Global variable.
Parameter to a function.
Pass a functor/lambda, get the value.
Environment variable.
Registry.
File.
Store it in a ping response.
VRAM texture that you write and then read.
Database.
REST server.
I'm pretty sure I could come up with a thousand ways.
But... why? Why do I want to write this book? Why has it been on my mind?
And something my friend wrote a few years ago: https://github.com/radian628/tram
Data storage and transmission has so much innovation going on!
It's very compiler-focused, but it's one piece of the puzzle.
You have the camera... I have the monitor... How about I flash QR codes? Oh, the camera is on top of the monitor, facing the room? Well, at night, I could FLASH the monitor full white in binary, and you can see the light on the chair and wall, right?
I'll leave the disc head on the sector number. You read the disc head position.
I'll exit my function on a time modulo a number to encode the remainder. You see what time you were called, and can decode the remainder.
I'll warm the CPU to a temperature... you read the temperature...
The list goes on and on...
"The Exfiltration Manual"...?