[fpc-pascal]The compiler messes up some pointers.

Jonas Maebe jonas at zeus.rug.ac.be
Sat Sep 22 21:57:04 CEST 2001

On Sat, 22 Sep 2001 yendor at nic.fi wrote:

> I have a problem where FPC modifies a pointer without me instructing it
> to do so.
> With stucture :
> Type
>   PAnsiString = ^AnsiString;
>   PPAnsiString = ^PAnsiString;
> Var
>   RowAP : ^PPAnsiString;
> I first create two rows (that are the ansistrings in a pointer of array of
> pointer of ansistring structure) and set some data into them. After that I
> add one row between these two and initialize it with exactly these rows :
>   GetMem(RowAP^[1], SizeOf(AnsiString));
>   Writeln(LongInt(RowAP^[2]));
>   RowAP^[1]^ := 'a';
>   Writeln(LongInt(RowAP^[2]));
> The program outputs two lines that are : "4284808" and "4284807" (the
> values differ of course, but not the difference) which isn't right. I can't
> reproduce this in other programs (haven't tried very extensively) but I
> don't think that I could ever mess up my program to produce this kind of
> thing.

You are doing very dangerous things. Ansistrings are reference counted,
which measn that the compiler normally does a lot of things behind your
back. With the above construction, it fails to do these things...

You have to intialize all memory you allocate this way for storing
ansistrings in explcitely with 0 before storing an ansistring in them, or
you may get random memory corruption.

I don't know whether the compile should do that automatically in this
case, it's possible it should (but it definitely doesn't currently)


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