Operating System in C: Device Drivers

Introduction

Device drivers are a crucial component of an operating system, acting as a bridge between hardware and software. Through device drivers, the operating system can control and manage hardware devices such as printers, graphics cards, and network adapters. In this article, we will delve into how to write simple device drivers using the C programming language, providing code examples to help beginners understand.

1. What is a Device Driver?

A device driver is a special type of software that allows the operating system to communicate with hardware. Each hardware component typically has a corresponding driver, enabling the operating system to access these components through a standard interface without needing to understand their specific implementation details.

1.1 Functions of a Driver

Initialization: Configuring and initializing hardware at startup.
Read and Write Data: Handling data requests from the application layer.
Interrupt Handling: Responding to interrupt signals from hardware.
Resource Management: Managing access to shared resources (such as memory).

2. Writing a Simple Character Device Driver

In a Linux environment, we will create a simple character device driver. Character devices allow for byte-wise reading and writing of data, suitable for input/output scenarios like keyboards and serial ports.

2.1 Environment Setup

Ensure your development environment has the following tools installed:

Linux kernel source code
GCC compiler
Make tool
Kernel headers

2.2 Creating the Basic Structure

First, we need to create a new C file, for example, simple_char_driver.c, and include the necessary header files:

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#define DEVICE_NAME "simple_char_device"
#define BUFFER_SIZE 256
static char message[BUFFER_SIZE];
static int device_open = 0;

2.3 Implementing Open, Close, and Read/Write Functions

Next, we define the open, close, and read/write functions:

static int device_open(struct inode *inode, struct file *file) {
    if (device_open) return -EBUSY; // Return busy status if already opened
    device_open++;
    return SUCCESS;
}
static int device_release(struct inode *inode, struct file *file) {
    device_open--; // Decrease open count
    return SUCCESS;
}
static ssize_t device_read(struct file *file, char __user *buffer, size_t length, loff_t *offset) {
    int bytes_read = strlen(message);
    if (*offset >= bytes_read || copy_to_user(buffer, message + *offset, bytes_read)) {
        return 0; // EOF or error occurred
    }
    *offset += bytes_read;
    return bytes_read; // Return number of bytes read
}
static ssize_t device_write(struct file *file, const char __user *buffer, size_t length, loff_t *offset) {
    snprintf(message, BUFFER_SIZE - 1, "%s", buffer); // Copy user space data to kernel space
    message[BUFFER_SIZE - 1] = '\0'; // Ensure string null terminator exists
    return length; // Return number of bytes written
}

2.4 Defining the File Operations Structure

We need to define a file_operations structure to associate our functions with file operations in the Linux kernel:

struct file_operations fops = {
   .open = device_open,
   .release = device_release,
   .read = device_read,
   .write = device_write,
};

2.5 Registering the Character Device

int init_module(void) {
   register_chrdev(240, DEVICE_NAME, &fops); // Register character device
   printk(KERN_INFO "Simple Char Device registered\n");
   return SUCCESS;
}
void cleanup_module(void) {
   unregister_chrdev(240, DEVICE_NAME); // Unregister character device
   printk(KERN_INFO "Simple Char Device unregistered\n");
}

Complete Code Example

Below is the complete code example, combining all the above parts:

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#define DEVICE_NAME "simple_char_device"
#define BUFFER_SIZE 256
#define SUCCESS 0
MODULE_LICENSE("GPL");
static char message[BUFFER_SIZE];
static int device_open = 0;
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char __user *, size_t , loff_t *);
static ssize_t device_write(struct file *, const char __user *, size_t , loff_t *);
struct file_operations fops = {
   .open = device_open,
   .release = device_release,
   .read = device_read,
   .write = device_write,
};
int init_module(void) {
   register_chrdev(240, "simple_char_device", &fops);
   printk(KERN_INFO "Simple Char Device registered\n");
   return SUCCESS;
}
void cleanup_module(void) {
   unregister_chrdev(240, "simple_char_device");
   printk(KERN_INFO "Simple Char Device unregistered\n");
}

Compiling and Loading the Module

To compile this module, you can use a Makefile to simplify the process. Create a file named Makefile with the following content:

obj-m += simple_char_driver.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

Then, run the following commands in the terminal to compile and load the module:

make          # Compile the module
sudo insmod simple_char_driver.ko # Load the module
dmesg | tail # View log information
sudo rmmod simple_char_driver # Unload the module
make clean      # Clean build artifacts

Conclusion

This article introduced how to write a simple Linux character device driver in C, including basic functionality implementation and registration process. This is just the beginning; the complexity in actual development will be higher, but mastering these foundational concepts is crucial for further learning. If you are interested, you can continue to explore more information about block storage, network protocol stacks, and more. I hope this article helps you take your first step.