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INOTIFY

Linux Programmer's Manual (7)
2014-05-08
 

NAME

inotify - monitoring filesystem events  

DESCRIPTION

The inotify API provides a mechanism for monitoring filesystem events. Inotify can be used to monitor individual files, or to monitor directories. When a directory is monitored, inotify will return events for the directory itself, and for files inside the directory.

The following system calls are used with this API:

*
inotify_init(2) creates an inotify instance and returns a file descriptor referring to the inotify instance. The more recent inotify_init1(2) is like inotify_init(2), but has a flags argument that provides access to some extra functionality.
*
inotify_add_watch(2) manipulates the "watch list" associated with an inotify instance. Each item ("watch") in the watch list specifies the pathname of a file or directory, along with some set of events that the kernel should monitor for the file referred to by that pathname. inotify_add_watch(2) either creates a new watch item, or modifies an existing watch. Each watch has a unique "watch descriptor", an integer returned by inotify_add_watch(2) when the watch is created.
*
When events occur for monitored files and directories, those events are made available to the application as structured data that can be read from the inotify file descriptor using read(2) (see below).
*
inotify_rm_watch(2) removes an item from an inotify watch list.
*
When all file descriptors referring to an inotify instance have been closed (using close(2)), the underlying object and its resources are freed for reuse by the kernel; all associated watches are automatically freed.

With careful programming, an application can use inotify to efficiently monitor and cache the state of a set of filesystem objects. However, robust applications should allow for the fact that bugs in the monitoring logic or races of the kind described below may leave the cache inconsistent with the filesystem state. It is probably wise to to do some consistency checking, and rebuild the cache when inconsistencies are detected.

 

Reading events from an inotify file descriptor

To determine what events have occurred, an application read(2)s from the inotify file descriptor. If no events have so far occurred, then, assuming a blocking file descriptor, read(2) will block until at least one event occurs (unless interrupted by a signal, in which case the call fails with the error EINTR; see signal(7)).

Each successful read(2) returns a buffer containing one or more of the following structures:


struct inotify_event {
    int      wd;       /* Watch descriptor */
    uint32_t mask;     /* Mask of events */
    uint32_t cookie;   /* Unique cookie associating related
                          events (for rename(2)) */
    uint32_t len;      /* Size of name field */
    char     name[];   /* Optional null-terminated name */
};

wd identifies the watch for which this event occurs. It is one of the watch descriptors returned by a previous call to inotify_add_watch(2).

mask contains bits that describe the event that occurred (see below).

cookie is a unique integer that connects related events. Currently this is used only for rename events, and allows the resulting pair of IN_MOVED_FROM and IN_MOVED_TO events to be connected by the application. For all other event types, cookie is set to 0.

The name field is present only when an event is returned for a file inside a watched directory; it identifies the file pathname relative to the watched directory. This pathname is null-terminated, and may include further null bytes ('\0') to align subsequent reads to a suitable address boundary.

The len field counts all of the bytes in name, including the null bytes; the length of each inotify_event structure is thus sizeof(struct inotify_event)+len.

The behavior when the buffer given to read(2) is too small to return information about the next event depends on the kernel version: in kernels before 2.6.21, read(2) returns 0; since kernel 2.6.21, read(2) fails with the error EINVAL. Specifying a buffer of size


    sizeof(struct inotify_event) + NAME_MAX + 1

will be sufficient to read at least one event.  

inotify events

The inotify_add_watch(2) mask argument and the mask field of the inotify_event structure returned when read(2)ing an inotify file descriptor are both bit masks identifying inotify events. The following bits can be specified in mask when calling inotify_add_watch(2) and may be returned in the mask field returned by read(2):
IN_ACCESS (*)
File was accessed (e.g., read(2), execve(2)).
IN_ATTRIB (*)
Metadata changed---for example, permissions (e.g., chmod(2)), timestamps (e.g., utimensat(2)), extended attributes (setxattr(2)), link count (since Linux 2.6.25; e.g., for the target of link(2) and for unlink(2)), and user/group ID (e.g., chown(2)).
IN_CLOSE_WRITE (*)
File opened for writing was closed.
IN_CLOSE_NOWRITE (*)
File not opened for writing was closed.
IN_CREATE (*)
File/directory created in watched directory (e.g., open(2) O_CREAT, mkdir(2), link(2), symlink(2), bind(2) on a UNIX domain socket).
IN_DELETE (*)
File/directory deleted from watched directory.
IN_DELETE_SELF
Watched file/directory was itself deleted. (This event also occurs if an object is moved to another filesystem, since mv(1) in effect copies the file to the other filesystem and then deletes it from the original filesystem.) In addition, an IN_IGNORED event will subsequently be generated for the watch descriptor.
IN_MODIFY (*)
File was modified (e.g., write(2), truncate(2)).
IN_MOVE_SELF
Watched file/directory was itself moved.
IN_MOVED_FROM (*)
Generated for the directory containing the old filename when a file is renamed.
IN_MOVED_TO (*)
Generated for the directory containing the new filename when a file is renamed.
IN_OPEN (*)
File was opened.

When monitoring a directory, the events marked with an asterisk (*) above can occur for files in the directory, in which case the name field in the returned inotify_event structure identifies the name of the file within the directory.

The IN_ALL_EVENTS macro is defined as a bit mask of all of the above events. This macro can be used as the mask argument when calling inotify_add_watch(2).

Two additional convenience macros are defined:

IN_MOVE
Equates to IN_MOVED_FROM | IN_MOVED_TO.
IN_CLOSE
Equates to IN_CLOSE_WRITE | IN_CLOSE_NOWRITE.

The following further bits can be specified in mask when calling inotify_add_watch(2):

IN_DONT_FOLLOW (since Linux 2.6.15)
Don't dereference pathname if it is a symbolic link.
IN_EXCL_UNLINK (since Linux 2.6.36)
By default, when watching events on the children of a directory, events are generated for children even after they have been unlinked from the directory. This can result in large numbers of uninteresting events for some applications (e.g., if watching /tmp, in which many applications create temporary files whose names are immediately unlinked). Specifying IN_EXCL_UNLINK changes the default behavior, so that events are not generated for children after they have been unlinked from the watched directory.
IN_MASK_ADD
Add (OR) events to watch mask for this pathname if it already exists (instead of replacing mask).
IN_ONESHOT
Monitor pathname for one event, then remove from watch list.
IN_ONLYDIR (since Linux 2.6.15)
Only watch pathname if it is a directory.

The following bits may be set in the mask field returned by read(2):

IN_IGNORED
Watch was removed explicitly (inotify_rm_watch(2)) or automatically (file was deleted, or filesystem was unmounted). See also BUGS.
IN_ISDIR
Subject of this event is a directory.
IN_Q_OVERFLOW
Event queue overflowed (wd is -1 for this event).
IN_UNMOUNT
Filesystem containing watched object was unmounted. In addition, an IN_IGNORED event will subsequently be generated for the watch descriptor.
 

Examples

Suppose an application is watching the directory dir and the file dir/myfile for all events. The examples below show some events that will be generated for these two objects.
fd = open("dir/myfile", O_RDWR);
Generates IN_OPEN events for both dir and dir/myfile.
read(fd, buf, count);
Generates IN_ACCESS events for both dir and dir/myfile.
write(fd, buf, count);
Generates IN_MODIFY events for both dir and dir/myfile.
fchmod(fd, mode);
Generates IN_ATTRIB events for both dir and dir/myfile.
close(fd);
Generates IN_CLOSE_WRITE events for both dir and dir/myfile.

Suppose an application is watching the directories dir1 and dir2, and the file dir1/myfile. The following examples show some events that may be generated.

link("dir1/myfile", "dir2/new");
Generates an IN_ATTRIB event for myfile and an IN_CREATE event for dir2.
rename("dir1/myfile", "dir2/myfile");
Generates an IN_MOVED_FROM event for dir1, an IN_MOVED_TO event for dir2, and an IN_MOVE_SELF event for myfile. The IN_MOVED_FROM and IN_MOVED_TO events will have the same cookie value.

Suppose that dir1/xx and dir2/yy are (the only) links to the same file, and an application is watching dir1, dir2, dir1/xx, and dir2/yy. Executing the following calls in the order given below will generate the following events:

unlink("dir2/yy");
Generates an IN_ATTRIB event for xx (because its link count changes) and an IN_DELETE event for dir2.
unlink("dir1/xx");
Generates IN_ATTRIB, IN_DELETE_SELF, and IN_IGNORED events for xx, and an IN_DELETE event for dir1.

Suppose an application is watching the directory dir and (the empty) directory dir/subdir. The following examples show some events that may be generated.

mkdir("dir/new", mode);
Generates an IN_CREATE | IN_ISDIR event for dir.
rmdir("dir/subdir");
Generates IN_DELETE_SELF and IN_IGNORED events for subdir, and an IN_DELETE | IN_ISDIR event for dir.
 

/proc interfaces

The following interfaces can be used to limit the amount of kernel memory consumed by inotify:
/proc/sys/fs/inotify/max_queued_events
The value in this file is used when an application calls inotify_init(2) to set an upper limit on the number of events that can be queued to the corresponding inotify instance. Events in excess of this limit are dropped, but an IN_Q_OVERFLOW event is always generated.
/proc/sys/fs/inotify/max_user_instances
This specifies an upper limit on the number of inotify instances that can be created per real user ID.
/proc/sys/fs/inotify/max_user_watches
This specifies an upper limit on the number of watches that can be created per real user ID.
 

VERSIONS

Inotify was merged into the 2.6.13 Linux kernel. The required library interfaces were added to glibc in version 2.4. (IN_DONT_FOLLOW, IN_MASK_ADD, and IN_ONLYDIR were added in glibc version 2.5.)  

CONFORMING TO

The inotify API is Linux-specific.  

NOTES

Inotify file descriptors can be monitored using select(2), poll(2), and epoll(7). When an event is available, the file descriptor indicates as readable.

Since Linux 2.6.25, signal-driven I/O notification is available for inotify file descriptors; see the discussion of F_SETFL (for setting the O_ASYNC flag), F_SETOWN, and F_SETSIG in fcntl(2). The siginfo_t structure (described in sigaction(2)) that is passed to the signal handler has the following fields set: si_fd is set to the inotify file descriptor number; si_signo is set to the signal number; si_code is set to POLL_IN; and POLLIN is set in si_band.

If successive output inotify events produced on the inotify file descriptor are identical (same wd, mask, cookie, and name), then they are coalesced into a single event if the older event has not yet been read (but see BUGS). This reduces the amount of kernel memory required for the event queue, but also means that an application can't use inotify to reliably count file events.

The events returned by reading from an inotify file descriptor form an ordered queue. Thus, for example, it is guaranteed that when renaming from one directory to another, events will be produced in the correct order on the inotify file descriptor.

The FIONREAD ioctl(2) returns the number of bytes available to read from an inotify file descriptor.  

Limitations and caveats

The inotify API provides no information about the user or process that triggered the inotify event. In particular, there is no easy way for a process that is monitoring events via inotify to distinguish events that it triggers itself from those that are triggered by other processes.

Inotify reports only events that a user-space program triggers through the filesystem API. As a result, it does not catch remote events that occur on network filesystems. (Applications must fall back to polling the filesystem to catch such events.) Furthermore, various pseudo-filesystems such as /proc, /sys, and /dev/pts are not monitorable with inotify.

The inotify API does not report file accesses and modifications that may occur because of mmap(2) and msync(2).

The inotify API identifies affected files by filename. However, by the time an application processes an inotify event, the filename may already have been deleted or renamed.

The inotify API identifies events via watch descriptors. It is the application's responsibility to cache a mapping (if one is needed) between watch descriptors and pathnames. Be aware that directory renamings may affect multiple cached pathnames.

Inotify monitoring of directories is not recursive: to monitor subdirectories under a directory, additional watches must be created. This can take a significant amount time for large directory trees.

If monitoring an entire directory subtree, and a new subdirectory is created in that tree or an existing directory is renamed into that tree, be aware that by the time you create a watch for the new subdirectory, new files (and subdirectories) may already exist inside the subdirectory. Therefore, you may want to scan the contents of the subdirectory immediately after adding the watch (and, if desired, recursively add watches for any subdirectories that it contains).

Note that the event queue can overflow. In this case, events are lost. Robust applications should handle the possibility of lost events gracefully. For example, it may be necessary to rebuild part or all of the application cache. (One simple, but possibly expensive, approach is to close the inotify file descriptor, empty the cache, create a new inotify file descriptor, and then re-create watches and cache entries for the objects to be monitored.)  

Dealing with rename() events

As noted above, the IN_MOVED_FROM and IN_MOVED_TO event pair that is generated by rename(2) can be matched up via their shared cookie value. However, the task of matching has some challenges.

These two events are usually consecutive in the event stream available when reading from the inotify file descriptor. However, this is not guaranteed. If multiple processes are triggering events for monitored objects, then (on rare occasions) an arbitrary number of other events may appear between the IN_MOVED_FROM and IN_MOVED_TO events.

Matching up the IN_MOVED_FROM and IN_MOVED_TO event pair generated by rename(2) is thus inherently racy. (Don't forget that if an object is renamed outside of a monitored directory, there may not even be an IN_MOVED_TO event.) Heuristic approaches (e.g., assume the events are always consecutive) can be used to ensure a match in most cases, but will inevitably miss some cases, causing the application to perceive the IN_MOVED_FROM and IN_MOVED_TO events as being unrelated. If watch descriptors are destroyed and re-created as a result, then those watch descriptors will be inconsistent with the watch descriptors in any pending events. (Re-creating the inotify file descriptor and rebuilding the cache may be useful to deal with this scenario.)

Applications should also allow for the possibility that the IN_MOVED_FROM event was the last event that could fit in the buffer returned by the current call to read(2), and the accompanying IN_MOVED_TO event might be fetched only on the next read(2).  

BUGS

In kernels before 2.6.16, the IN_ONESHOT mask flag does not work.

As originally designed and implemented, the IN_ONESHOT flag did not cause an IN_IGNORED event to be generated when the watch was dropped after one event. However, as an unintended effect of other changes, since Linux 2.6.36, an IN_IGNORED event is generated in this case.

Before kernel 2.6.25, the kernel code that was intended to coalesce successive identical events (i.e., the two most recent events could potentially be coalesced if the older had not yet been read) instead checked if the most recent event could be coalesced with the oldest unread event.  

SEE ALSO

inotifywait(1), inotifywatch(1), inotify_add_watch(2), inotify_init(2), inotify_init1(2), inotify_rm_watch(2), read(2), stat(2), fanotify(7)

Documentation/filesystems/inotify.txt in the Linux kernel source tree  

COLOPHON

This page is part of release 3.66 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
DESCRIPTION
Reading events from an inotify file descriptor
inotify events
Examples
/proc interfaces
VERSIONS
CONFORMING TO
NOTES
Limitations and caveats
Dealing with rename() events
BUGS
SEE ALSO
COLOPHON

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Time: 21:43:04 GMT, July 12, 2014