概述
inotify — a powerful yet simple file change notification system.
inotify是linux内核2.6.13以后支持的一种特性,功能是监视文件系统的变化,在监视到文件系统发生变化
以后,会向相应的应用程序发送变化事件。
inotify是一种文件系统的变化通知机制,如文件增加、删除等事件可以立刻让用户得知,该机制是著名的
桌面搜索引擎项目beagle引入的。
监测事件
头文件:#include <sys/inotify.h>
Supported events suitable for MASK parameter of inotify_add_watch.
/* File was accessed. 文件被访问。*/
#define IN_ACCESS 0x00000001
/* FIle was modified. 文件被修改。*/
#define IN_MODIFY 0x00000002
/* Metadata changed. 文件属性被修改,如chmod、chown、touch等。*/
#define IN_ATTRIB 0x00000004
/* Writtable file was closed. 可写文件被关闭。*/
#define IN_CLOSE_WRITE 0x00000008
/* Unwrittable file closed. 不可写文件被关闭。*/
#define IN_CLOSE_NOWRITE 0x00000010
/* 文件被关闭。*/
#define IN_CLOSE (IN_CLOSE_WRITE | IN_CLOSE_NOWRITE)
/* File was opened. 文件被打开。*/
#define IN_OPEN 0x00000020
/* File was moved from X. 文件被移走,如mv。*/
#define IN_MOVED_FROM 0x00000040
/* File was moved to Y. 文件被移来,如mv、cp。*/
#define IN_MOVED_TO 0x00000080
/* Moves. 文件被移动。*/
#define IN_MOVED (IN_MOVED_FROM | IN_MOVED_TO)
/* Subfile was created. 创建新文件。*/
#define IN_CREATE 0x00000100
/* Subfile was deleted. 文件被删除,如rm。*/
#define IN_DELETE 0x00000200
/* Self was deleted. 自删除,即一个可执行文件在执行时删除自己。*/
#define IN_DELETE_SELF 0x00000400
/* Self was moved. 自移动,即一个可执行文件在执行时移动自己。*/
#define IN_MOVE_SELF 0x00000800
Events sent by the kernel.
/* Backing fs was unmounted. 宿主文件系统被unmount。*/
#define IN_UNMOUNT 0x00002000
/* Event queued overflowed. 在内核中,事件的数据超过了
* inotify_device中的max_events。*/
#define IN_Q_OVERFLOW 0x00004000
/* File was ignored. 表示系统把该文件对应的watch从inotify实例中
* 删除,因为文件已经被删除了!*/
#define IN_IGNORED 0x00008000
Special flags.
/* Only watch the path if it is a directory. */
#define IN_ONLYDIR 0x01000000
/* Do not follow a sym link. */
#define IN_DONT_FOLLOW 0x02000000
/* Add to the mask of an already existing watch. */
#define IN_MASK_ADD 0x20000000
/* Event occurred against dir. */
#define IN_ISDIR 0x40000000
/* Only send event once. */
#define IN_ONESHOT 0x80000000
All events which a program can wait on.
#define IN_ALL_EVENTS (IN_ACCESS | IN_MODIFY | IN_ATTRIB | IN_CLOSE_WRITE \
| IN_CLOSE_NOWRITE | IN_OPEN | IN_MOVED_FROM | IN_MOVED_TO \
| IN_CREATE | IN_DELETE | IN_DELETE_SELF | IN_MOVE_SELF)
API函数
inotify主要提供如下API。
(1)创建inotify实体
/* Create and initialize inotify instance. */
extern int inotify_init (void) __THROW;
inotify_init()在内核中创建一个实体:inotify_device,并返回一个文件描述符。
使用:int fd = inotify_init();
(2)创建监视器
/* Add watch of object NAME to inotify instance FD. Notify about events
* specified by MASK.
*/
extern int inotify_add_watch (int __fd, const char * __name, uint32_t __mask)
__THROW;
inotify_add_watch用于向inotify_device中的监视器列表添加监视器:inotify_watch。
创建监视器要提供:
(1) inotify实例inotify_device的文件描述符:fd
(2) 监视目标路径:name
(3) 监视事件列表:mask
如果成功,返回监视器描述符wd,否则返回-1。
(3)删除监视器
/* Remove the watch specified by WD from the inotify instance FD. */
extern int inotify_rm_watch (int __fd, uint32_t __wd) __THROW;
用于从inotify_device的监视器列表中删除一个监视器。
读取事件
为了确定哪些文件系统事件发生,需要用read系统调用读取inotify_init()返回的文件描述符。
read()会返回一个或多个inotify_event。
/* Structure describing an inotify event. */
struct inotify_event
{
int wd; /* Watch descriptor. */
unit32_t mask; /* Watch mask */
unit32_t cookie; /* Cookie to synchronize two events. */
unit32_t len; /* Length (including NULLs) of name. */
char name __flexarr; /* Name. */
};
inotify_event为文件系统事件。
其中wd为被监视目标的watch描述符,mask为事件掩码,name为监视目标的路径名,
len为name的长度。
每个notify_event的结构体大小为:sizeof(struct inotify_event) + len。
通过inotify_event可以看出:
(1) 监视目标(什么文件/目录) —> name,len
(2) 监视器 —> wd
(3) 监视目标的什么事件 —> mask
通过read()可以一次性获得多个事件,只要提供的buf足够大。
size_t len = read(fd, buf , MAX_BUF_SIZE);
len为实际获得的事件集总长度。
可以在函数inotify()返回的文件描述符fd上使用select()或poll()、epoll(),也可以在fd上使用ioctl命令
FIONREAD来得到当前队列的长度。
close(fd)将删除所有添加到fd中的watch并做必要的清理。
内核实现简述
Each inotify instance is represented by an inotify_handle structure.
Inotify's userspace consumers also have an inotify_device which is
associated with the inotify_handle, and on which events are queued.
Each watch is associated with an inotify_watch structure. Watches are chained
off of each associated inotify_handle and each associated inode.
See fs/notify/inotify/inotify_fsnotify.c and fs/notify/inotify/inotify_user.c for
the locking and lifetime rules.
无论是目录还是文件,在内核中都对应一个inode结构,inotify系统再inode结构中增加了两个字段:
#ifdef CONFIG_INOTIFY
struct list_head inotify_watches; /* watches on this inode */
struct semaphore inotify_sem; /* protects the watches list */
#endif
Reference
1. documentation \ filesystems \ inotify.txt
2. include \ linux \ inotify.h
3. http://www.ibm.com/developerworks/cn/linux/l-inotifynew/
4. http://tianyapiaozi.blogbus.com/logs/61783047.html