How to properly read BTN_0 in C

The manufacturer of the device connected a key to the BTN_0, which code is 0x100. It is astonishing I have difficulties finding the way to read status of it. They said it is a key like keyboard ENTER key, but of course it does not read using fgetc().
I found a number of source examples for shell, but no source examples for C. As I understood the task assumes accessing events through /dev/input/eventX device file. But there're a number of them, and I must figure out what device file to use for BTN_0. Someone suggests to look into /proc/bus/input/devices but I do not see how to connect them to BTN_0 key.
Please advise. Thank you.

Update: I found this which was actually recommended by the manufacturer. Questions:

  1. what would be the best handover of the button change event from shell script to C program? File operation?
  2. Is there any way to read BTN_0 state using some system call in C?