CS 100 Lab 4: Concurrent Programming

For this lab you will be analyzing a number of different concurrent programs (each consisting of some initial values and the bodies of 2 or more threads) alongside their output. Assume that the bodies of the threads (the separate functions) run concurrently and non-deterministically. Most of the exercises will ask you to determine how they arrived at their output (which may be unexpected), and some may also ask you to make suggestions as to how to "fix" their behavior.

When explaining how they arrive at the given output, you may either sketch a diagram and/or outline the sequence in English (e.g., "First, t1 runs lines number W-X, then t2 runs line number Y-Z, etc.").

Exercise 1

x = 0

1   def t1():
2       a = x
3       a = a + 5
4       x = a

5   def t2():
6       b = x
7       b = b + 10
8       x = b

Exercise 2

x = 5
y = 10
z = 30
sum = 0

1   def t1():
2       val1 = sum + x
3       sum = val1

4   def t2():
5       val2 = sum + y
6       sum = val2

7   def t3():
8       val3 = sum + z
9       sum = val3

Exercise 3

x = 0
turn = ''

1   def t1():
2       turn = 't1'
3       while turn == 't2':
4           pass
5       a = x
6       a = a + 5
7       x = a
8       turn = 't2'

9   def t2():
10      turn = 't2'
11      while turn == 't1':
12          pass
13      b = x
14      b = b + 10
15      x = b    
16      turn = 't1'

Exercise 4

s = ''
t1_state = 'Running'
t2_state = 'Waiting'

1   def t1():
2       for _ in range(5):
3           while t2_state == 'Running':
4               pass
5           t1_state = 'Running'
6           s = s + '@'
7           t1_state = 'Waiting'

8   def t2():
9       for _ in range(5):
10          while t1_state == 'Running':
11              pass
12          t2_state = 'Running'
13          s = s + '#'
14          t2_state = 'Waiting'

Submission

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