Q. Write a pseudo code to find sum of two functions?

Let's write a pseudo code to find sum of two functions f(A) + f(B). In first algorithm we will not use locking.

Process A                                Process B

 sum = 0                                           :

    :                                                   :

  fork B                                     sum = sum+ f(B)

   :                                                   :

 sum = sum + f(A)                         end B

    :

    join B   

   :

 end A

If process A executes statement sum = sum + f(A) in addition writes results in main memory followed by computation of sum by process B then we attain correct result however consider case when B executes statement sum = sum + f(B) before process A can write result in the main memory. Then sum comprises only f(B) that is incorrect. To avoid such variation we use locking.  

Process A                                Process B

 sum = 0                                           :

    :                                                   :

    :                                                  lock sum               

  fork B                                     sum = sum + f(B)

   :                                                   unlock sum

  lock sum                                        :

 sum = sum + f(A)                         end B

unlock sum 

   :

    join B   

   :

 end A

In this case every time a process obtains sum variable it locks it sothat no other process can access that particular variable that guarantees consistency in results.