Is it safe to pass (synchronously) stack-allocated memory to other thread?

Question

Recently I heard that memory in the stack is not shared with other thread and memory in the heap is shared with other threads.

I normally do:

HWND otherThreadHwnd;
DWORD commandId;
// initialize commandId and otherThreadHwnd

struct MyData {
  int data1_;
  long data2_;
  void* chunk_;
};

int abc() {
  MyData myData;
  // initialize myData
  SendMessage(otherThreadHwnd,commandId,&myData);
  // read myData
}

Is it alright to do this?

Answer 1

I think 2 different issues are being confused by whoever you "heard that memory in the stack is not shared with other thread":

1. object lifetime - the data on the stack is only valid as long the thread doesn't leave the scope of the variable's name. In the example you giove, you're handling this by making the call to the other thread synchronously.

2. memory address visibility - the addresses pspace for a process is shared among the various threads in that process. So variables addressable by one thread are addressable by other threads in that process. If you are passing the address to a thread in a different process, the situation is quite different and you'd need to use some other mechanism (which might be to ensure that the memory block is mapped into both processes - but that I don't think that can normally be done with stack memory).

Answer 2

Yes, it is safe in this instance.

Data on the stack only exists for the lifetime of the function call. Since SendMessage is a synchronous, blocking call, the data will be valid for the duration of that call.

This code would be broken if you replace SendMessage with a call to PostMessage, SendNotifyMessage, or SendMessageCallback, since they would not block and the function may have returned before the target window received the message.

Answer 3

Yes, it is okay.

SendMessage is working in blocking mode. Even if myData is allocated in stack, its address is still visible to all threads in the process. Each thread has its own private stack; but data in the stack can be explicitly shared, for example, by your code. However, as you guess, do not use PostThreadMessage in such case.

Answer 4

What you heard about is "potential infringement of privacy", which is sharing the data on one thread's private stack with another thread.

Although it is not encouraged, it is only a "potential" problem--with correct synchronization, it can be done safely. In your case, this synchronization is done by ::SendMessage(); it will not return until the message is processed in the other thread, so the data will not go out of scope on the main thread's stack. But beware that whatever you do with this pointer in the worker thread, it must be done before returning from the message handler (if you're storing it somewhere, be sure to make a copy).

Answer 5

As others have said already, how you have it written is just fine, and in general, nothing will immediately fail when passing a pointer to an object on the stack to another thread as long as everything's synchronized. However, I tend to cringe a little when doing so because things that seem threadsafe can get out of their intended order when an exception occurs or if one of the threads is involved with asynchronous IO callbacks. In the case of an exception in the other thread during your call to SendMessage, it may return 0 immediately. If the exception is later handled in the other thread, you may have an access violation. Yet another potential hazard is that whatever's being stored on the stack can never be forcibly disposed of from another thread. If it gets stuck waiting for some callback, object, etc, forever and the user has decided to cancel or quit the application, there is no way for the working thread to be sure the stalled thread has tidied up whatever objects are on its stack.

My point is this: In simple scenarios as you've described where everything works perfectly, nothing ever changes, and no outside dependencies fail, sharing pointers to the local stack is safe - but since allocating on the heap is really just as simple, and it gives you the opportunity to explicitly control the object's lifetime from any thread in extenuating circumstances, why not just use the heap?

Finally, I strongly suggest that you be very careful with the void* chunk_ member of your MyData structure, as it is not threadsafe as described if it's copied in the other thread.