内核下读写文件

内核中操作文件函数与用户层read，write这样API不同，需要使用内核空间专用的一套函数。

 

文件API

struct file *filp_open(const char *filename, int flags, int mode);

打开文件

int filp_close(struct file *filp, fl_owner_t id);

关闭文件，参数id为POSIX线程ID，一般设NULL

 

struct file
{
     struct path    f_path;    //文件路径
     const struct file_operations *f_op;  //文件操作结构，定义了读写等众多操作函数
     spinlock_t   f_lock;  /* f_ep_links, f_flags, no IRQ */
     ...
};

/*
 * NOTE:
 * read, write, poll, fsync, readv, writev, unlocked_ioctl and compat_ioctl
 * can be called without the big kernel lock held in all filesystems.
 */
struct file_operations {
  struct module *owner;
  loff_t (*llseek) (struct file *, loff_t, int);
  ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); //读
  ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); //写
  ...
}

 

 

测试

static void read_file( void )
{
  char szBuff[210] = {};
  mm_segment_t oldfs;
   
  // 打开文件
  fp = filp_open("/etc/redis/test.conf",O_RDWR,0);
  if(!fp || IS_ERR(fp))
  {
    printk(KERN_ALERT "fail to open test.conf\n");
    return -1;
  }
   
  // read
  if(!fp->f_op || !fp->f_op->read)
  {
    printk(KERN_ALERT "Can't read file\n");
    return -1;
  }
   
  oldfs = get_fs();  // 获取原先地址空间设置
  set_fs(KERNEL_DS);  // 设置为内核空间
   
  if(fp->f_op->read(fp,szBuff,210,&fp->f_pos) == 0)// 执行fp的read函数
  {
    printk(KERN_ERR "Error in read file\n");
    return -1;
  }
   
  set_fs(oldfs);// 还原为原先地址空间设置
  printk(KERN_ALERT "%s\n",szBuff);
   
  // 关闭文件 
  filp_close(fp);
  return 0;
}