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Understanding Promela Programs and Message Passing Channels

Learn the intricacies of Promela programs and message passing channels, including system states, process automata, asynchronous interleaving, and more! Dive into the world of message passing operations and avoid race conditions.

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Understanding Promela Programs and Message Passing Channels

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  1. The Meaning of Promela Programs init x==1 y==? x==1 y==2 init processautomata/Kripke structure program/systemasynchronous interleaving product of automata proctype A() {x=1;y=2} proctype B() {x=3} program: byte x,y; init {run A(); run B()} process has a local state system has a global state x==3 y==? init x==3 y==? x==1 y==? x==1 y==2 x==3 y==? x==1 y==? x==3 y==2 x==1 y==2

  2. Count==0 Count==1 proctype A() {bit count =0; do :: (count == 0) -> count=count+1 :: (count!=0) -> count=count-1 od } init { run A()}

  3. [] [0] [0,0] chan ch = [2] of bit; proctype A() {bit x=0; do :: ch!x :: ch?x od } init {run A()} OR proctype A() {do :: ch!x od} proctype B() {do :: ch?x od} init {atomic{ run A(); run B()}}

  4. Message Passing Channels • allow transfer of data from one process to another chan one = [16] of int chan two = [1] of {bool, int, int} • writing a message • one!154 • queue!x • two!false,19393,2 only executable when the channel is not full • reading a message • one?var1 • two?on,num1,num2 only executable when the channel is not empty

  5. Message Passing • prefefined operations len(channel) - returns no. of messages len of empty channel blocks full - returns status (i.e. is channel full) empty - returns status (i.e. is channel empty) Also, nfull and nempty.

  6. Message Passing • channels can also be passed as messages! • Proctype A(chan q1) • { chan q2; • q1?q2; • q2!123 • } • Proctype B(chan q3) • {int x; • q3?x; • printf(“x = %d\n”, x) • } • init • {chan qname = [1] of {chan}; • chan qforb = [1] of {int}; • run A(qname); run B(qforb); • qname!qforb • }

  7. Message Passing • non-destructive read q1?[message]; message == somethinggood -> something else q1?[m] returns 1if q1?m is executable, 0 otherwise. No side-effects, receive is evaluated, not executed. • race conditions - BE CAREFUL! (len (channelA) < Max) -> channelA!message concurrently with channelA! Message (len (channelA) > 0) -> channelA?x concurrently with channelA? x

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