Question

I have a DLL which provided a decoding function, as follows:

function MyDecode (Source: PChar; SourceLen: Integer; var Dest: PChar; DestLen: Integer): Boolean; stdcall; 

The HostApp call “MyDecode”, and transfer into the Source, SourceLen and Dest parameters, the DLL returns decoded Dest and DestLen.
The problem is: The HostApp impossible to know decoded Dest length, and therefore would not know how to pre-allocated Dest’s memory.

I know that can split “MyDecode” into two functions:

function GetDecodeLen (Source: PChar; SourceLen: Integer): Integer; stdcall;  // Return the Dest's length
function MyDecodeLen (Source: PChar; SourceLen: Integer; var Dest: PChar): Boolean; stdcall; 

But, My decoding process is very complicated, so if split into two functions will affect the efficiency.

Is there a better solution?

---

Yes Alexander, this may be a good solution.
HostApp code:

//... 
MyDecode(....) 
try 
  // Use or copy Dest data 
finally 
  FreeDecodeResult(...) 
end;

DLL code:

function MyDecode(...): Boolean;
begin
  // time-consuming calculate

  // Allocate memory
  GetMem(Dest, Size);   
  // or New()?
  // or HeapAlloc()?
end;

procedure FreeDecodeResult(Dest: PChar);
begin
  FreeMem(Dest);
  // or Dispose(Dest); ?
  // or HeapFree(Dest); ?
end;

Maybe I should change Dest’s type to Pointer.

Which is a better allocate memory method? GetMem/New, or HeapAlloc?

Answer 1

You can split “MyDecode” into two routines by other way:

function  MyDecode(Source: PChar; SourceLen: Integer; out Dest: PChar; out DestSize: Integer): Boolean; stdcall;
procedure FreeDecodeResult(Dest: PChar); stdcall;

I.e. – you allocate memory in MyDecode instead of asking a caller to do that.