【Operating System】——An interesting question on Process Creation

In the book Operating System Concepts- 9th Edition - Chapter 3 - Page 117 - Page 120 it says:

How do we understand this concept? There is an answer getting from Stack Overflow:

operating system - Creation of A New Process in an OS - Stack Overflow

Whenever a processor(CPU) runs a program , it stores the line number of the code it is executing (more formally address pointing to that instruction).The computer stores it in a register (kind of variable) called as stack pointer. So the stack pointer(SP) will store the current instruction processor has to execute (or run). This way computer track which instruction should be executed .

When a program runs , it is allocated some small memory in the computer's main memory.Here's where all our code along with important registers(including SP) which help processor to keep track of program when it's running.To a process is uniquely identified by Process ID(PID).

Now let me come to your question. Whenever we call fork , a copy of the program from which you called fork is created. This copy is called as the "child process" and our original process is known as parent process.

When the copy created , all the memory that your program has been allocated is copied to some other place in memory (which is now child process's memory).So an identical running program(process) is created.

Now this copied memory contains the SP of the parent process , so whenever processor runs the program it directly runs the program from the same fork call line (since SP will store this line as current instruction when the process is created).Since our fork call was successful , it has to return a non-negative value (denoting success of fork system call)So it return 0 to the child process and child process ID to the parent(since the current Process ID was pointing here too).

Returning Child Process ID to parent makes a good deal since it's better that parent can keep a track of child process created from it.Too returning child a 0 make the deal even better as we have to return a non-negative number and other positive number may be some process's PID.

Run cd /proc in your linux system . All the directories having some numeral name are pid of some process(which may be active/inactive).Read more about it to clear the concept.