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
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, ...