[fpc-pascal] Library.StringFunction() : PChar => NO

[Fred van Stappen]

> Date: Fri, 14 Mar 2014 23:11:14 +0100
> From: pascaldragon at googlemail.com
> To: fpc-pascal at lists.freepascal.org
> Subject: Re: [fpc-pascal] Library.StringFunction() : PChar => NO
> 
> On 14.03.2014 22:07, Fred van Stappen wrote:
> >  > It's not a problem, as long as you provide an API to dispose the memory
> >  > used by the returned PChar.
> >  > The responsibility of calling this API is delegated
> >  > to the application.
> >
> > Yep, with pleasure,... but how to provide an API (and what do you mean
> > with "provide an API to dispose the memory") ?
> >
> > => In short, what must i code to do that ?
> 
> Inside your library you normally do this when creating a string result:
> 
> === code begin ===
> 
> function InLibStringFunction: PChar; // of course declared as cdecl ;)
> const
>    MyString: AnsiString = 'Hello World'; // this string could also come 
> frome somewhere else in your library
> begin
>    Result := GetMem(Length(MyString) + 1);
>    Move(@MyString[1], Result, Length(MyString));
>    Result[Length(MyString)] := 0;
> end;
> 
> === code end ===
> 
> Note: if your MyString is valid through the complete lifetime it could 
> be used you can also use "Result := PChar(MyString);" instead, but if 
> your string is e.g. a variable inside the function or a variable inside 
> another function you must copy its content.
> 
> If you now allocated a PChar using GetMem you add this function as well 
> and export it from the library:
> 
> === code begin ===
> 
> procedure InLibFreeString(aStr: PChar);
> begin
>    FreeMem(aStr);
> end;
> 
> === code end ===
> 
> One could of course now extend the API with additional checks. E.g. so 
> that you know that the string was really allocated by the library and 
> not by e.g. the program using it. But maybe the heap manager already 
> checks this, I don't know currently.
> 
> Regards,
> Sven

Yep, Sven, excellent and hyper clear.

Hum, i think that this code must go somewhere in wiki.
If you agree i will add it into fpc wiki.

Many thanks.

Fred.
 		 	   		  
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