Monitor Linux File System Events with Inotify

By Eli M. Dow
2005-05-31


Why inotify?
There are many reasons to use inotify instead of dnotify. The first is that dnotify requires opening one file descriptor for each directory that you intend to watch for changes. This can become quite costly when you are monitoring several directories at once, since it is possible to reach a per-process file descriptor limit.

Additionally, the file descriptor pins the directory, disallowing the backing device to be unmounted, which causes problems in scenarios involving removable media. When using inotify, if you are watching a file on a file system that is unmounted, the watch is automatically removed and you receive an unmount event.

The second reason dnotify is inferior to inotify is a bit more complex. It is important to note that straightforward file system-monitoring granularity using the dnotify infrastructure exists only at the directory level. For more fine-grained monitoring with dnotify, application programmers are forced to keep a cache of stat structures relating to each directory being monitored. This user space cache of stat structures is needed to determine exactly what change in the directory occurred when a notification signal is received. When notifications are obtained, a list of stat structures is generated and compared with the last known state. Clearly, this technique is sub-optimal.

An additional advantage of inotify is that it uses a file descriptor as the basic interface to let application developers use select and poll to watch the device. This allows for efficient multiplexed I/O and integration with Glib's mainloop. In contrast, dnotify uses signals with which programmers often have more difficulty and that they find to be less than elegant.

Inotify solves these issues by providing a more elegant API that uses minimal file descriptors and ensures finer granularity of monitoring. Communication with inotify is provided through a device node. For these reasons it should be your clear choice when monitoring files on Linux 2.6 platforms.

Tutorial Pages:
» This next-gen, dnotify replacement meets file system event-monitoring needs in the 2.6 kernel
» Why inotify?
» Installing inotify
» Using inotify in a simple application
» Conclusion
» Resources


First published by IBM DeveloperWorks

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