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+% The Linux Kernel
+
+[back](index.md)
+
+TODO: A word about how the kernel is in charge of enforcing security
+
+TODO: Talk about `dmesg`
+
+Role
+----
+
+A kernel is a piece of software that is loaded at boot time. It is a
+component that is responsible for "orchestrating" the OS in different
+manners. There are different types of kernel in this world, and they may
+have different responsibilities. We will be focusing solely on the role
+of the Linux kernel here.
+
+### Hardware control and abstraction
+
+One goal for a kernel is to be the "glue" between the hardware and
+other software.
+
+For example, let's suppose two pieces of software want to store data in
+a hard drive. Without hardware control, if these two softwares were to
+run concurrently, they both would try to control the spinning hard
+drive, potentially trying to make it go in different directions.
+
+Without hardware control, each software would have to consciously and
+constantly collaborate with each other to avoid conflict when accessing
+each piece of hardware.
+
+With the Linux kernel accessing the hardware is gated by an API: each
+software that wants to access the hard drive has to go use system calls
+(e.g.: `read(2)` / `write(2)`), and the kernel will then schedule
+these tasks sequentially.
+
+Another important point, is that without the Linux kernel, each piece of
+software would have to care if the hard drive is a spinning disk, an
+SSD, or a USB stick, or if is connected using a SATA cable, a USB cable,
+etc.
+
+The Linux kernel allows us to not care about this, and provides us with
+the filesystem abstraction, and mount points. To the software developer
+doing a `read` on a file doesn't change if the file is stored on an
+SSD or even on a network filesystem, because the kernel will check that
+for us, and apply the appropriate logic.
+
+### Multitasking
+
+Another big role of the Linux kernel is to allow multiple programs to
+run concurrently: the CPU doesn't have knowledge of processes or
+threads, it just runs binary code.
+
+Let's say we have a CPU with 4 cores. That means that the CPU can truly run
+only 4 tasks in parallel.
+
+As a user with this CPU, this doesn't impact me that I'm running a graphical
+interface, a web browser, a mail client, a terminal window, and a video player.
+Each of these 5 applications feels responsive even if the CPU can only run 4 of
+them at the time.
+
+This is because the kernel is responsible for switching the tasks that the CPU
+runs (a.k.a. scheduling). Theses tasks / applications are switched quickly and
+in such a way that they feel responsive to the user.
+
+The Linux kernel also provides developers with another abstraction to
+execute tasks concurrently: threads. Threads are akin to having multiple
+processes, except that when scheduled, they keep the same memory region
+for their heap.
+
+Without the kernel, the software developer would have to care about the
+CPU architecture, how much cores it has, and would have to find the
+right assembly instructions for the given CPU to run code in parallel,
+and if they want to run more tasks concurrently than the amount of
+available CPU cores, they would have to implement their own task
+scheduler.
+
+Thankfully we do not have to care about that, because the Linux kernel
+cares about it for us instead.
+
+Sources
+-------
+
+The official sources of the Linux kernel can be found in
+[kernel.org](https://www.kernel.org)[^distrib-patches].
+
+[^distrib-patches]:
+    Note that these are the *original* sources. Most GNU/Linux
+    distributions incorporate additional patches to their releases.
+
+The official programming language of the Linux kernel is C. It is
+heavily documented in the `Documentation` subdirectory, using the
+sphinx documentation system.
+
+
+Versioning
+----------
+
+TODO: this section
+
+Dependencies
+------------
+
+To compile a Linux kernel whose version is greater than 5.2, one needs
+the following dependencies:
+
+<!--
+A C compiler:: GCC, or Clang Perl 5:: An old scripting language bc:: A
+command line calculator net-tools:: Some networking-related command-line
+tools (contains for example the `ifconfig` command) openssl:: A well
+known cryptography library rsync:: A command line utility to copy /
+synchronize files and directories gmp:: A library for doing arithmetic
+with arbitrary precision mpc:: A library for doing complex arithmetic
+with arbitrary precision mpfr:: A library for doing floating-point
+arithmetic with arbitrary precision libelf:: A library for reading ELF
+file. ELF is the standard executable format used for Linux. utillinux::
+A set of system utilities for Linux (contains for example the `dmesg`
+command) bison:: A parser generator flex:: A lexical analyser generator
+cpio:: A command-line utility for CPIO compressed archives
+-->
+
+TODO: Also refer to their Debian, Ubuntu, etc. packages.
+
+Configuring the kernel
+----------------------
+
+```sh
+make nconfig
+```
+
+Building the kernel
+-------------------
+
+```sh
+make -j8
+```
+
+Running the kernel
+------------------
+
+TODO: link the QEMU article
+
+
+```c
+#include <stdio.h>
+
+/**
+ * \brief The entry point of the program.
+ *
+ * \param argc the number of command line arguments
+ * \param argv the command line argument, as an array of NULL terminated strings
+ */
+int main(int argc, char** argv) {
+        for(int i = 0; i < argc; ++i) {
+                printf("Argument %i: %s\n", i, argv[i]);
+        }
+}
+```