Q. Example of shared programming using library routines?

Think about subsequent set of statements 

Process A                                  Process B              

   :                                                    :

fork B;                                              :

   :                                                     :

Join B;                                            end B;             

In the above group of statements process A creates a child process B by statement fork B. Then A and B carry on their computations separately till A reaches join statement At this phase if B is already completed then A continues executing subsequent statement or else it waits for B to complete. 

In the shared memory model a general problem is to synchronize the processes. It can be possible that more than one process is trying to concurrently edit the same variable. To solve this problem numerous synchronization mechanism such as test_and_set, semaphores and monitors have been used. We will not go in the details of these mechanisms. Rather we will symbolize them by a pair of two processes known as lock and unlock. Whenever a process P locks a common variable then only P can use that variable.

Other simultaneous processes have to wait for common variable till P calls the unlock on that variable. Let's see effect of locking on output of a program when we don't employ lock and when we employ lock.