关于字符和块设备一些杂谈

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我是靠谱客的博主想人陪大象，最近开发中收集的这篇文章主要介绍关于字符和块设备一些杂谈，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

当我们对文件进行读写是调用file->f_op 最终调用的是inode->i_fop
而inode->i_fop 在具体的文件系统已经进行初始化
以下摘抄fs/ext2

if (S_ISREG(inode->i_mode)) {
		inode->i_op = &ext2_file_inode_operations;
		if (ext2_use_xip(inode->i_sb)) {
			inode->i_mapping->a_ops = &ext2_aops_xip;
			inode->i_fop = &ext2_xip_file_operations;
		} else if (test_opt(inode->i_sb, NOBH)) {
			inode->i_mapping->a_ops = &ext2_nobh_aops;
			inode->i_fop = &ext2_file_operations;
		} else {
			inode->i_mapping->a_ops = &ext2_aops;
			inode->i_fop = &ext2_file_operations;
		}
	} else if (S_ISDIR(inode->i_mode)) {
		inode->i_op = &ext2_dir_inode_operations;
		inode->i_fop = &ext2_dir_operations;
		if (test_opt(inode->i_sb, NOBH))
			inode->i_mapping->a_ops = &ext2_nobh_aops;
		else
			inode->i_mapping->a_ops = &ext2_aops;
	} else if (S_ISLNK(inode->i_mode)) {
		if (ext2_inode_is_fast_symlink(inode)) {
			inode->i_op = &ext2_fast_symlink_inode_operations;
			nd_terminate_link(ei->i_data, inode->i_size,
				sizeof(ei->i_data) - 1);
		} else {
			inode->i_op = &ext2_symlink_inode_operations;
			if (test_opt(inode->i_sb, NOBH))
				inode->i_mapping->a_ops = &ext2_nobh_aops;
			else
				inode->i_mapping->a_ops = &ext2_aops;
		}
	} else {
		inode->i_op = &ext2_special_inode_operations;
		if (raw_inode->i_block[0])
			init_special_inode(inode, inode->i_mode,
			   old_decode_dev(le32_to_cpu(raw_inode->i_block[0])));
		else 
			init_special_inode(inode, inode->i_mode,
			   new_decode_dev(le32_to_cpu(raw_inode->i_block[1])));
	}

可以看出只要文件的类型不是ACL索引活数据，不是普通文件，不是目录，不是符号连接，就属于特殊文件，就要通过init_special_inode()来初始化其inode结构：
例如我们的字符设备驱动块设备驱动文件节点这些

这里说如果是字符设备就调用字符设备如果块设备就调用块设备

void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
{
	inode->i_mode = mode;
	if (S_ISCHR(mode)) {
		inode->i_fop = &def_chr_fops;
		inode->i_rdev = rdev;
	} else if (S_ISBLK(mode)) {
		inode->i_fop = &def_blk_fops;
		inode->i_rdev = rdev;
	} else if (S_ISFIFO(mode))
		inode->i_fop = &def_fifo_fops;
	else if (S_ISSOCK(mode))
		inode->i_fop = &bad_sock_fops;
	else
		printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o) for"
				  " inode %s:%lun", mode, inode->i_sb->s_id,
				  inode->i_ino);
}

字符设备操作

const struct file_operations def_chr_fops = {
	.open = chrdev_open,
};

这里通过kobject来检测具体字符驱动然后把字符设备操作函数通过fops_get(p->ops)赋值到filp->f_op

static int chrdev_open(struct inode *inode, struct file *filp)
{
	p = inode->i_cdev;
	if (!p) {
		struct kobject *kobj;
		int idx;
		spin_unlock(&cdev_lock);
		kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);
		if (!kobj)
			return -ENXIO;
		new = container_of(kobj, struct cdev, kobj);
		spin_lock(&cdev_lock);
		/* Check i_cdev again in case somebody beat us to it while
		   we dropped the lock. */
		p = inode->i_cdev;
		if (!p) {
			inode->i_cdev = p = new;
			list_add(&inode->i_devices, &p->list);
			new = NULL;
		} else if (!cdev_get(p))
			ret = -ENXIO;
	} else if (!cdev_get(p))
		ret = -ENXIO;
	spin_unlock(&cdev_lock);
	cdev_put(new);
	if (ret)
		return ret;
	ret = -ENXIO;
	filp->f_op = fops_get(p->ops);
	if (!filp->f_op)
		goto out_cdev_put;
	if (filp->f_op->open) {
		ret = filp->f_op->open(inode,filp);
		if (ret)
			goto out_cdev_put;
	}
}