Process IDs

Figure 4.1: UNIX System initialization

• Each process in a Linux system is identified by its unique process ID, pid.
• Process IDs are 16-bit numbers that are assigned sequentially by Linux as new processes are created.
• Every process also has a parent process, you can think of the processes on a Linux system as arranged in a tree, with the init process at its root.The parent process ID, or ppid, is simply the process ID of the process's parent.
• A program can obtain the process ID of the process it's running in with the getpid() system call, and it can obtain the process ID of its parent process with the getppid() system call.

  #include <stdio.h>
  #include <unistd.h>
  
  int main ()
  {
    printf ("The process id is %d\n", (int) getpid ());
    printf ("The parent process id is %d\n", (int) getppid ());
    return 0;
  }
  

  if you invoke it every time from the same shell, the parent process ID (that is, the process ID of the shell process) is the same.