In Pascal, all calculations are performed using the base type (which is longint in FPC), so if w > 255, then w shl 8 > high(word) and will result in a range check error when assigning to swapWord. Even if we calculated everything using words, you'd still get an overflow error, this time when doing the shl.

Jonas

Hi,

Why does the following result in a runtime error ?

program example;
const
	THE_PORT = 9999;
function swapWord(w: Word): Word;
begin
	swapWord := (w SHL 8) OR (w SHR 8);
end;
var
	port:	Word;
begin
	port := swapWord(THE_PORT)
end.

$ ./swapword
Runtime error 201 at 0x08052AA2
0x08052AA2
0x08052AC2
0x080480B0

cheers
James

Use a typecast:

function swapWord(w: Word): Word;
begin
	swapWord := Word((w SHL 8) OR (w SHR 8));
end;

Jonas

Turn range checking off for this piece of code or do
function swapWord(w: Word): Word;
   begin
      swapWord := ((w SHL 8) OR (w SHR 8)) and $ffff;
   end;

swapWord := ((w AND $FF) SHL 8) OR (w SHR 8);

How can I correct this ?
This bit of code was taken directly from server.pp in one of the old 2002 mailing threads...

cheers
James

change to
  swapword word (w shl 8) OR word (w shr 8);

or temporarily disable {$R+} there.

How can I correct this ? This bit of code was taken directly from server.pp in one of the old 2002 mailing threads...

But was it under {$R+} there?

Excellent thank you :)

James

Least now I know the reason :)

cheers
James

This is a range check error.
(w SHL 8) OR (w SHR 8)
is evaluated as a longint. i.e. the result may not fit in a longint. And I think it should be:
((w and $FF) SHL 8) OR (w SHR 8)

Michael.

Doesn't system contain a routine called 'swap' that does this exact thing??

This program would seem to work in both Delphi and FPC1.1:

program test;
{$APPTYPE CONSOLE}
uses
  sysutils;
var
  i, j: word;
begin
  i := $9988;
  writeln( IntToHex(i, 4) ); //output '9988'
  j := swap( i );
  writeln( IntToHex(j, 4)); //output '8899'
  readln;
end.

No need to go to the trouble of writing your won routine!! ;-)

Matt

I didn't write it, one of the FPC developers did last year in the mailing list :)

cheers
James