Rendez-Vous

1
Rendez-Vous

• Logical extension of
• chan buffer N of byte
• is
• chan port 0 of byte
• Channel port is a rendez-vous port (binary
  handshake).
• Two processes, a sender and receiver, can
  synchronise
• e.g. port!2 is blocked until there is a
  corresponding
• port?msg ready to execute
• then both will synchronise.

2
Dijkstras Semaphore using Rendez-vous

• define p 0
• define q 1
• chan sema 0 of bit
• proctype semaphore()
• byte count 1
• do (count 1) -gt sema!p count 0
• (count 0) -gt sema?v count 1
• od
• proctype user()
• do sema?p
• / critical section /
• sema!v
• / non-critical section /
• od

3
Synchronous vs Asynchronous Communication

• define msgtype
• chan name x of byte,byte
• proctype A()
• name!msgtype(124)
• name!msgtype(121)
• proctype B()
• byte state
• name?state
• init
• atomic run A() run B() / created at the
  same time /
• Behaviour
• x0
• A and B will synch on transfer of 124, then A
  will block.

4
An Interesting Way to Compute the Factorial
Function

• Proctype fact(int n chan p)
• / calculate factorial n, communicating result
  via p /
• chan child 1 of int
• / for result from fact n-1 /
• int result
• if (n lt 1) -gt p!1
• (ngt2) -gt run fact(n-1, child)
• child?result
• p!(nresult)
• fi
• init / factorial 5 /
• chan result 1 of int
• int answer
• run fact(5, result)

5
Assertions

• Assertions are statements about the program state
  that can be embedded in the program.
• assert (condition)
• E.g. assert (state 1)
• assert (x gt y)
• Extremely useful! For
• run-time behavioural audit
• program invariants
• But be careful,
• assertions abort the program if the condition
  evaluates to 0, i.e. it is false.

6
Assertions

• Common ways to use assertions
• idle assert(arm up) ..
• receiver assert (full(inchannel))
• parcel 0 ..
• sender assert (empty(outchannel))
• parcel 1 ...
• inchannel?x assert x last_value 1 ..

7
Factorial Function

• Proctype fact(int n chan p)
• / calculate factorial n, communicating result
  via p /
• chan child 1 of int
• / for result from fact n-1 /
• int result
• if (n lt 1) -gt p!1
• (ngt2) -gt assert (empty child)
• run fact(n-1, child)
• assert (full(child))
• child?result
• p!(nresult)
• fi
• (assert full(p))
• init / factorial 5 /
• chan result 1 of int