What is the file type of a kernel (Linux kernel for example)?
I have been very interested in the Linux Kernel and operating systems in general. The thing i was wondering was, what is the file type or extension that a kernel has? It obviously doesn’t have a .exe or .out extension because they are used for applications installed on operating systems. Is a kernel just a Binary file? Bonus question: I know that the Linux Kernel source code is divided over many .c files, but i was wondering if, when compiling, all these files are compiled into a single binary file or are the linked externally? (I hope my queston(s) are not to vague)
To answer your bonus question, compile a kernel and you’ll see (the process is verbose enough than you can tell what’s going on, but not so verbose as to flood you with gcc output). As for the rest of your post, the kernel is a binary file, and usually has no extension.
Try the file command on the kernel executable. (Sorry, off the top of my head, I don’t know the full path, and I’m not on a Linux machine at the moment) file knows more than a file name extension, with details such as architecture, etc.
2 Answers 2
The Linux Kernel (or any other OS Kernel) is just a binary image containing machine code for the target architecture. It is kinda like a statically linked executable, because there’s no operating system to link any dependecy before it’s running,so that once loaded in the main memory, it is able to execute without any other helper. This doesn’t mean that it cannot load any other module dynamically. In Linux, this behavior is easily seen when you load a module from userspace (it’s different process from loading a .so file tough).
That image may be compressed before being stored in the filesystem, and that’s why you may get something like this output from «file»:
file /boot/vmlinuz-2.6.39-400.215.7.el6uek.x86_64
/boot/vmlinuz-2.6.39-400.215.7.el6uek.x86_64: Linux kernel x86 boot executable bzImage, version 2.6.39-400.215.7.el6uek.x86_64 , RO-rootFS, swap_dev 0x3, Normal VGA
How to View and Browse the Linux Kernel Source?
This article briefly explains the Linux kernel is and how to browse its source code.
This tutorial is optimized for inexperienced Linux users and programmers who want to read the kernel source code or become Linux kernel developers. The Linux kernel is universal; therefore, the present document is useful for all Linux distributions.
By reading this article, you will understand what the kernel is, how it is structured and how to browse through its components. This article was initially written in 2019 and updated in 2022.
What Is the Linux Kernel and How To Browse It
The Linux kernel is the main component allowing interaction between the software and the hardware with the help of device drivers. You can think about the Linux kernel as a translator allowing communication between programs, the hardware, and the user interface.
Learning how to browse the Linux kernel is the first step for any programmer who wants to become a kernel developer. It is also useful for programmers or users who want to read its source code or modify it.
The Linux Kernel Structure
The Linux kernel is structured with directories and subdirectories known as “kernel tree”, users can browse to access specific sections of the whole code. This allows users and developers to focus on specific parts of the code.
Except for the Slackware Linux distribution, whose kernel is maintained by a single person, the rest of the Linux distributions’ huge developers’ community divide tasks based on the kernel tree. In contrast, several developers focus on drivers, others focus on security, and others on virtualization.
The kernel source tree is the following structure described through which users can browse the source code:
Viewing and Browsing the Linux Kernel Source Code
Usually, when we install Linux, we don’t include the source code. We can install Linux and keep the source code in our system. But most users discard it to download a smaller ISO image. After all, you can download the source code later if you need it.
If the kernel source code is present in your system, you can find it under the /usr/src/linux- directory, where must be replaced with the source code kernel version you are dealing with.
You also can find the last kernel source code at https://github.com/torvalds/linux.
As you can see in the previous screenshot or at the provided URL, within the source code tree, we can find the following main directories and files:
- arch: This directory contains source codes for different system architectures supported by the Linux kernel.
- block: The block I/O layer directory contains code for the management of block devices (such as hard disks, DVDs, floppy disks, etc.) and their requests.
- certs: The certs directory contains certificates and signs files to allow module signatures for the kernel to load signed modules. This can be useful to prevent malicious code from running within kernel modules like rootkits.
- COPYING: This file has information about licenses for the source code
- CREDITS: A partial listing of people who have contributed to the linux kernel source code
- Crypto: This directory contains cryptographic ciphers, which handle cryptographic and compression tasks.
- Documentation: As its name suggests, this directory includes all documentation about all kernel components.
- drivers: The drivers directory contains source code for device drivers to support hardware. Inside this directory, you’ll find subdirectories for each hardware, such as video, Bluetooth, etc. (any supported hardware).
- Fs: This directory contains sources for file systems, including virtual file systems, plus read and write file systems.
- include: This directory contains C headers for kernel files, such as functions to compile code.
- init: The init directory contains source code related to the boot process and the initialization of the Linux kernel. The source code is stored in a file named mainc.c within the /init directory. The code initializes the kernel and some initial processes.
- ipc: Inter-Process Communication, such as signals and pipes.
- Kbuild: The Kbuild file indicates object files for vmlinuz.
- Kconfig: The Kconfig file is used to disable or enable kernel features. Users deal with running the “make menuconfig” command.
- kernel: The kernel directory includes core subsystems, such as scheduler signal handling code.
- lib: This directory includes library routines, common string operations, hardware-dependent operations, debugging routines, and command-line parsing code.
- LICENSES: It contains the licenses to be applied to the kernel source and individual source files, which may have different licenses. Inside this directory, you will find four main subdirectories containing supported licenses like LGPL-X, Apache license, etc.
- MANTAINERS: The MAINTAINERS file shows the kernel maintainers’ list and instructions to submit kernel changes.
- Makefile: This file reads the .config file and builds modules and vmlinux.
- mm: The Linux kernel manages hardware and virtual memory (swap). This is the directory where the source code for memory management is stored.
- net: The net directory contains the network stack. Here, you can find code related to communication protocols, such as IP, TCP, and UP.
- README: The README file refers to guides for kernel developers in the Documentation directory previously described.
- Samples: The samples directory includes kernel and configuration files sample code.
- scripts: Here are stored the necessary scripts to build the Linux kernel.
- security: The security subdirectory contains the Security-Enhanced Linux (SELinux) security framework.
- sound: Here, you can find the sound subsystem, including sound drivers, driver architectures, and code related to sound, such as ALSA.
- tools: Here are tools for compressed kernel development, such as ACPI, cgroup, USB testing tools, vhost test module, GPIO, IIO and SPI tools, and Inter energy policy tool.
- usr: here, usr stores the initramfs, which roots the filesystem and init in the kernel memory cache. Code included allows you to execute code in user space in the boot process when the kernel isn’t fully loaded.
- virt: The virtualization directory contains the KVM (Kernel Virtual Machine) module for hypervisors, including architectures.
These are all top files and directories included in the main kernel source directory. You can browse each section depending on your needs. Once you understand the purpose of all files and directories listed above, subdirectories are pretty intuitive.
Conclusion
As you can see, this article was written for novel and medium users to understand the basics of the Linux kernel. Yet, some knowledge on development under Linux, hardware, and operating system characteristics is necessary for practical use. The previous lecture is excellent as an introduction to the Linux kernel structure and Linux distribution. Keep in mind that the Linux kernel evolves periodically. That’s why we keep the previously described content up-to-date.
We hope you found this article useful. Keep reading LinuxHint for more Linux professional articles.