Ten minute hacks: Process pause & resume

I’m old school and still rocking an old X220 laptop because I didn’t like the new ones. My battery life isn’t as great as I’d like it to be, but it gets worse when some “webapp” (which I’d much rather have as a native GTK+ app) causes Firefox to rev my CPU with their websocket (hi gmail!) poller.

This seems to happen most often on planes or when I’m disconnected from the internet. Since it’s difficult to know which tab is the offending one, and since I might want to keep that tabs state anyway, I decided to write a little shell script to pause and resume misbehaving processes.

After putting it into my ~/bin/ and running chmod u+x ~/bin/pause-continue.sh on it, you can now:

james@computer:~$ pause-continue.sh firefox
Stopping 'firefox'...
[press any key to continue]
Continuing 'firefox'...
james@computer:~$ echo $?        # error codes work iirc
0
james@computer:~$

The code is trivially simple, with an added curses hack to make this 13% more fun. It sends a SIGSTOP signal initially, and then when you press a key it resumes it with SIGCONT. Here it is the code.

You should obviously substitute in the name of the process that you’d like to pause and resume. If your process breaks because it didn’t deal well with the signals, then you get to keep both pieces!

This should help me on my upcoming travel! I’ll be presenting some of my mgmtconfig work at DevConf.cz, FOSDEM and CfgMgmtCamp.eu! CfgMgmtCamp will also have a short mgmt track (looking forward to seeing Felix present!) and we’ll be around to hack on the 8th during fringe (the day after the official camp) if you’d like help to get your patch merged! I’m looking forward to it!

Happy hacking!

James

Ten minute hacks: Hacking airplane headphones

I was stuck on a 14 hour flight last week, and to my disappointment, only one of the two headphone speakers were working. The plane’s media centre has an audio connector that looks like this:

airplane-headphones-jack

Someone should consider probing this USB port.

The hole to the left is smaller than a 3.5mm headphone jack, and designed for a proprietary headphone connector that I didn’t have, and the two holes to the right are part of a different proprietary connector which match with the cheap airline headphones to provide the left and right audio channels.

airplane-headphones-connected

Completely reversible, and therefore completely ambiguous. Stereo is so 1880’s anyways.

By reversing the connector, I was quickly able to determine that the headphones were not faulty, because this swapped the missing audio channel to the other ear. It’s also immediately obvious that since there are no left vs. right polarity markings on either the receptacle or the headphones, there’s a 50% chance that you’ll get reverse stereo.

With the fault identified, and lots of time to kill, I decided to try to hack a workaround. I borrowed some tweezers from a nearby passenger, and slowly ripped off some of the exterior plastic to expose the signal wires. To my surprise there were actually four wires, instead of three using a shared ground.

airplane-headphones-separated

Headphone wires stripped, exposed and ready for splicing.

With a bit of care this only took about five minutes. The next step was to “patch” the working positive and ground wires from the working channel, into the speaker from the broken channel. I did this by trial and error using a bit of intuition to try to keep both speakers in phase.

airplane-headphones-spliced

After a twist splice and using paper as an insulator.

A small scrap of paper acted as an insulator to prevent short circuits between the positive and negative wires. Lastly, a figure eight on a bight was tied to isolate the weak splice from any tension, thus preventing damage and disconnects.

airplane-headphones-knotted

All wrapped up neatly and tied with a knot.

The finished product worked beautifully, despite now only providing monaural audio and is about five centimetres shorter, which is still perfectly usable since the seats hardly recline. The flight staff weren’t angry that I had cannibalized their headphones, but also didn’t understand how my contraption was able to solve the problem.

This fun little ten minute hack helped provide some distraction in economy class, and maybe it will be useful to you since I doubt they’ve repaired the media system in the seat! If you work for Emirates, let me know and I’ll give you the seat and flight number.

Happy hacking!

James