linux内核情景分析之命名管道

管道是一种"无名","无形文件,只可以近亲进程使用,不可以再任意两个进程通信使用,所以只能实现"有名","有形"的文件来实现就可以克服其缺点,这里的有名:一个文件应该有文件名,使得任何进程可以通过文件名或者路径找到该文件,有形指的是文件的inode应该存在与磁盘或者其他文件熊截止上.使得任何进程可以再任何时间都可以建立联系.

命名管道的创建用mknod,当然也可以使用mknod函数创建命名管道.建立了这样的节点,进程就可以通过打开一个文件一样打开整个民命管道.

命名管道与匿名管道的不同之处仅仅在于打开的过程,分析如何通过open系统调用与filfo文件取得联系.进程在内核通过sys_open进入filp_open(),然后在open_namei()调用一个函数path_walk(),根据文件的路径在文件系统中找到代表这文件的inode,将磁盘的inode读取内存,根据文件类型,将inode的i_op和i_fop指针设置为对应的inode_operation与file_operation,但对于FIFO这样特殊的文件则用调用init_special_inode加以初始化,可见文件类型不属于ACL或者数据,不是普通文件,不是目录,不是符号连接,就属于FIFO文件

if (inode->i_ino == EXT2_ACL_IDX_INO ||
	    inode->i_ino == EXT2_ACL_DATA_INO)
		/* Nothing to do */ ;
	else if (S_ISREG(inode->i_mode)) {
		inode->i_op = &ext2_file_inode_operations;
		inode->i_fop = &ext2_file_operations;
		inode->i_mapping->a_ops = &ext2_aops;
	} else if (S_ISDIR(inode->i_mode)) {
		inode->i_op = &ext2_dir_inode_operations;
		inode->i_fop = &ext2_dir_operations;
	} else if (S_ISLNK(inode->i_mode)) {
		if (!inode->i_blocks)
			inode->i_op = &ext2_fast_symlink_inode_operations;
		else {
			inode->i_op = &page_symlink_inode_operations;
			inode->i_mapping->a_ops = &ext2_aops;
		}
	} else 
		init_special_inode(inode, inode->i_mode,
				   le32_to_cpu(raw_inode->i_block[0]));

接下来查看init_special_inode

void init_special_inode(struct inode *inode, umode_t mode, int rdev)
{
	inode->i_mode = mode;
	if (S_ISCHR(mode)) {
		inode->i_fop = &def_chr_fops;
		inode->i_rdev = to_kdev_t(rdev);
	} else if (S_ISBLK(mode)) {
		inode->i_fop = &def_blk_fops;
		inode->i_rdev = to_kdev_t(rdev);
		inode->i_bdev = bdget(rdev);
	} else if (S_ISFIFO(mode))//是fifo文件,设置文件操作为def_fifo_fops
		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 imode (%o)\n", mode);
}

/*
 * Dummy default file-operations: the only thing this does
 * is contain the open that then fills in the correct operations
 * depending on the access mode of the file...
 */
struct file_operations def_fifo_fops = {
	open:		fifo_open,	/* will set read or write pipe_fops */
};

sys_open->filp_open->dentry_open->fifo_open()

static int fifo_open(struct inode *inode, struct file *filp)
{
	int ret;

	ret = -ERESTARTSYS;
	lock_kernel();
	if (down_interruptible(PIPE_SEM(*inode)))
		goto err_nolock_nocleanup;

	if (!inode->i_pipe) {//第一次打开fifo文件,该管道的缓冲页面没分配,以后打开会跳过
		ret = -ENOMEM;
		if(!pipe_new(inode))
			goto err_nocleanup;
	}
	filp->f_version = 0;

	switch (filp->f_mode) {
	case 1:
	/* //只读
	 *  O_RDONLY
	 *  POSIX.1 says that O_NONBLOCK means return with the FIFO
	 *  opened, even when there is no process writing the FIFO.
	 */
		filp->f_op = &read_fifo_fops;
		PIPE_RCOUNTER(*inode)++;//读端的记录++
		if (PIPE_READERS(*inode)++ == 0)//表示刚打开读端,很可能有写进程睡眠,那就要唤醒写进程
			wake_up_partner(inode);

		if (!PIPE_WRITERS(*inode)) {//如果不存在写端
			if ((filp->f_flags & O_NONBLOCK)) {//并且不阻塞
				/* suppress POLLHUP until we have
				 * seen a writer */
				filp->f_version = PIPE_WCOUNTER(*inode);//写端计数
			} else //不存在写端并且阻塞,那就之产生一般,需要睡眠,等待生产者进程将其唤醒
			{
				wait_for_partner(inode, &PIPE_WCOUNTER(*inode));
				if(signal_pending(current))
					goto err_rd;
			}
		}
		break;
	
	case 2:
	/*只写
	 *  O_WRONLY
	 *  POSIX.1 says that O_NONBLOCK means return -1 with
	 *  errno=ENXIO when there is no process reading the FIFO.
	 *///不存在读端并且设置了不阻塞,直接return错误
		ret = -ENXIO;
		if ((filp->f_flags & O_NONBLOCK) && !PIPE_READERS(*inode))
			goto err;

		filp->f_op = &write_fifo_fops;//设置专用写操作
		PIPE_WCOUNTER(*inode)++;
		if (!PIPE_WRITERS(*inode)++)//如果写端第一次创建,很可能有读进程在睡眠
			wake_up_partner(inode);//将其唤醒

		if (!PIPE_READERS(*inode)) {//如果不存在读端并且阻塞
			wait_for_partner(inode, &PIPE_RCOUNTER(*inode));//休眠等待读进程将其唤醒
			if (signal_pending(current))
				goto err_wr;
		}
		break;
	
	case 3:
	/*可读可写
	 *  O_RDWR
	 *  POSIX.1 leaves this case "undefined" when O_NONBLOCK is set.
	 *  This implementation will NEVER block on a O_RDWR open, since
	 *  the process can at least talk to itself.
	 *///相当于一个进程同时打开命名管道的两端,所以不需要等待,只要任何一端是第一次打开,就唤醒在睡眠的进程
		filp->f_op = &rdwr_fifo_fops;

		PIPE_READERS(*inode)++;
		PIPE_WRITERS(*inode)++;
		PIPE_RCOUNTER(*inode)++;
		PIPE_WCOUNTER(*inode)++;
		if (PIPE_READERS(*inode) == 1 || PIPE_WRITERS(*inode) == 1)
			wake_up_partner(inode);
		break;

	default:
		ret = -EINVAL;
		goto err;
	}

	/* Ok! */
	up(PIPE_SEM(*inode));
	unlock_kernel();
	return 0;

err_rd:
	if (!--PIPE_READERS(*inode))
		wake_up_interruptible(PIPE_WAIT(*inode));
	ret = -ERESTARTSYS;
	goto err;

err_wr:
	if (!--PIPE_WRITERS(*inode))
		wake_up_interruptible(PIPE_WAIT(*inode));
	ret = -ERESTARTSYS;
	goto err;

err:
	if (!PIPE_READERS(*inode) && !PIPE_WRITERS(*inode)) {
		struct pipe_inode_info *info = inode->i_pipe;
		inode->i_pipe = NULL;
		free_page((unsigned long)info->base);
		kfree(info);
	}

err_nocleanup:
	up(PIPE_SEM(*inode));

err_nolock_nocleanup:
	unlock_kernel();
	return ret;
}

注意FIFO文件的inode节点存在磁盘,但那只是一个节点,而文件的数据之存在内存缓冲区.与普通管道一样

来自为知笔记(Wiz)