Problem with threading and execv

[Motoma]

Hello everyone,

I am having a tiny problem putting together a small script. I tried throwing together a multi-threaded ping utility, but the threads are not behaving the way I anticipated...Perhaps one of you could look and see why this is happening.

Python 2.5.2, Ubuntu 8.04

#! /usr/bin/env python                                                               
 
import os, sys, threading
 
class PingThread(threading.Thread):
    host = ''
 
    def __init__(self, host):
        self.host = host
        threading.Thread.__init__(self)
 
    def run (self):
        os.execv('/bin/ping', ['/bin/ping', self.host])
 
print "PingThread('google.com').start()"
PingThread('google.com').start()
print "PingThread('yahoo.com').start()"
PingThread('yahoo.com').start()
print "Main thread ended."
 

If someone could point out my misuse of the Thread class, I would appreciate it.

Thanks,
Motoma

[Motoma]

It appears that my problem is not with the utilization of the Thread class; rather, my error was with my misunderstanding of the way python's exec*() functions work.

In C, one way to spawn a new process is to fork() and exec*() in the new thread. However, when running exec*() commands in python, it appears that the entire python process is taken over by the exec*() command, not just that particular thread. My solution to this issue is to utilize the os.system() call, rather than the os.exec*() call.

Working example:

#! /usr/bin/env python                                                               
 
import os, sys, threading
 
class PingThread(threading.Thread):
    host = ''
 
    def __init__(self, host):
        self.host = host
        threading.Thread.__init__(self)
 
    def run(self):
        os.system('/bin/ping %s' %(self.host))
 
print "PingThread('google.com').start()"
PingThread('google.com').start()
print "PingThread('yahoo.com').start()"
PingThread('yahoo.com').start()
print "Main thread ended."
 

[weazzle]

It appears that my problem is not with the utilization of the Thread class; rather, my error was with my misunderstanding of the way python's exec*() functions work.

In C, one way to spawn a new process is to fork() and exec*() in the new thread. However, when running exec*() commands in python, it appears that the entire python process is taken over by the exec*() command, not just that particular thread. My solution to this issue is to utilize the os.system() call, rather than the os.exec*() call.
[/code]

Actually, these work the same in both C and Python. You are confusing threads and processes. The fork() system call creates a new process, you then must determine which process is the child, and explicitly replace the current program using one of the exec* functions. If you simply launch a new thread, the thread is running as part of the same process, so it will naturally take over the entire process when you call an exec* function. The important thing to understand here is that a Thread is a different construct than a process.

The os.system() call is nice and easy, however, if you want to set up special inter-process communication, you should really be using the fork(), exec*() and potentially dup2(). dup2() allows you to map a file descriptor to another. For example, if you want the new process to read from the parent process instead of stdin, you need to remap its stdin file descriptor to a pipe from the parent process using dup2() before launching the program.

Example (grep data in parent):

#include os
#include sys
 
(pipe_read, pipe_write) = os.pipe()
 
pid = os.fork()
if pid: #parent process
    os.close( pipe_write )
    os.dup2( pipe_read, sys.stdin.fileno() )
    os.execv('/bin/grep', ['/bin/grep', expression])
else: #parent process
    os.close( pipe_read )
    os.write( pipe_write, text )
 

The above example is not complete, you would need a way to retrieve the data from the child process, potentially via another pipe that redirects its stdout.