[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

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1. #! /usr/bin/env python                                                               
2.  
3. import os, sys, threading
4.  
5. class PingThread(threading.Thread):
6.     host = ''
7.  
8.     def __init__(self, host):
9.         self.host = host
10.         threading.Thread.__init__(self)
11.  
12.     def run (self):
13.         os.execv('/bin/ping', ['/bin/ping', self.host])
14.  
15. print "PingThread('google.com').start()"
16. PingThread('google.com').start()
17. print "PingThread('yahoo.com').start()"
18. PingThread('yahoo.com').start()
19. print "Main thread ended."
20.  

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:

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1. #! /usr/bin/env python                                                               
2.  
3. import os, sys, threading
4.  
5. class PingThread(threading.Thread):
6.     host = ''
7.  
8.     def __init__(self, host):
9.         self.host = host
10.         threading.Thread.__init__(self)
11.  
12.     def run(self):
13.         os.system('/bin/ping %s' %(self.host))
14.  
15. print "PingThread('google.com').start()"
16. PingThread('google.com').start()
17. print "PingThread('yahoo.com').start()"
18. PingThread('yahoo.com').start()
19. print "Main thread ended."
20.  

[weazzle]

Originally Posted by 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.
[/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):

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1. #include os
2. #include sys
3.  
4. (pipe_read, pipe_write) = os.pipe()
5.  
6. pid = os.fork()
7. if pid: #parent process
8.     os.close( pipe_write )
9.     os.dup2( pipe_read, sys.stdin.fileno() )
10.     os.execv('/bin/grep', ['/bin/grep', expression])
11. else: #parent process
12.     os.close( pipe_read )
13.     os.write( pipe_write, text )
14.  

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.