7.2 POSIX Process Management API

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 Memory Layout

Process Memory Layout

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 process created using fork holds copies of all memory available to the parent process
This memory is a copy
Memory is not shared as with threads

Multiprocessing

Can be used to improve performance of parallel computations
Provides different characteristics than threads

Running new programs

exec can be used to cause a process to begin running a new program

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

int main() {
  execl("/bin/echo", "echo", "Hello, world!", NULL);

  printf("An error has occurred");
}

Fork and Exec

fork creates a copied process
exec can be used to cause only the child to run a new program

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

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

  if (!pid) {
    printf("Child launching xterm\n");
    execl("/usr/bin/xterm", "xterm", NULL);
  } else {
    printf("Parent process complete\n");
  }
}

Killing processes

kill can be used to send a signal to a specified process

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

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

  if (!pid) {
    printf("Child launching xterm\n");
    execl("/usr/bin/xterm", "xterm", NULL);
  } else {
    sleep(3);
    printf("Killing %d\n", pid);
    kill(pid, 9);
    printf("Parent process complete\n");
  }
}