The C Language in Operating Systems: Applications of C in the Linux Kernel

In the field of operating systems, the C language is undoubtedly a crucial programming language. Especially in the development of the Linux kernel, C serves as the primary programming language, allowing users to interact with hardware at a lower level. In this article, we will delve into the applications of C in the Linux kernel, providing specific examples to help beginners understand how to use C for operating system-related programming.

1. Why Choose C Language?

1.1 Efficiency

The C language provides control capabilities close to the underlying hardware, making programs run more efficiently. Compared to other high-level programming languages, using C can generate smaller and faster executable files.

1.2 Portability

Most Unix-like operating systems (including Linux) are written in C. This makes software developed in C easily portable across different platforms.

1.3 Low-level Operations

Through pointers and direct memory access, programmers can have better control over data structures and computer architecture, which is essential for low-level drivers and resource management.

2. Overview of the Linux Kernel

The Linux kernel is a multitasking, multi-user operating system core with high security. Its functions include:

Process management
Memory management
File system management
Device driver management, etc.

3. Writing a Simple Linux Kernel Module

To demonstrate the application of C in the Linux kernel, we will create a simple “Hello, World!” kernel module, which is a good starting point for learning kernel module development.

3.1 Environment Setup

Ensure you have the following tools installed:

GCC (GNU Compiler Collection)
Make tool
Linux Kernel Headers (matching the current Kernel version)

3.2 Creating the Module File

First, create a new folder in your working directory, for example, helloworld:

mkdir helloworld && cd helloworld

Then, create a source file named hello.c in that directory and add the following code:

#include <linux/init.h>
#include <linux/module.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple Hello World Module");
static int __init hello_init(void) {
    printk(KERN_INFO "Hello, World!\n");
    return 0;
}
static void __exit hello_exit(void) {
    printk(KERN_INFO "Goodbye, World!\n");
}
module_init(hello_init);
module_exit(hello_exit);

Source Code Explanation:

#include <linux/init.h>: Includes the header for functions needed to initialize and exit the module.
#include <linux/module.h>: Includes methods related to loading and unloading modules.
MODULE_LICENSE, MODULE_AUTHOR, and MODULE_DESCRIPTION are macros used to provide information about this module.

hello_init() function will be called when the module is loaded, while hello_exit() will be called upon unloading. In these two functions, we confirm their successful invocation by printing messages.

3.3 Creating a Makefile

Next, create a file named Makefile in the same directory with the following content:

obj-m += hello.o
all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Makefile Explanation:

This Makefile is used to build our kernel module, where:

obj-m indicates which object files we want to generate, here it is “hello.o”.
The all rule specifies what should be done when the make command is run; it calls the kernel build system for compilation.
The clean rule is used to remove all temporary files generated during the build to keep the environment tidy.

3.4 Compiling and Loading the Module

Open a terminal and run the following command in your working directory to compile the module:

make

Once completed, you can use the following command to insert your Hello World module into the kernel:

sudo insmod hello.ko

To view the generated messages, you can use the command:

dmesg | tail

You should see the message “Hello, World!” output. Here, you have successfully run code using C in the Linux kernel!

To remove the module, you can run the following command:

sudo rmmod hello && dmesg | tail

You should also see the message “Goodbye, World!” output, indicating that the module has been successfully unloaded.

3.5 Cleaning Up the Build Environment

Finally, to ensure you have a clean workspace, you can execute the following command to delete the compiled platform object files and other unnecessary content:

make clean

Conclusion

Through the above example, we have initially experienced the C calling mechanism and how to create, compile, and manage the most basic unit in Linux—custom modules. In your future learning journey, you will gradually master more complex issues such as scheduling, synchronization, device drivers, etc., all of which rely on extensive practice of C’s control capabilities over low-level mechanisms. If you want to learn more about the Linux kernel or other related topics, consider expanding your projects or reading more literature to broaden your horizons.