如何写一个最简单的字符设备驱动？

字符设备驱动在Linux系统中是很常见的。字符设备驱动是Linux内核中用于管理字符设备的模块。字符设备以字节流形式进行数据传输，通常不支持随机访问。

常见的例子包括键盘、鼠标和串口等。驱动程序通过实现file_operations结构体中的函数（如open、read、write等）与用户空间交互，提供设备的操作接口。

字符设备在文件系统中以设备文件形式呈现，用户通过文件I/O函数访问设备。驱动程序还需在内核中注册设备，分配主次设备号，并在/dev目录下创建设备文件。

实例代码

// simple_cdev.c

#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/minmax.h>
#include <linux/module.h>
#include <linux/uaccess.h>

#define DEV_NAME "simple_cdev"
#define BUFFER_SIZE 1024

typedef struct {
    int major;                // 主设备号
    int minor;                // 从设备号
    char buffer[BUFFER_SIZE]; // 缓冲区
} simple_cdev_t;

simple_cdev_t my_cdev = {0};

static int _open(struct inode *inode, struct file *file) {
    pr_info("simple_cdev_open\n");
    return 0;
}

static ssize_t _read(struct file *file, char __user *userbuf, size_t size,
                     loff_t *offset) {
    pr_info("simple_cdev_read\n");

    size = min(size, BUFFER_SIZE);

    unsigned long remain_len = copy_to_user(userbuf, my_cdev.buffer, size);

    if (remain_len > 0) {
        pr_info("copy_to_user failed\n");
        return -EIO;
    }

    return size;
}

static ssize_t _write(struct file *file, const char __user *userbuf,
                      size_t size, loff_t *offset) {
    pr_info("simple_cdev_write\n");

    size = min(size, BUFFER_SIZE);
    unsigned long remain_len = copy_from_user(my_cdev.buffer, userbuf, size);

    if (remain_len > 0) {
        pr_err("copy_from_user failed\n");
        return -EIO;
    }

    return size;
}

static int _release(struct inode *inode, struct file *file) {
    pr_info("simple_cdev_release\n");
    return 0;
}

// 文件处理函数，每当访问设备文件，都会调用这里的回调函数
static struct file_operations fops = {
    .open = _open,
    .read = _read,
    .write = _write,
    .release = _release,
};

static int __init simple_cdev_init(void) {
    pr_info("simple_cdev_init\n");

    // 动态申请主设备号
    my_cdev.major = register_chrdev(0, DEV_NAME, &fops);
    if (my_cdev.major < 0) {
        pr_err("simple_cdev register_chrdev failed!\n");
        return -EIO;
    }

    my_cdev.minor = 0;

    pr_info("simple_cdev major = %d, minor = %d\n", my_cdev.major,
            my_cdev.minor);

    return 0;
}

static void __exit simple_cdev_exit(void) {
    pr_info("simple_cdev_exit\n");
    unregister_chrdev(my_cdev.major, DEV_NAME);
}

module_init(simple_cdev_init);
module_exit(simple_cdev_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("heng30");
MODULE_VERSION("v0.0.1");
MODULE_DESCRIPTION("A simple simple_cdev kernel module");

Makefile编译脚本

#!/bin/sh

top-dir = $(shell pwd)
kernel-version = $(shell uname -r)
kernel-dir ?= /lib/modules/$(kernel-version)/build

obj-m = simple_cdev.o

all:
        make -C $(kernel-dir) modules M=$(top-dir)

clean:
        rm -f *.o *.ko *.mod *.mod.c *.order *.symvers
        make -C $(kernel-dir) clean m=$(top-dir)

测试

• 安装驱动：insmod simple_cdev.ko

[58512.143558] simple_cdev: loading out-of-tree module taints kernel.
[58512.143563] simple_cdev: module verification failed: signature and/or required key missing - tainting kernel
[58512.143981] simple_cdev_init
[58512.143984] simple_cdev major = 240, minor = 0

• 创建字符设备文件simple_cdev：mknod /dev/simple-cdev c 240 0。240是主设备号，0是次设备号。

• 向simple_cdev文件写入内容：echo "hello" > /dev/simple-cdev

[58984.196903] simple_cdev_open
[58984.196917] simple_cdev_write
[58984.196919] simple_cdev_release

• 从simple_cdev文件读取内容： head -n 1 /dev/simple-cdev

[59354.674957] simple_cdev_open
[59354.674972] simple_cdev_read
[59354.674994] simple_cdev_release

• 移除驱动：rmmod simple_cdev.ko