Date   : Mon, 04 Jul 2005 11:39:53 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: This program will self destruct

Greg Cook wrote:

> You may know that the actual parameters of PROCs and FNs are
> automatically made LOCAL but restored in reverse order, which is why
> the ! fields must not overlap.  But the ? operator really goes wrong.

Interesting, thanks.

You're right about the ? operator - I've lost work through not knowing this
in the past.  Take the following, perfectly legitimate looking program:

10 FOR N%=0 TO 2 STEP 2
20 P%=&900
30 [OPT N%
40 OPT FNequb(32)
50 ]
60 NEXT
70 END
80 :
90 DEF FNequb(?P%):P%=P%+1:=N%

When I tried running this just now, it overwrote the four bytes following
'OPT' on line 40 with zeroes, causing the same 'self-destruct' effect.  The
question is: why does this happen?  It seems odd that there's a bug in the
? operator, which appears not to happen at all with the ! operator.  It's
as if it thinks that ?P% is 4 bytes long instead of 1, though I can't
imagine why it trashes mid-program memory.  Even the $ operator, with its
variable length value, appears to work OK.

(Of course, the above bit of code wouldn't actually work like an EQUB even
if Basic wasn't broken... since ?P% is 'local' to FNequb, its value would
be restored to its original value when exiting the function - something I
hadn't considered when trying to use that code years ago.  Good job it
broke really!)

Rich


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