Author: Simone

POSIX Signals are not just bad, they’re awful

Simone Leave a comment

It all started with a problem which seem to be simple and already seen around in the real world:

I just need a simple script to restart a process if it crashes.

Simple enough right? So simple that I wanted to use a short bash script to do it. Turns out it’s not really that simple, especially if your thought process continues with

And I will use signals to control it.

Okay, now it’s really a bad idea which can fail for a lot of reasons:

 1. Checking if a process has crashed is not idiot-proof

The idiot being the one that writes a C program that exits main() with “return(-1);” or any other negative value. The exit status is a signed byte, thus a negative value will wrap around to a > 127 value and be indistinguishable from a process crash.

A program exiting with “return(-119);” has the same exit status as one killed with kill -9 (SIGKILL). The fun!

 2. You can’t always control signals in bash

According to POSIX specs, bash can’t change the signal handler for signals which were ignored in the parent process. So this script would be unusable if you happen to launch it from a process which has the relevant signals ignored. That’s where I got the idea to use a one-line python wrapper around my bash script to enable all signals before giving control to bash.

 3. Using a signal wrapper spawns a second process

It turned out quickly that it wasn’t a great idea. Using a python wrapper around a bash script will obviously generate a second process, one for bash and one for python.
So in just one move I spawned three more problems:

– the python wrapper has the process name you would expect (the wrapper name) and has every signal enabled, i.e. any signal would kill it
– the bash script has a different process name which makes it counter-intuitive which one is the process to send signals to
– the bash script starts after the python wrapper so if two wrappers start simultaneously, we have a funny race condition to deal with.

So, I just decided to drop bash and rewrite everything in python instead.

 4. There is no standard signal to ask a process to restart

Usually it’s SIGHUP, but it’s not universally true. If your controlled process can be restarted with a signal, that signal should be SIGHUP, but no guarantees.

 5. Signals are not setup immediately at startup

When your control script starts, all signals have their default handler. So for example if you launch your control script and then immediately decide to restart the controlled process (with a SIGHUP signal), it may happen that the controller gets killed instead, and the child process is left with no control.

 6. SIGKILL can’t be handled

If your control script is killed with SIGKILL, the child process is left running with no control.

 7. The child process exiting will have interesting race conditions

Say for example that the restart behaviour will be to send SIGHUP to the child process, wait for its exit, and launch the child process again.
If your control script is asked to restart the child process several times in a row, it might be sending several signals to a PID, which after the first SIGHUP might not correspond to any running process (not so bad) or correspond to a different process recently spawned (definitely not good).

In certain *NIX flavours you can setup a signal handler that will fire when the child process exits, but based on my research this is not true for every OS out there.

 8. There is no alternative to signals

I might have finished this list with better news, but unfortunately the only way to ask a process to terminate is, yes, signals.

The DOs:

 Use a lockfile to ensure a single instance of your controller is running.

 Use process groups.

 Handle SIGCHLD if your OS uses it to signal child processes exiting.

 Use systemd if your OS has it. It might get kind of long to config properly (especially for a dynamic list of processes), though.

PHP not parsing POST data?

Simone 1 Comment

If your PHP (or any other server side code) is receiving a single $_POST field of raw data, looking more or less like this, instead of receiving your form fields nicely:

——WebKitFormBoundaryVmWPyJJIBR3n18Wk
Content-Disposition: form-data; name=”form-file-1″; filename=”my document.doc”
Content-Type: application/octet-stream
——WebKitFormBoundaryVmWPyJJIBR3n18Wk
Content-Disposition: form-data; name=”form-field-1″

form-value-1
——WebKitFormBoundaryVmWPyJJIBR3n18Wk–

(that WebKitFormBoundary is probably the sign that you are using Chrome)

Then, chances are your javascript is overriding the content-type header. Check out the request headers, and if you find anything looking like this:

Content-Type:application/x-www-form-urlencoded;

or really anything else which is not this:

Content-Type:multipart/form-data; boundary=—-WebKitFormBoundaryVmWPyJJIBR3n18Wk

then you need to fix the content-type header. It might be simpler than you think; try setting it to `false` (in jQuery / Prototype / …) or disable anything javascript is setting there. The browser will take care of it better than what JS is doing.

In my case, I was getting this problem because I was using the js FormData object to extract form data and then sending it the same way it used to do with “regular” form data. Turns out the content-type header should be set differently. Maybe I saved you a couple of hours of searching.

How to not drive a motor with an Arduino

Simone 2 Comments

If you are like me, when you were an absolute beginner, you tried to wire a motor up to some batteries, saw the motor was spinning, and thought out loud: “It’s working! How hard could it be to turn it on and off with an Arduino?”

Actually a little more than it seemed at first. There are several problems I encountered and slowly fixed, and I’ll try to sum up in this post the kind of iterative reasoning that led to a decent circuit design.

motor1

So, first thing you try is to attach the battery poles to the motor, keeping the wires in place with your fingers. Everything seems to work fine! So what’s next? What’s the most obvious attempt to control it?

motor2

The second step would be to add a switch. Pushing the switch makes it run, release the switch and it stops. Perfect: now you just have to use an Arduino to open and close that switch.
motor3
This might be the most obvious change for a beginner, but also a very very bad idea. All the current is passing through the Arduino which is, in best cases, limited at 40mA, while even a very small motor will probably need way more than that. This might work for very very small motors but overall you risk damaging the pins.

motor4

So a “better” (but still bad) idea is to use a transistor instead! The configuration above is called emitter-follower. After all, transistor are like switches, and you have a lot of 2n2222 lying around, and they are a buck a kilogram, so why not make use of them? And maybe this circuit is kind of working, but it has at least two issues you should be considering now:

  • What is the current required by the motor? Check the datasheet, and remember it probably needs a way higher amount when it is starting from a still position. The 2n2222 has a maximum rating of 500mA which might not be enough even if your motor is rated to consume only, say, 100mA. A cheap option would be to use a Darlington pair (basically, more transistors), but a better choice would be to use a different transistor which can bear a bigger current, like a MOSFET (e.g. a IRF510, still cheap and easily available) or possibly a relay. (As you probably know there are several models of BJTs, MOSFETs and relays and I’m pretty sure someone skilled enough could write a book about proper choice of this part).
  • the other issue is that a part of the current is still sourced through the Arduino, so before burning something for good, a nice idea would be to add a resistor for protection, so that the maximum current flowing out of the Arduino is limited. (This is probably not an issue if using a MOSFET).

motor5

This will add some protection to the Arduino, but really, why risk? We can change the circuit so that none of the current flowing from the Arduino actually goes to the motor.

motor6

This configuration, called common emitter, is getting better, but there’s still something needed to be done. The motor is a typical inductive load, and when the transistor tries to open the circuit, the motor will try to keep going, forcing a current in the transistor (which is bad and will likely damage it) and/or in the Arduino (which will cause random resets and damage the pins). Another improvement is to add a flyback diode, so that the current has somewhere to go when the circuit is open.

motor7

There. It probably isn’t finished here, but it’s getting on the safe side. Then you might want to drive the motor in reverse, vary its speed, but this will probably be discussed in another post.

(Credits: graphics in this page were drawn with Fritzing.)

My experience with Soylent alternatives in Europe so far

Simone 1 Comment

Unfortunately in Europe there are no Soylent resellers. There are, however, quite a bunch of less famous alternatives. I’ve been asked about them a couple of times so I felt it was worth a blog entry.

I’ve been trying Soylent alternatives since November 2015, replacing about 30-40% of my meals. I had the chance to try Jimmi Joy (formerly Joylent), Huel, Mana, Nano, Jake and Futricio (formerly SoylentLife) and here are some of the differences between them.

Jimmi Joy: it is the one I’ve been recommending to everyone wanting to try out Soylent alternatives. Jimmi Joy is probably the one with the less barriers to entry. It is one of the cheapest, it comes with a handy bottle, it leaves a good fullness sensation and has a good variety of the best flavors. Cons: it tastes really better if you either blend it or leave it to rest for 8 hours or more, and variety is limited to its 5/6 flavors as there is no “unflavoured” option. Jimmi Joy also offers a “sports” version, and their bars are legit.

Mana: I’m more cautious in recommending it to beginners, since I feel it needs a little more experience to appreciate. Pros: very good ingredients (almost completely natural, and all of them are laboratory tested), has a good fullness sensation, and is almost unflavored, meaning you can add whatever flavor you want; this gives Mana the best flavor variety, in a way. Cons: it’s one of the most expensive, their bottle is not up to par with the others, and preparing it needs some more dedication.

Huel: I find this an excellent product, and the mix of different flours (oats, pea, and flaxseed) is very interesting. Pros: free shirt, good bottle, 30-30-40 macros ratio, cheap, comes in unflavoured variety, best “fullness” sensation yet, can be used for baking (not tried). Cons: does not dissolve easily (a spoon helps a lot), it is REALLY flavourless, needs a lot of flavours added (which they luckily sell along it). It is a great product if you want to stick to the unflavoured variety (and add some flavors yourself, but again, it’s dedication for such a product), otherwise the vanilla variety gets old after a while.

Jake: I have been trying their Sports variety because of the added protein contents, but after all it didn’t make me feel like I really needed them. Pros: good bottle included, single portion bags are very easy to prepare, and good vanilla flavor. Cons: only vanilla is available, and it didn’t make me feel really “full”. After a while I was craving for something different. Overall I think I could recommend it for casual use, but not for every day.

Nano: it is very similar to Jake (they share vanilla, pea flour, and crushed flaxseed as ingredients). Pros: less flavoured than Jake, you can actually add other flavours for variety, better “full” sensation than Jake, single portion bags. Cons: the included bottle is hard to clean, and it never makes me feel really “full”, just like Jake.

Nano Veggie: it is a unique product as it’s meant to be consumed hot. It has a quite interesting spicy tomato soup consistency. Pros: unique taste, single portion bags. Cons: only one flavour, and the same downsides as regular Nano.

Futricio: I was not satisfied with this. Pros: bottle included, single portion bags, 5 flavors available. Cons: flavors are not as good as Joylent ones, and it didn’t really make me feel “full”.

There are a couple of others I did not try out: Queal, Bertrand, and so on. Other reviews have not encouraged me to do so, but if somebody wants to trade bags, I’m open.

I also compiled a list of ingredients easily found in Italy, and attempted to make a recipe out of those (with an simple optimization tool I wrote), but still haven’t had a chance to try it out.

Anagrhash, a anagram generator & reverse hash searching tool

Simone Leave a comment

Anagrhash is a tool which can generate anagrams, and test them against a hash.

It is a tool I wrote many years ago for educative purposes, to try out some different concurrency models and inter-thread communication methods in C, and get some basics of programming in the GNU/Linux environment.

It serves as an anagram generator, and can shuffle letters, but can also be set to shuffle whole words in a sentence, use tokens from a list (words which you don’t want to appear simultaneously), and specify separators between tokens (like spaces or commas).

It can also test anagrams which satisfies an exact target hash, or a hash specified with a regular expression.

You can get it on my GitHub page.

xboxdrv 0.8.8 for the Raspberry Pi Zero

Simone Leave a comment

If you, like me, have just found out that GameStop xbox360 controllers, even when they have the same BB070-B-EU model, from the same shop, same look, same everything, … but they happen to have different chipsets, and not all of them work properly with xboxdrv 0.8.5; you might appreciate an update from the version which is offered in the Raspbian repositories.

In my case, the incompatible model was showing up with magic numbers 0e6f:0401 in `lsusb`. The GameStop xbox360 controllers which works with xboxdrv 0.8.5 shows up as 0e6f:0301.

Compiling it took quite a long time (about two hours), so here’s a precompiled package. You can get xboxdrv 0.8.8 in a .deb package for ARM here.

The online replacement vs repair decision helper

Simone 1 Comment

I wrote this tool to help decision-making when you have have to repair or replace something.
It uses a pair of jeans as an example, but it’s meant to be generic. Just replace its parameters and let it calculate.

(I’m assuming as a currency and one as a time unit.
I also assume a %/year growth rate of your assets.)

You are thinking about replacing a:
You have owned pair of jeans for: year(s)
Replacing the pair of jeans would cost you: $
Your pair of jeans is costing you in repairs: $ / year
While the pair of jeans is undergoing repairs, you have a backup plan which costs you: $ / year
Do you feel the pair of jeans at risk of a major, catastrophic, sudden failure?
Not really.
Possibly… but maybe an ice cream would cheer me up again. (+5 $ / year)
I think I’ll have to pay a couple of fine dinners if that happens. (+50 $ / year)
Well, that would cost me about $.
Would a new pair of jeans have significant advantages, or technological advancements, over the old one?
Not really. I’m kind of emotionally bonded to the old one.
For starters, it wouldn’t look old. (+15% value over the old one.)
It has an improvement or two. (+30% value over the old one.)
The new one is way better! (+% value over the old one.)

Bluetooth low-energy temperature beacon using the nRF24L01: cheap and compatible with existing smartphone apps!

Simone 8 Comments

The reason I did this project is because I have a slight overheating issue in a server room; I was wondering if there would be some way to check another room’s temperature from my desk. That’s how I came up with a “wireless thermometer” that can send readings to my Android phone, and my colleagues can use it too!

You might have heard about the nRF24L01: it is a cheap (0.80€) radio frequency module that, back in 2013, Dmitry Grinberg was able to use to “fake” a BTLE beacon. Real Bluetooth 4.0 modules are, nowadays, about 5€ each.

I wondered if I could use it to make a BTLE compatible temperature beacon. Turns out there are 3 major BTLE beacon protocols: iBeacon (by Apple), Eddystone (by Google), and AltBeacon (by Radius Networks). And well, none of them can be emulated with the nRF24L01, since it is only able to send a 16-byte payload (among other limitations), and all three of those protocols require bigger PDUs.

BUT!!! Although I didn’t find a name for it, Nordic Semiconductors Bluetooth ICs (namely the nRF8001 and nRF51822) have their own protocol they use to send telemetry data (which means: temperature, battery level and device ID); turns out this protocol is simple enough to be emulated by the nRF24L01 as well, although with some limitations. They also are so nice to distribute a suite of Android and iOS apps to work with them; the most relevant apps are nRF Master Control Panel (useful for debugging BTLE devices) and nRF Temp 2.0 (a temperature logger; I think it was meant to track device overheating, but hey). You can also download the source code from the app page!

wpid-wp-1444076649110.jpeg  wpid-wp-1444076692964.jpeg
However, the temperature in my room is quite boring, as shown from those screenshots of Nordic Semiconductors apps. (nRF Master Control Panel on the left, nRF Temp 2.0 on the right).

The temperature beacon hardware is very simple. Just wire up your microcontroller to a nRF24L01 and a temperature sensor of your choice (I used a DHT11 I had lying around).

wpid-wp-1444076633700.jpegIt’s more about how you wire them up, I guess.

 rf24-pin
(fabacademy.org)

  1.  GND -> GND on the Arduino
  2. VCC -> 3.3v on the Arduino
  3. CE -> PIN 9 (see below)
  4. CSN -> PIN 10 (see below)
  5. SCK -> PIN 13 on the Arduino Uno
  6. MOSI -> PIN 11 on the Arduino Uno
  7. MISO -> PIN 12 on the Arduino Uno
  8. IRQ -> not used

Be especially careful about the power pin: altough the nRF24L01 can work with your 5v Arduino, you must power it at 3.3v!

While SCK/MOSI/MISO are pretty much wired to 13-11-12 pins in order to use SPI, CE and CSN can be moved to a pin of your choice.

About the DHT11, just wire its output pin to A0, and the other two to ground/+5v.

On the software side, I did a couple modifications on floe’s BTLE library to allow mimicking of the nRF8001 and nRF51822; you can find the BTLE library here. With this library, you can now create your temperature beacon. (Notice I used pins 9 and 10 as CE and CSN, as mentioned previously).

Did you know? Most Arduinos have an onboard temperature sensor! The reasoning behind this probably is that a device malfunction would overheat it. Unfortunately this can’t be easily used as an ambient temperature sensor, because the chip temperature is widely influenced by its heat dissipation. That’s why I used a DHT11. (You can read more about this on Arduino Playground here).

Among other limitations, this setup is not able to use more than 3 of the 40 BT 4.0 channels, and broadcast a name longer than 8 characters (remember that 16 byte limit?). I’m pretty sure it could be used to broadcast the battery level as well.

Sadly, it does not seem somebody has thought of writing a standard protocol for broadcasting some other non-temperature parameter (atmosphere pressure, light, noise level, etc). To a certain extent, nRF Master Control Panel can be used to receive arbitrary data from the Arduino (as float or as integer value), but it’s not suitable for logging and combined with the 16 byte limit it’s quite limited (BT also has a 2 bytes overhead for every field you want to send, hence: 8 bytes device name + 4 bytes temperature + 2*2 bytes of overhead = 16 bytes). If I’m wrong, let me know!

I hope you enjoyed this article!