In UNIX, as we’ve seen, each process is identified by its process identifier,

which is a unique integer. A new process is created by the fork() system call. The new process consists of a copy of the address space of the original process. This mechanism allows the parent process to communicate easily with its child process. Both processes (the parent and the child) continue execution at the instruction after the fork(), with one difference: the return code for the fork() is zero for the new (child) process, whereas the (nonzero) process identifier of the child is returned to the parent.

         Typically, the exec() system call is used after a fork() system call by one of the two processes to replace the process’s memory space with a new program. The exec() system call loads a binary file into memory (destroying the memory image of the program containing the exec() system call) and starts its execution. In this manner, the two processes are able to communicate and then go their separate ways. The parent can then create more children; or, if it has nothing else to do while the child runs, it can issue a wait() system call to move itself off the ready queue until the termination of the child.

 eg:

 

#include 
    
     #include 
     
      #include 
      
       int main(){pid t pid;/* fork a child process */pid = fork();if (pid < 0) { /* error occurred */fprintf(stderr, "Fork Failed");return 1;}else if (pid == 0) { /* child process */execlp("/bin/ls","ls",NULL);}else { /* parent process *//* parent will wait for the child to complete */wait(NULL);printf("Child Complete");}}return 0;