[fpc-pascal] Pointer

[Jonas Maebe]

On 15 nov 2005, at 17:11, Luis Del Aguila wrote:

> Somebody can help me.
> I do not understand, that I am making bad.
> The program is:
>
> Program ProbandoMemoria;
> {$R+}
> Var
>   PLPunteros: ^Pointer;
>   NuevoTamanio : integer;
>   elementos : longint;
>   x,b : ^integer;
> Begin
>   elementos := 3;
>   NuevoTamanio:=Sizeof(Pointer)*elementos;
>   ReallocMem(PlPunteros,NuevoTamanio);
>   Writeln('Direccion : ',longint(Plpunteros)  );
>   new(x);
>   x^:=7;
>   Plpunteros^:=x;
>   b:=Plpunteros^;
>   Writeln('Direccion : ',longint(b),' valor : ',b^);
>   new(x);
>   x^:=15;
>   (Plpunteros+4)^:=x;

In this statement (and several next ones) you are writing past the  
memory allocated for Plpunteros. Adding a value to a pointer,  
increases the value of said pointer by this value times the size of  
the elements the pointer points to. Since Plpunteros = ^Pointer, the  
above writes X to the address "pointer(plpunteros)+4*sizeof(pointer)".

What you want is "(Plpunteros+1)^:=x;". Or, if you are in fpc or  
objfpc mode, you can also use "plpunteros[1]:=x;"

>   b:=(Plpunteros+4)^;
>   Writeln('Direccion : ',longint(b),' valor : ',b^);
>   new(x);
>   x^:=25;
>   (Plpunteros+8)^:=x;
>   b:=(Plpunteros+8)^;
>   Writeln('Direccion : ',longint(b),' valor : ',b^);
>
> //This I do not understand.
> //Why ' b^ ' does not have the 7?

Probably because one of the previous out-of-bounds writes overwrote  
the value.


Jonas