How to play sounds at different tones in C without using any external library? I know there are dozens of sound libraries in C that allows you to play sound but what I want to know is how does that work behind? How do you tell the computer to play a certain note at a certain tone/frequency?
I know it's possible on windows using the sound() function but I can't find any documentation talking about Linux, all that I found is the beep() function (or write(1, "\a", 1)) that outputs the default terminal beep but I can't figure out how to play different sounds.
The Linux kernel native audio API is ALSA (Advanced Linux Sound Architecture).
Example of raw audio playback with ALSA:
https://gist.github.com/ghedo/963382/815c98d1ba0eda1b486eb9d80d9a91a81d995283
However, ALSA is a low-level API that is not recommended to be used directly by higher-level applications.
A modern system audio API for GNU/Linux would be either PulseAudio (the current default on Ubuntu), or the newer and arguably better PipeWire (the default on Fedora).
Example of raw audio playback with PipeWire that generates audio "from scratch":
https://docs.pipewire.org/page_tutorial4.html
How do you tell the computer to play a certain note at a certain tone/frequency?
Sound is a mechanical vibration that propagates through the air (or another medium). It can be represented digitally as a sequence of numerical values representing air pressure at a given sampling rate. To play a given tone/frequency, generate a sine wave of that frequency (at the playback sampling rate) and use the sound API of your choice to play it.
See the PipeWire tutorial above for an example generating a 440Hz tone.
About PulseAudio/PipeWire:
These libraries are typically part of the OS and exposed as system APIs (so they are not "external libraries" if that means some library to ship with your program or to ask users to install), and they should be used by applications to play audio.
Behind the scene, these libraries handle audio routing, mixing, echo-canceling, recording, and playback to the kernel through ALSA (or using Bluetooth, etc). Everything that users and developers expect from the system audio layer.
Until recently, PulseAudio was the de-facto universal desktop system audio API, and many apps still use the PulseAudio API to play audio on GNU/Linux.
PipeWire includes compatibility with PulseAudio, so that apps using the PulseAudio API will keep working in the foreseeable future.
Example of raw audio playback with PulseAudio:
https://freedesktop.org/software/pulseaudio/doxygen/pacat-simple_8c-example.html