How can I return a PChar from a DLL function to a VB6 application without risking crashes or memory leaks?

Question

I have to create a DLL which is used by a VB6 application. This DLL has to provide several functions, some of them must return strings.

This is the VB6 declaration:<

Answer 1

I would say that whoever allocates the memory must also free it in this case. You will run into problems with other scenarios. So the most safe and clean way would be:

1. The DLL allocates memory (because it knows how much) and returns the PChar to caller
2. After the caller is done with it, it calls FreePointer back to the DLL
3. DLL frees the memory in the FreePointer exported function

The setup would be like this:

unit DLL;

interface

uses
  SysUtils;

function Execute(const Params: PChar): PChar; stdcall;
procedure FreePointer(const P: PChar); stdcall;

exports Execute;
exports FreePointer;

implementation

function Execute(const Params: PChar): PChar; stdcall;
var
  Size: Cardinal;
begin
  Size := Calculate the size;
  GetMem(Result, Size);

  ...do something to fill the buffer
end;

procedure FreePointer(const P: PChar); stdcall;
begin
  FreeMem(P);
end;

end.