4. 3.17 What are the benefits and the disadvantages of each of the following? Consider both the system level and the programmer level.

a. Synchronous and asynchronous communication

b. Automatic and explicit buffering

c. Send by copy and send by reference

d. Fixed-sized and variable-sized messages

Which of the following components of program state are shared across threads in a multithreaded process?

1. Register values
2. Heap memory
3. Global variables
4. Stack memory