在Linux内核里面，设备（device）主要分为字符设备，块设备，网络设备，字符设备驱动是Linux驱动基础，在看《Linux 设备驱动开发详解》这本书的过程中，把字符设备相知识记录整理如下。

字符设备驱动的组成

字符设备驱动模块加载和卸载函数

//设备结构体 struct xxx_dev_t{ struct cdev cdev; //...... }xxx_dev; static struct xxx_dev *xxx_dev; const struct file_operations xxx_fops = { .owner = THIS_MODULE, .read = xxx_read, .write = xxx_write, //...... } stattic int __init xxx_init(void) { //...... cdev_init(&xxx_dev->cdev, &xxx_fops);  xxx_dev->cdev.owner = THIS_MODULE; //获取字符设备号 alloc_chrdev_region(&xxx_dev_no, 0, DEVICE_COUNT, DEVICE_NAME); //注册设备  ret = cdev_add(&xxx_dev->cdev, xxx_dev_no, 1); //...... } static void __exit xxx_exit(void) { //注销设备 cdev_del(&xxx_dev->cdev); //注销设备号 unregister_chrdev_region(xxx_dev_no,1); //...... } 

字符驱动操作成员函数

static int xxx_open(struct inode *inode, struct file *filp) { //...... return 0; } static int xxx_release(struct inode *inode, struct file *filp) { //...... return 0; } static long xxx_ioctl(struct file *filp, unsigned int cmd, unsigned long arg) { //... switch (cmd) { case xxx_cmd1: //...... break; default: return -EINVAL; } return 0; } static ssize_t xxx_read(struct file *filp, char __user *buf,                               size_t size, loff_t *ppos) { //... copy_to_user(buf, xxx, count); //... return count; } static ssize_t xxx_write(struct file *filp, const char __user *buf,                                size_t size, loff_t *ppos) { //... copy_from_user(xxx, buf, count); //... return count; } static loff_t globalmem_llseek(struct file *filp, loff_t offset, int orig) { //...... switch (orig) { case 0:/*SEEK_SET  Seek from beginning of file*/ //......         filp->f_pos = (unsigned int)offset;         ret = filp->f_pos; break; case 1: /*SEEK_CUR Seek from current position*/ //......         filp->f_pos += offset;         ret = filp->f_pos; break; case 2:/* SEEK_END Seek from end of file */ //......      filp->f_pos -= offset;         ret = filp->f_pos; break; default:         ret = -EFAULT; break; } return ret; } 

DEMO测试

驱动Demo完整代码

通过以下命令加载和查看驱动

 $: make     $: sudo demsg -c     $: sudo insmod multi_globalmem.ko     $: lsmod     $: sudo mknod /dev/globalmem c 230 0   //创建设备节点     $: sudo dmesg 

用户空间测试代码如下：

#include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #define DEVICE_NAME "/dev/globalmem" #define MAX_SIZE 1020 #define MEM_CLEAR 0x01 static void usage(char *name) { fprintf(stderr, "Usage:%s [message]n", name); } int main(int argc, char **argv) { if (argc != 2) { usage(argv[0]); return -1; } char *msg = argv[1]; char buf[MAX_SIZE]; //open the file int fd = open(DEVICE_NAME, O_RDWR); if (fd < 0) { fprintf(stderr, "open %s failedn", DEVICE_NAME); return -1; } //clear the file if (ioctl(fd, 1, 0) < 0) { fprintf(stderr, "clear %s filedn", DEVICE_NAME); close(fd); return -1; } //write message to file if (write(fd, msg, strlen(msg)) <= 0) { fprintf(stderr, "write %s errorn", DEVICE_NAME); } if (lseek(fd, -strlen(msg), SEEK_CUR) < 0) { fprintf(stderr, "seek %s errorn", DEVICE_NAME); } //read from file if (read(fd, buf, MAX_SIZE) <= 0) { fprintf(stderr, "read %s failed!n", DEVICE_NAME); } else { fprintf(stderr, "read suceess: buf=%sn", buf); } close(fd); return 0; } 

使用以下命令测试：

 $: gcc test_globalmem.c -o test_globalmem     $: sudo ./test_globalmem message 

Debug message 如下：