Kernel Korner - Intro to inotify

Applications that watch thousands of files for changes, or that need to know when a storage device gets disconnected, need a clean, fast solution to the file change notification problem. Here it is.

John McCutchan and I had been working on inotify for about a year when it was finally merged into Linus' kernel tree and released with kernel version 2.6.13. Although a long struggle, the effort culminated in success and was ultimately worth every rewrite, bug and debate.

What Is inotify?

inotify is a file change notification system—a kernel feature that allows applications to request the monitoring of a set of files against a list of events. When the event occurs, the application is notified. To be useful, such a feature must be simple to use, lightweight with little overhead and flexible. It should be easy to add new watches and painless to receive notification of events.

To be sure, inotify is not the first of its kind. Every modern operating system provides some sort of file notification system; many network and desktop applications require such functionality—Linux too. For years, Linux has offered dnotify. The problem was, dnotify was not very good. In fact, it stank.

dnotify, which ostensibly stands for directory notify, was never considered easy to use. Sporting a cumbersome interface and several painful features that made life arduous, dnotify failed to meet the demands of the modern desktop, where asynchronous notification of events and a free flow of information rapidly are becoming the norm. dnotify has, in particular, several problems:

  • dnotify can watch only directories.

  • dnotify requires maintaining an open file descriptor to the directory that the user wants to watch. First, this open file descriptor pins the directory, disallowing the device on which it resides from being unmounted. Second, watching a large number of directories requires too many open file descriptors.

  • dnotify's interface to user space is signals. Yes, seriously, signals!

  • dnotify ignores the issue of hard links.

The goal, therefore, was twofold: design a first-class file notification system and ensure that all of the deficiencies of dnotify were addressed.

inotify is an inode-based file notification system that does not require a file ever be opened in order to watch it. inotify does not pin filesystem mounts—in fact, it has a clever event that notifies the user whenever a file's backing filesystem is unmounted. inotify is able to watch any filesystem object whatsoever, and when watching directories, it is able to tell the user the name of the file inside of the directory that changed. dnotify can report only that something changed, requiring applications to maintain an in-memory cache of stat() results and compare for any changes.

Finally, inotify is designed with an interface that user-space application developers would want to use, enjoy using and benefit from using. Instead of signals, inotify communicates with applications via a single file descriptor. This file descriptor is select-, poll-, epoll- and read-able. Simple and fast—the world is happy.

Getting Started with inotify

inotify is available in kernel 2.6.13-rc3 and later. Because some bugs were found and subsequently fixed right after that release, kernel 2.6.13 or later is recommended. The inotify system calls, being the new kids on the block, might not yet be supported in your system's version of the C library, in which case the header files listed in the on-line Resources will provide the necessary C declarations and system call stubs.

If your C library supports inotify, all you should need is the following:


#include <sys/inotify.h>

If not, grab the two header files, stick them in the same directory as your source files, and use the following:

#include "inotify.h"
#include "inotify-syscalls.h"

The following examples are in straight C. You can compile them the same as any other C application.

Initialize, inotify!

inotify is initialized via the inotify_init() system call, which instantiates an inotify instance inside the kernel and returns the associated file descriptor:

int inotify_init (void);

On failure, inotify_init() returns minus one and sets errno as appropriate. The most common errno values are EMFILE and ENFILE, which signify that the per-user and the system-wide open file limit was reached, respectively.

Usage is simple:


int fd;

fd = inotify_init ();
if (fd < 0)
        perror ("inotify_init");

______________________

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support on 64-bit machines

vishalj's picture

Do i need to make some changes to get it working on 64 bit machines .

read() length, alignment

Anonymous's picture

fyi, something that wasn't clear in the article is the requirements on the read() call.

I was playing with inotify, and I tried to call read an event in 2 parts:
struct inotify_event evt_hdr;
ssize_t nr = read(fd, &evt_hdr, sizeof(evt_hdr));

read() returned EINVAL
( at least on 2.6.23.17-88.fc7 )

When I restructured the code to read the entire event,
(similar to the article example) it works.

It would seem also that the read buffer must have proper alignment for a
struct inotify_event as well, so just declaring a char buffer on the stack
for the read() destination isn't necessarily going to work, either.

otherwise, nice article, nice feature!
Thanks

Inotify does not work

Inotify user's picture

I have registered Inotify to a file for listening IN_MODIFY | IN_DELETE | IN_DELETE_SELF. But it does not work when I fwrite() to the file. This doest not happen always.

Seems to be a bug in iNotify

changes

Anonymous's picture

Seems changes are coming, not primary in inotify and the old dnotify, but in the whole system of fs/notify.
After a long search I think there is a guy called Eric Paris (RedHat) that is currently in charge of notify structures in kernel.
He is actively commiting to the kernel git repository, which can be followed always at http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git.

As Eric Paris claims he is preparing the whole thing for upcoming "fanotify" system, which will be more powerfull, sending the possibility to decide which file accessse should be granted to the userspace (if I got right the idea).

http://lwn.net/Articles/311350/
http://lwn.net/Articles/339253/

Which is of course fantastic, but for many cases we dont need this power, and the inotify does the job at right level. I just needed a little add-on to provide the uid in inotify_event, but now it seems we will have to wait for the fanotify to be introduced first, and after to be decided the future of inotify ...

Inotify

aveugle's picture

Is there any possibility to obtain the uid of the trigger for certain inotify_event?

P.S. in general - is inotify a living project or it is shut-down?

how to check for status of the inotify

Anonymous's picture

i have a problem and hope i get a response from you.
i have a script that will create an sql file to the watch directory of my inotify. upon detection, my C program then executes the sql statment using OCI functions. is there any way i can throw back the result/status to my script if the execution was successful or not?

i have a problem and hope i

Anonymous's picture

i have a problem and hope i get a response from you.

currently, my C program with the inotify is already running. this C program will wait base on its watch directory.

i then have a script that will create an sql file to the watch directory of my inotify. upon detection, my C program then executes the sql statment using OCI functions. is there any way i can throw back the result/status to my script if the execution was successful or not?

*** note: my script did not call my C program which is already running from the start

Not able to notify changes apart from /tmp

Anonymous's picture

Hi,

I am facing some different behaviour using inotify for notifying file changes.

I added file notification for some file in /tmp directory with IN_MODIFY flag.
when i modified contents in this file, i am able to read the notifications of this change and the mask value for this notification is IN_MODIFY(2).

If i use same file with same source code in other directory(ex : /home), iam getting mask value as IN_IGNORED flag(3276. After this any modifications to the file, i am not getting any notifications.

This is some strange behaviour.

Kernel : 2.6.15-1.2054_FC5

Thank you for this, it was

Anonymous's picture

Thank you for this, it was very useful.

One annoyance for someone coming across this: while I created the inotify using
IN_MODIFY | IN_CREATE | IN_DELETE

When the file was deleted, it returned only IN_IGNORED (because the file was deleted, the inotify was removed), not IN_DELETED

inotify for /proc

madhav's picture

Hi,
There is a problem in my application. I am trying to monitor /proc for process creation/deletion, but i am not able to monitor the /proc directory in particular. The inotify will monitor all the sub directories in the /proc and other directories, but not in /proc, i.e. for process creation/deletion. i am using linux kernel-2.6.20-hardened. There was a bug previously reported in v2.6.16 regarding this issue, but i think it is fixed in further versions. if not please may i know so i will be able to patch it myself.
Please help..
Waiting for the reply

How to install inotify

Anonymous's picture

Hi..
can u pls send me the full procedure to download as wel as how to install inotify... it s very userful for me ...

Thanks

Refer this on how to install

AG's picture

Refer this on how to install and how to use for various scenarios. http://inotify.aiken.cz/?section=inotify&page=faq&lang=en

Monitoring whole directory tree

Anonymous's picture

Can you please help me?
I want to monitor the whole directory tree, using inotify.
I have done monitoring directory which notify me changes in file within that perticular directory, creation of directories into it, butI am unable to get the changes done within its subdirectories.
Please help me for this.

How would one get the user associated with an event.....

rdifalco's picture

If I wanted to write a utility that would list each time a file I was interested in was modified and by who, how would I do that? Could I do that with inotify? I'm guessing no.

INOTIFY a very helpful tool

Ioan PREDESCU's picture

I tested INOTIFY in conjunction with POSIX queues (mq_open…) on my CentOS 4.3 – 2.6.18 and it seems to work OK, I am really impressed by this ‘new’ feature:

Just a small ‘observation’ as the code piece from below is going to drastically decrease the performances (stack is exhausted as ‘event’ is an in loop declaration) :-)) :

while (i < len) {
struct inotify_event *event;
…………………

Thanks to INOTIFY creators and contributors,
John

you're flat out wrong about the stack exhausted <EOM>

Anonymous's picture

Code Feedback from gcc and a.out

Anonymous's picture

Thanks for posting this information and the examples. Was very stuck trying to convert to inotify without this article, and now have a working program thanks to your help. Feedback on this article accumulated from the process:

  • Unbalanced parenthesis in the #define for BUF_LEN
  • In that same code snippet, the while loop is conditional, based on (!len). That loop needs to be invoked when len is true and non-negative. As originally posted, the loop isn't invoked when an event is received from the read().
  • IN_CREATE is missing from Table 1, although it is referenced earlier in the article.

Thanks! inotify ROCKS!

wrong /proc path

Anonymous's picture

As of 2.6.17, it is NOT

/proc/sys/filesystem/inotify/max_user_watches

but

/proc/sys/fs/inotify/max_user_watches

Nice and helpful article

Manoj Awasthi's picture

Thanks for the same.

Special files

Alberto's picture

Hello!

Is inotify supposed to work on special files in /sys ?

I tried monitoring normal files and it works great, but when i try to monitor files on sysfs, strace shows that the executable is blocked on the read() call.

Thanks!
Alberto

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