Received data has zeros...

I have a few mega bytes of data and fixed size buffer (4MB).

So the sender thread iterates sending (MPI_Send) a fixed sized data at a time until it sends all the data.
The receiver knows how many bytes it will receive before beginning and iterates (MPI_Recv) using the same fixed sized buffer (4MB)

Sometimes the receiver receives data with all zeros only when it receives the last remaining data (possibly smaller than 4MB)

 - I checked that the sender sends the correct data. ('CheckContents' on the below code)
 - I checked that the receiver received the correct amount of bytes.

Sender(int bytes_to_send) {
   char* buffer = new char[4MB];
   while (;) {
      memcpy_from_some_other_buffer_into (buffer);
      CheckContents(buffer);
      MPI_Send(buffer, 4MB or remaining bytes);
      CheckContents(buffer);
   }
   delete [] buffer;
}

Receiver has the exactly same loop form except for the fact that it uses 'MPI_Recv'.

- 1. Is it possible that MPI_Send() still has the reference to the 'buffer' that I delete right after the last call?
- 2. Is there any method that I can use to debug this problem?

Can the following flags affect the program's correctness?
I_MPI_FABRICS
I_MPI_FALLBACK
MPICH_ASYNC_PROGRESS
I_MPI_PIN
I_MPI_DYNAMIC_CONNECTION