[fpc-pascal]Dereferencing Nil Pointers

Jonas Maebe jonas at zeus.rug.ac.be
Sun Dec 29 15:14:10 CET 2002

On zondag, dec 29, 2002, at 06:03 Europe/Brussels, Mark Emerson wrote:

> I did not find "Attempted to dereference a NIL pointer" as a runtime
> error in FPC (except for error 204 which involves calls to dispose or
> Freemem).
> For example:
> var
>   x : ^char;
>   y : char;
> begin
>   x := nil;
>   y := x^;
> end.
> should produce a runtime error at the second statement.  This was a
> deficiency in Turbo Pascal.

Actually, it's a deficiency in 16 bit Dos, which can't do zero page 
protection. It's still a problem under Windows when running 32 bit Dos 
programs, because for some strange reason Windows doesn't allow DPMI 
programs to turn on zero page protection (running this program under 
Dos/go32v2 does produce a segmentation fault). In all other cases, this 
will cause a segmentation fault (run time error 216).


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