Pointer across threads

[Chris via Digitalmars-d-learn]

The following

struct DATA {
  short* data;
  size_t len;
}

// data and len are provided by a C function
// ...
auto data = mymodule.getSpeechData();
// cast to immutable, because of concurrency
immutable short* tmp = cast(immutable)(data.data);
auto proc = spawn(&processData, thisTid);
send(processData, tmp, data.len);

However, processData never receives "tmp". Is this because tmp 
and data.data are still pointing to the same address? If this is 
the case, why doesn't the compiler warn me? Or is it something 
else (pointer not visible across threads?).

Is there a work around? (I wanted to avoid having to convert 
short* to short[] and .idup it.)