seed div 26) mod 26 + 1;
   seed := seed mod 10000;

      {swap bytes at inx1 and inx2...}

   c := filename [inx1];
   filename [inx1] := filename [inx2];
   filename [inx2] := c;
   end;

filename [9] := ' ';

fid := fopen (filename, 3, 0);

if fid = 0 then
   begin
   buf := 'Failed to open file!';
   print (buf, -strlen (buf), 0);
   terminate;
   end;

len := fread (fid, buf, -80);
if ccode = cce then
   begin
   setstrlen (buf, len);
   print (buf, -len, 0);
   end;

fclose (fid, 0, 0);
fid := 0;

end.