readValue needs to be passed as a reference, since the contents will be modified by the i2c_read function. That's likely causing the segmentation fault.

Or change readValue to a point and allocate two bytes before passing it to i2c_read

See the documentation for the i2c_read function for more details: http://docs.onion.io/omega2-docs/i2c-c-library.html#i2c-c-read-multiple-bytes

@Matt-Gessner-0 while it is technically possible to do some development on the omega, it is quite impractical. You will quickly run out of capacity and patience and you'll end up using a more suitable build environment. Save youself the time and heartache. Take a look at the cross compiling docs it is easy to set up a build environment and use the better resources of a proper development platform

=== Version Info ===
Omega firmware: v0.2.1 b199
onion-os - 1.0.5-1

Wow, thank you so much! This is exactly what I was looking for.

If you have a build systen installed You can tweak the kernel setup by using "make kernel_menuconfig"

Look under Device Drivers -> I2C Support, there is a several debugging options you can set in order to assist with debugging i2c.

With raspberry pi1 (the 2011 model) the screen are working but not with omega that not recognize them.
I will probably resell my omega and various accessories because the assistance is not very helpful :-/

@Michal-Dobrzynski Please do not forget to remove the pin3-pin4 short circuit if you want to plug the Oled Expansion to any genuine Onion Omega Expansion Board Header directly because Omega2(+) may be damaged if you set GPIO11 to Low.

There are I2C level shifters on the OLED Expansion.
Pin4 of the Expansion Header provides the voltage to the lower voltage section for those level shifters.
But pin4 of the Expansion Header is connected to GPIO11 on the Omega2(+) board.
So @Onion must set GPIO11 to High (~3.3V). It's a hidden simple hack that @Onion forgot to mention for example here.
You could find some further technical details here.

@Maximilian-Gerhardt Thank You.

I have been fighting against an error where it looked like the linker was not finding my the shared libonioni2c.so. Your suggestion about first renaming the current files /usr/lib/libonioni2c.so to /usr/lib/libonioni2c.so.XX and /usr/lib/liboniondebug.so to /usr/lib/liboniondebug.so.XX was a good suggestion.

My error changed to libonioni2c.so not found. So it shows me the linker was finding the correct shared object but there was something wrong with that libonioni2c.so share object. then I copied the shared objecs from github https://github.com/gamer-cndg/omega2-libs and put them in /usr/lib and everything worked!!!

2 weeks I have wasted.

Now I don't know what is in those shared objects or what is different but at least I can continue building my project.

From my 2 weeks of trying everything I have concluded that the shared object libraries provided in the standard distribution for the 2+ libonioni2c.so and/or liboniondebug.so are defective.

Sorry, my fault :-) 194 is ok!

Thank you @György-Farkas for detailed answer.
I've tried this ADC (on demo board) on Arduino and also received nothing.
I think that this board is broken.
I bought ADS1115 and now I can smoothly read from photodiode :)

If somebody will need that in future guys from Control Everything Community made working python example for Omega2.
(or at least it's working in my case with FW: 0.2.0 b191)

@Lazar-Demin Great to see it working. If you need a gateway you can also use a ESP8266 NodeMCU board with another RFM95 module and configure and flash the https://github.com/things4u/ESP-1ch-Gateway-v5.0 firwmare on it, also see http://things4u.github.io/. Needs a little bit of fiddling around with the configuration and pinout; Also be aware that the EUI of the TTN gateway must be the one that the firmware spits out on startup (it's made up of the ESP8266's MAC address and two 0xff in the middle).

you can try this python lib
https://github.com/bhaskar-anil429/MCP9600/blob/master/MCP9600.py
or this c
https://github.com/bhaskar-anil429/MCP9600/blob/master/MCP9600.c
You can replace i2c read write with onion syntax.