How do I pass and retrieve memory stream from my Application to/from DLL?

Question

Suppose I have a TMemoryStream I need to pass to my DLL and get back TMemoryStream (Bitmap stream) from the DLL.

I was thinking my DLL would ha

Answer 1

Two obvious options, assuming the callee is to allocate the memory:

1. Use a shared heap

For instance you can use the COM heap. In the callee your write:

OutBuffSize := ...; // you know what this value is
OutBuff := CoTaskMemAlloc(OutBuffSize);
// populate the buffer

The caller destroys this with CoTaskMemFree. You can use LocalAlloc, or HeapAlloc if you prefer, it doesn't really matter.

2. Use the callee's heap and export a deallocator

Here you use the native heap of the callee:

OutBuffSize := ...; // you know what this value is
GetMem(OutBuff, OutBuffSize);
// populate the buffer

You also need to export a deallocator:

procedure DeallocateMemory(Ptr: Pointer); stdcall;
begin
  FreeMem(Ptr);
end;

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Another option that I rejected is to use a shared memory manager. I tend to avoid that because it constrains the caller to be a Delphi program.

To fill a stream from a buffer call WriteBuffer:

Stream.WriteBuffer(Buff^, BuffSize);

where Buff is a pointer to the buffer.