Casting pointer to TBytes

[Jacek Laskowski 57]

I have a method (from external code) that takes an array of TBytes as a parameter:

procedure Something(const aData : TBytes);

and I need to pass to it a (read-only) memory area, allocated by GetMem().

 

Is it possible to cast this to TBytes (or some other trick) without copying the memory to a separate variable of type TBytes?

 

[David Heffernan 2345]

That's not an array of TBytes. It's not clear to me what your question is yet. 

[Cristian Peța 103]

You can not because a dynamic array is not just the data. There is also the length and reference count.

But you can pass a pointer and length of that memory,

[Lars Fosdal 1792]

By "need to pass it to a (read only) memory area" - what do you actually mean?
Do you want to get data from aData or put it to aData?
 

Edit: ok, I read that again.

 

What you want to do, is to allocate your TBytes array instead of using GetMem.

SetLength(MyData, Size);

Whatever fills your GetMem block, should also be able to fill @MyData[0] with the length you have preallocated?
You then pass that as Something(MyData);

[Jacek Laskowski 57]

Simplified version of the question: is it possible to pass a memory area (pointer) to a method that accepts an array of bytes (TBytes) without copying?

 

If it's not possible, there's no trick, that's too bad. I'm looking for another solution. Thanks for the help...

[Anders Melander 1782]

6 minutes ago, Jacek Laskowski said:

is it possible to pass a memory area (pointer) to a method that accepts an array of bytes (TBytes) without copying?

No - for the reason Cristian stated.

[David Heffernan 2345]

You need to have a function that accepts a pointer to the byte array, and its length. Then you can pass your raw memory pointer, or pass Pointer(Bytes), Length(Bytes) when you have a TBytes to hand.

[Lars Fosdal 1792]

Nothing beats pointers to arrays for InterOp - even today.

[Remy Lebeau 1393]

3 hours ago, Jacek Laskowski said:

Simplified version of the question: is it possible to pass a memory area (pointer) to a method that accepts an array of bytes (TBytes) without copying?

The only way to do that is to make the memory area MIMIC a real TBytes, by adjusting the size of the GetMem() allocation to include a fake TBytes header whose reference count is set to 1 or higher so the method doesn't try to free the allocated memory internally as it passes around and uses the TBytes.  For example:

type
  // these are defined only in the System unit's implementation, so
  // they need to be defined manually in your code...
  PDynArrayRec = ^TDynArrayRec;
  TDynArrayRec = packed record
  {$IFDEF CPUX64}
    _Padding: LongInt; // Make 16 byte align for payload..
  {$ENDIF}
    RefCnt: LongInt;
    Length: NativeInt;
  end;

var
  MemBlk: Pointer;
  MyByteArr: PByte;
begin
  //GetMem(MyByteArr, DesiredSize);
  GetMem(MemBlk, SizeOf(TDynArrayRec) + DesiredSize);
  try
    PDynArrayRec(MemBlk).RefCnt := 1;
    PDynArrayRec(MemBlk).Length := DesiredSize;
    MyByteArr := PByte(MemBlk) + SizeOf(TDynArrayRec);

    // fill MyByteArr as needed ...

    Something(TBytes(MyByteArr));

    // ...
    
  finally
    //FreeMem(MyByteArr);
    FreeMem(MemBlk);
  end;
end;

If that is not an option, and changing the method (or calling a different method) is also not possible, then your only remaining options are to either:

 

- copy the GetMem() data to a temp TBytes, and then copy it back afterwards if needed.

 

- change the GetMem() allocation to TBytes to begin with.

Edited February 9, 2021 by Remy Lebeau