Kernel Module实战指南(三)：编写字符型设备讲解Linux Kernel Module的驱动程序

Introduction

Linux Kernel Module：。open(), release(), read(), write()，。

file_operations

Linux，，。linux/fs.h，file_operations。

struct file_operations {
  ...
  ssize_t(*read) (struct file *, char __user *, size_t, loff_t *);
  ssize_t(*write) (struct file *, const char __user *, size_t, loff_t *);
  int (*open) (struct inode *, struct file *);
  int (*release) (struct inode *, struct file *);
  ...
};

，，。
，file_operations，register_chrdev()。

code

#include 
#include 
#include 
#include 

int init_module(void);
void cleanup_module(void);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
// 
#define DEVICE_NAME "csprojectedu"

static int major_version;
static int device_is_open = 0;
static char msg[1024];
static char *pmsg;
// ，read(), write(), open(), release()
static struct file_operations fops = {
  .read = device_read,
  .write = device_write,
  .open = device_open,
  .release = device_release
};
// 
int init_module() {
  // fops
  major_version = register_chrdev(0, DEVICE_NAME, &fops);
  if (major_version < 0) {
    printk(KERN_ALERT "Register failed, error %d.\n", major_version);
    return major_version;
  }
  // ，/dev/csprojectedu
  printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, major_version);
  return 0;
}
// 
void cleanup_module() {
  unregister_chrdev(major_version, DEVICE_NAME);
}
// ，cat /dev/csprojectedu 
static ssize_t device_read(struct file *filp, char *buffer, size_t length, loff_t *offset) {
  int bytes = 0;
  if (*pmsg == 0) {
    return 0;
  }
  while (length && *pmsg) {
    put_user(*(pmsg++), buffer++);
    length--;
    bytes++;
  }
  return bytes;
}
// ，
static ssize_t device_write(struct file *filp, const char *buff, size_t length, loff_t *offset) {
  return -EINVAL;
}
// 
static int device_open(struct inode *inode, struct file *file) {
  static int counter = 0;
  if (device_is_open) {
    return -EBUSY;
  }
  device_is_open = 1;
  sprintf(msg, "Device open for %d times.\n", ++counter);
  pmsg = msg;
  try_module_get(THIS_MODULE);
  return 0;
}
// /
static int device_release(struct inode *inode, struct file *file) {
  device_is_open = 0;
  module_put(THIS_MODULE);
  return 0;
}

Makefile

$ make
$ sudo insmod csprojectedu.ko
$ dmesg
[115465.083271] &apos;mknod /dev/csprojectedu c 250 0&apos;.
$ sudo mknod /dev/csprojectedu c 250 0
$ cat /dev/csprojectedu
Device open for 1 times.
$ cat /dev/csprojectedu
Device open for 2 times.
$ sudo rm /dev/csprojectedu
$ sudo rmmod csprojectedu.ko