A question about terminal control in Python...

trist007 · 03-29-2014, 04:26 PM

This is a wrapper program that uses the xmp binary to play mod files at random. However, I would like to have another process running so that if a good song is playing I can write the name of the filename to a file for future reference.

Code:

#!/usr/bin/python
# -*- coding:utf-8 -*-

"""
wrapper for xmp
to play random files
"""

import random
import os
import sys


def main():
	files = os.listdir('/Users/rgonzale/Mods')
	command = "/usr/local/bin/xmp /Users/rgonzale/Mods/%s" % (files[random.randint(0,(len(files) - 1))])

	pid = os.fork();
	if pid > 0:
		os.system(command);
	if pid == 0:
		c = sys.stdin.read(1);
		if c == 'e':
			print "hope this works"

if __name__ == "__main__":
	main()

When the parent runs the command it has stdin, stdout, and stderr to the pty that I ran the process from. The character 'e' is not a character that xmp will do anything with from stdin, I checked in the man page. When I have the child try and read a char from stdin, I can't get it to print "hope this works".

At the point where the os.fork() occurs, do both processes have stdin, stdout, and stderr all tied to the pty I am in? When it is forked, I understand that the child has an exact copy of all the file descriptors which should include stdin, stdout, and stderr that are similar to those of the parent.

I think I need to be using the termios module. However, the documentation for the Python termios module is slim to none. It is also a little hard to grasp. Could somebody give me an example of how I would use that module in my specific case?

jlinkels · 03-29-2014, 05:23 PM

I have no experience with Python forking, but right after the fork, both processes share the same pty. So to debug, write this piece in your code:

Code:

pid = os.fork();
	if pid > 0:
		print "this is the parent"
	if pid == 0:
		print "this is the child"

Both sentences should appear on screen. Which appears first is undetermined.
Once you see this, it is easier to take it from there.

jlinkels

trist007 · 03-29-2014, 07:17 PM

Ah ok thank you. Turns out I just needed to flush stdout continuously so that the output of the xmp process that the child ran continues to go to the screen.

trist007 · 03-29-2014, 08:42 PM

I got what I was looking for.

1. Continuously flush stdout so that the output of the child process can continuously be outputted to the terminal.

2. non-blocking console input so that I can add a filename of a song to a favorites file.

Code:

def isData():
	return select.select([sys.stdin], [], [], 0) == ([sys.stdin], [], [])

old_settings = termios.tcgetattr(sys.stdin)
try:
	tty.setcbreak(sys.stdin.fileno())
	i = 0
	while 1:
		print i
		i += 1

		if isData():
			c = sys.stdin.read(1)
			if c == '\x1b':         # x1b is ESC
				break

finally:
	termios.tcsetattr(sys.stdin, termios.TCSADRAIN, old_settings)