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

[Sven Barth]

Am 14.03.2014 23:46 schrieb "Fred van Stappen" <fiens at hotmail.com>:
>
> > 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.

I thought that it was mentioned there somewhere already... If nit feel free
to add it. That's what a wiki is good for afterall. ^^

Regards,
Sven
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