Possible to force Delphi threadvar Memory to be Freed?

Question

I have been chasing down what appears to be a memory leak in a DLL built in Delphi 2007 for Win32. The memory for the threadvar variables is not freed if the threads still e

Answer 1

As you've already determined, thread-local storage will get released for each thread that gets detached from the DLL. That happens in System._StartLib when Reason is DLL_Thread_Detach. For that to happen, though, the thread needs to terminate. Thread-detach notifications occur when the thread terminates, not when the DLL is unloaded. (If it were the other way around, the OS would have to interrupt the thread someplace so it could insert a call to DllMain on the thread's behalf. That would be disastrous.)

The DLL is supposed to receive thread-detach notifications. In fact, that's the model suggested by Microsoft in its description of how to use thread-local storage with DLLs.

The only way to release thread-local storage is to call TlsFree from the context of the thread whose storage you want to free. From what I can tell, Delphi keeps all its threadvars in a single TLS index, given by the TlsIndex variable in SysInit.pas. You can use that value to call TlsFree whenever you want, but you'd better be sure there won't be any more code executed by the DLL in the current thread.

Since you also want to free the memory used for holding all the threadvars, you'll need to call TlsGetValue to get the address of the buffer Delphi allocates. Call LocalFree on that pointer.

This would be the (untested) Delphi code to free the thread-local storage.

var
  TlsBuffer: Pointer;
begin
  TlsBuffer := TlsGetValue(SysInit.TlsIndex);
  LocalFree(HLocal(TlsBuffer));
  TlsFree(SysInit.TlsIndex);
end;

If you need to do this from the host application instead of from within the DLL, then you'll need to export a function that returns the DLL's TlsIndex value. That way, the host program can free the storage itself after the DLL is gone (thus guaranteeing no further DLL code executes in a given thread).