介绍个特殊的字符设备，主设备号为1的设备

1 char Memory devices

1 = /dev/mem Physical memory access

2 = /dev/kmem Kernel virtual memory access

3 = /dev/null Null device

4 = /dev/port I/O port access

5 = /dev/zero Null byte source

6 = /dev/core OBSOLETE - replaced by /proc/kcore

7 = /dev/full Returns ENOSPC on write

8 = /dev/random Nondeterministic random number gen.

9 = /dev/urandom Faster, less secure random number gen.

10 = /dev/aio Asynchronous I/O notification interface

11 = /dev/kmsg Writes to this come out as printk's, reads

export the buffered printk records.

12 = /dev/oldmem OBSOLETE - replaced by /proc/vmcore

主设备号为1 的设备严格上都不是真正的字符设备，都是和mem有关系的MEM_MAJOR 1

例如：/dev/null 内核中有些信息不适合在屏幕上显示，又不适合重定向到磁盘中，这时候

/dev/null起到直接把数据消失的作用。

实现代码原理如下：

最终执行是在file_operations中的，所以从文件操作开始：主设备号为1的如下：

static const struct file_operations memory_fops = {

.open = memory_open,

.llseek = noop_llseek,

};

static int memory_open(struct inode *inode, struct file *filp)

{

int minor;

const struct memdev *dev;

minor = iminor(inode);  //获取次设备号

if (minor >= ARRAY_SIZE(devlist)) //主设备号为1的devlist设备列表 ，判断是否在范围内

return -ENXIO;

dev = &devlist[minor];    //从列表中获取设备指针

if (!dev->fops)           //判断文件操作是否有效

return -ENXIO;

filp->f_op = dev->fops;      //赋值文件操作指针

filp->f_mode |= dev->fmode;

if (dev->fops->open)        //如果非空，执行自己的open操作/dev/null为空

return dev->fops->open(inode, filp);

return 0;

}

static const struct file_operations null_fops = {

.llseek = null_lseek,

.read = read_null,

.write = write_null,

.read_iter = read_iter_null,

.write_iter = write_iter_null,

.splice_write = splice_write_null,

};

上面filp->f_op = dev->fops;     后就可进行读写操作了。

读写操作如下：

static ssize_t read_null(struct file *file, char __user *buf,

size_t count, loff_t *ppos)

{

return 0;

}

static ssize_t write_null(struct file *file, const char __user *buf,

size_t count, loff_t *ppos)

{

return count;

}

没错，什么也没做直接返回了。从而掩盖事实。做了啊，实际什么也没做。

起到一些消息不得不管理，统一空处理的作用。