The comm.send() and comm.recv() functions are blocking functions, which means that they block the caller until the buffered data involved can be used safely. Also, in MPI, there are two management methods of sending and receiving messages:

• Buffered mode: The flow control returns to the program as soon as the data to be sent has been copied to a buffer. This does not mean that the message is sent or received.
• Synchronous mode: The function only gets terminated when the corresponding receive function begins receiving the message.