Is there a way to perform two-way communication? This pseudo-code seems to imply that you can only send or receive, but not both at the same time. Consider a concrete manifestation of the message-passing model where the message channel is the Internet - it's possible to send and receive at the same time (considering the processing time for sending/receiving is much less than the underlying network latency), is that understanding correct?

That's a very interesting point that got me thinking. Networks on Chip (NoC's) do function a lot like the internet. Imagine that all your nodes of communication are arranged in a mesh/torus structure and passing around messages along the edges of the mesh/torus. From what I know, sending and receiving can be done simultaneously at each node; each node behaves like an Internet router. The only difference is that in NoC's, there is no requirement for an acknowledgment being sent from receiver to sender (as is the case with TCP) - on such a small-scale device, packets are guaranteed to reach always.

However, this course might be referring to some different communication mechanisms. And I am curious to know more about the intricacies of the systems discussed in this course.

Do the threads have any notion of which other threads their "neighbor threads" are, provided that they're not in a shared address space?

Need to interleave send and receive to avoid blocking