fork is a system call used for creating child processes of a

parent process.it returns the process id of the created

child process.after that pid(process id ) is checked if it

is negative,it means no child process is created,pid==0

implies the id of the newly created process and pid>0 is the

id of child process given to the parent process.the

statements following fork system call are executed by both

the parent and child process.and one more thing,the parent

and child process have the exact copy of address space but

it exist separately for the two processes.