Programming Python, Second Edition

Forking Processes

Forked processes are the traditional way to structure parallel tasks, and are a fundamental part of the Unix tool set. Forking is based on the notion of copying programs: when a program calls the fork routine, the operating system makes a new copy of that program in memory, and starts running that copy in parallel with the original. Some systems don’t really copy the original program (it’s an expensive operation), but the new copy works as if it was a literal copy.

After a fork operation, the original copy of the program is called the parent process, and the copy created by os.fork is called the child process. In general, parents can make any number of children, and children can create child processes of their own -- all forked processes run independently and in parallel under the operating system’s control. It is probably simpler in practice than theory, though; the Python script in Example 3-1 forks new child processes until you type a “q” at the console.

Example 3-1. PP2E\System\Processes\fork1.py

# forks child processes until you type 'q'

import os

def child(  ):
    print 'Hello from child',  os.getpid(  )
    os._exit(0)  # else goes back to parent loop

def parent(  ):
    while 1:
        newpid = os.fork(  )
        if newpid == 0:
            child(  )
        else:
            print 'Hello from parent', os.getpid(  ), newpid
        if raw_input(  ) == 'q': break

parent(  )

Python’s process forking tools, available in the os module, are simply thin wrappers over standard forking calls in the C library. To start a new, parallel process, ...