[fpc-devel] GetMem(SomePointer, 0) <> nil

[Sergei Gorelkin]

Jonas Maebe пишет:
> 
> On 01 Mar 2011, at 07:32, Paul Ishenin wrote:
> 
>> I noticed today that if I want to allocate 0 bytes I get not a nil but 
>> a valid pointer. Why?
> 
> The current code that causes this comes from this (cvs) revision:
> 
> revision 1.16
> date: 1999-09-17 19:14:12 +0200;  author: peter;  state: Exp;  lines: 
> +284 -966;
>   + new heap manager supporting delphi freemem(pointer)
> 
> I guess the reason was to be able to
> a) give a run time error if someone tries to do freemem(nil)

Delphi allows freemem(nil) without error, consistent with TObject.Free

> b) nevertheless allow freeing all pointers that have been allocated 
> using getmem() (even if the size was 0 bytes), for backwards 
> compatibility with FPC code from before that revision (FPC also returned 
> a non-nil pointer in that case)
> 
>> This is incompatible with delphi.

I remember a discussion some time ago, the conclusion was it's compatible with TP
rather than with Delphi.
> 
> If it is changed, it will also require changes in cmem (and probably 
> also in heaptrc), because at least on Mac OS X libc also returns a valid 
> pointer for malloc(0).
> 
Not necessary, if we have only one GetMem function (currently we have several - getmem, fpc_getmem, 
getmemory and maybe something else, each calling MemoryManager independently) and write it like this:

if size=0 then
   result := nil
else
   result := MemoryManager.GetMem(size);

>> And I suppose cause crashes and memory leaks not only in my delphi 
>> code which I'm porting to FPC/Lazarus.
> 
> That sounds very much like badly written code.
> 
Fully agree.

Sergei