1. CIS 720
Message Passing

2. Message passing systems
Send statements
Receive statements

3. Process naming
P ! m(para_list): send message m with parameters para_list to P.
send(m(para_list, P)
P ? m(para_list): receive message m from P, and assign the received parameters to variables in para_list.
receive(m(para_list), P)

4. Channel naming send(m(para_list), ch) or ch!m(para_list)
receive(m(para_list), ch) or ch?m(para_list)

5. A: amount of synchronization and Buffering

6. Minimal synchronization: 1
Non-blocking send: 1 + 6
Blocking send:
Reliable blocking send:
Synchronous send:

7. Synchronous communication
Send statement:
A ! m(expr): the send statement blocks until the receive statement is also enabled.
Receive statement
B ? m(var): the receive statement blocks until the send statement is also enabled.
When matching send and receive statements are enabled, then var is assigned the value of expr

8. A pair of send and receive statements are matching if
The send statement appears in the process named in the receive statement.
The receive statement appears in the process named in the send statement.
var = expr is a valid assignment statement
The type of the messages match

9. P1: P2:
y := 2 z := 1
P2 ! y P1 ? w
P2 ? y P1 ! w + z

10. P1: P2: P3: P3 ? y P3 ! w + z P1 ! x y := 2 z := 1 x = 1
P2 ! y P1 ? W P2 ? x
P3 ? y P3 ! w + z P1 ! x

11. P1: P2:
y := 2 z := 1
P2 ! y P1 ! w
P2 ? y P1 ? w + z
Deadlock

12. Array copying program A[0..N-1] B[0..N-1] i = 0; P1: P2: od od do do
i < n P2 ! A[i]; i < n  P1 ? B[i]
i = i + 1
od od

13. Array copying program A[0..N-1] B[0..N-1] i = 0; P1: P2: od od do do
i < n P2 ! A[i]; i < n  P1 ? B[i]
i = i + 1
od od
This could deadlock because when i = n-1, P2 could check “i < n” before i is incremented.

14. Array copying program A[0..N-1] B[0..N-1] i = 0; j = 0 P1: P2: od od
do do
i < n P2 ! A[i]; j < n  P1 ? B[j];
i = i j = j + 1
od od

15. Satisfaction condition
one must show the following satisfaction condition for each pair of matching send and receive statements:
P P2
: :
{P} {Q}
P2!expr P1?var
{U} {V}

16. P1: P2: {true} {true} {w = 2} {z = 1 /\ w = 2} P2 ? y P1 ! w + z
y := 2 z := 1
{y = 2} {z = 1}
P2 ! y P1 ? w
{w = 2} {z = 1 /\ w = 2}
P2 ? y P1 ! w + z
{y = 3} {y = 3}
{ y = 2 /\ z = 1}
w = y
{ w = 2 /\ z = 1}
{ w = 2 /\ z = 1}
y = w + z
{ y = 3}

17. Guarded Communication
P ? x /\ bool  action
Guard evaluates to true if
bool is true
executing P ? x will not cause delay
Guard evaluates to false if bool is false
Guard blocks if bool is true but executing P ? x will cause delay

18. Mutual exclusion C: P1: P2: do do do P1?req  P1?rel C!req; C!req
[] cs cs
P2?req  P2?rel C!rel; C!rel
od od od

19. Example….
P1: P2:
v1 = 1; v2 = ? v3 = ?; v4 = 2
if if
fi fi

20. Example…. P1: P2: v1 = 1; v2 = ? v3 = ?; v4 = 2 if if
P2 ! v1  P2 ? v2 P1 ! v4  P1 ? v3
[] []
P2 ? v2  P2 ! v1 P1 ? v3  P1 ! v4
fi fi

21. Computing the minimum value
D: num=0; m = Max; A: B: C: do
A ? v  m = min(m,v);num D!a; D!b D!c
[] D?a D?b D?c
B ? v  m = min(m,v);num++ []
C ? v  m = min(m,v);num++
num = 3  A!m; B!m; C!m od

22. Computing the minimum value
A: a=initial value; num1 = 0 do
B ? v  a = min(a,v);num1++ []
C ? v  a = min(a,v);num1++
B ! a  skip; num1++
C ! a  skip; num1++
num1 = 4  exit; od
B: b=initial value; num2 = 0 do
A ? v  b = min(b,v);num2++ []
C ? v  b = min(b,v);num2++
A ! a  skip; num2++
C ! a  skip; num2++
num2 = 4  exit; od

23. Centralized Semaphore
C: A: B:
do do do
A ? p  A ? v C!p; C!p
[] cs cs
B ? p  B ? v C!v; C!v
od od od

24. Centralized Semaphore
C: sem =1 A: B:
do do do
sem > 0; A ? p sem C!p; C!p
[] cs cs
sem> 0; B ? p  sem C!v; C!v
[] od od
sem=0; A ? v  sem++ []
sem=0; B ? v  sem++ od

25. Dining philosophers problemdo
hungry
acquire left and right fork
eat
release forks
sleep od