Notes

Parent and Child Process

All the processes in an operating system are created when a process executes the fork() system call except the startup process. The process that used the fork () system call is the parent process. In other words, a parent process is one that creates a child's process. A parent process may have multiple child processes but a child process only one parent process.

On the success of a fork () system call, the PID of the child process is returned to the parent process and 0 is returned to the child process. On the failure of a fork () system call, -1 is returned to the parent process and a child process is not created.

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
void main() {
int pid,pid1,pid2;
pid=fork();
if(pid==-1)
{
printf("ERROR IN PROCESS CREATION n");
exit(1);
}
if(pid!=0) {
pid1=getpid();
printf("n the parent process ID is %dn", pid1);
}
else {
pid2=getpid();
printf("n the child process ID is %dn", pid2);

}
}

PID and PPID

In Linux, an executable stored on a disk is called a program, and a program loaded into memory and running is called a process. When a process is started, it is given a unique number called process ID (PID) that identifies that process to the system. If you ever need to kill a process, for example, you can refer to it by its PID.

In addition to a unique process ID, each process is assigned a parent process ID (PPID) that tells which process started it. The PPID is the PID of the process’s parent.

For example, if process1 with a PID of 101 starts a process named process2, then process2 will be given a unique PID, such as 3240, but it will be given the PPID of 101. It’s a parent-child relationship. A single parent process may spawn several child processes, each with a unique PID but all sharing the same PPID

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

int main()

{

int pid, pid1, pid2;

pid = fork(); // variable pid will store the value returned from fork() system call

// If fork() returns zero then it means it is child process.

if (pid == 0) {

// First child needs to be printed later hence this process ismade to sleep for 3 secs.

sleep(3);

// This is first child process getpid() gives the process id and getppid() gives the

// parent id of that process.

printf("child[1] --> pid = %d and ppid = %dn", getpid(), getppid());

}

else {

pid1 = fork();

if (pid1 == 0) {

sleep(2);

printf("child[2] --> pid = %d and ppid = %dn", getpid(), getppid());

}
else {

pid2 = fork();

if (pid2 == 0) {

// This is third child which is needed to be printed first.

printf("child[3] --> pid = %d and ppid = %dn", getpid(), getppid());

}

// If value returned from fork() in not zero and >0 that means this is parent process.

else {

// This is asked to be printed at last hence made to sleep for 3 seconds.

sleep(3);

printf("parent --> pid = %dn", getpid());

}

}

}

return 0;

}