Fork

#include <unistd.h>
#include <stdio.h>

int main() {
  int i;
  i = fork();

  printf("%d\n", i);
}

Process

Closest to the informal idea of a running program

Process

One or more threads
Virtual memory accessible to those threads
Other access rights
Resource allocation context
Other context (e.g. working directory)

Process Identification

Each process has a process ID number (PID)
All PIDs are positive integers

Process Creation

Processes are created as a fork of another
The only exception is the initial process created by the OS on boot

Fork

The fork system call can be called from a thread to create a new process
The parent calling fork sees the process ID of the child as a return value
The child process sees a return value of 0
The child process is otherwise an exact copy of the parent

#include <unistd.h>
#include <stdio.h>

int main() {
  int pid = fork();

  printf("PID: %d\n", pid);
}

Memory

A forked process sees a copy of its parent’s memory
Clobbering the other processes memory is not possible
Communication via memory modification is not available

Signals

We need a way to communicate between processes
One way to do this is to send signals

#include <unistd.h>
#include <stdio.h>
#include <signal.h>

int main() {
  int pid = fork();

  if (pid) {
    kill(pid, SIGKILL);
    printf("Killed child process\n");
  }

  printf("Done\n");
}