inotify — monitoring file system events
The inotify API
provides a mechanism for monitoring file system 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), inotify_add_watch(2), inotify_rm_watch(2), read(2), and close(2).
inotify_init(2) creates an inotify instance and returns a file descriptor referring to the inotify instance.
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.
inotify_rm_watch(2) removes an item from an inotify watch list.
When all file descriptors referring to an inotify instance have been closed, the underlying object and its resources are freed for re-use by the kernel; all associated watches are automatically freed.
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.
Each successful read(2) returns a buffer containing one or more of the following structures:
struct inotify_event { int wd;uint32_t mask;uint32_t cookie;
events (for rename(2)) */uint32_t len;char 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 only
used for rename events, and allows the resulting pair of
IN_MOVE_FROM and IN_MOVE_TO events to be connected by the
application.
The name field is
only present 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 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(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.
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_ACCESSFile was accessed (read) (*).
IN_ATTRIBMetadata changed (permissions, timestamps, extended attributes, etc.) (*).
IN_CLOSE_WRITEFile opened for writing was closed (*).
IN_CLOSE_NOWRITEFile not opened for writing was closed (*).
IN_CREATEFile/directory created in watched directory (*).
IN_DELETEFile/directory deleted from watched directory (*).
IN_DELETE_SELFWatched file/directory was itself deleted.
IN_MODIFYFile was modified (*).
IN_MOVE_SELFWatched file/directory was itself moved.
IN_MOVED_FROMFile moved out of watched directory (*).
IN_MOVED_TOFile moved into watched directory (*).
IN_OPENFile 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 IN_MOVE, which equates to
IN_MOVED_FROM|IN_MOVED_TO, and IN_CLOSE which 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_FOLLOWDon't dereference
pathnameif it is a symbolic link (since Linux 2.6.15).IN_MASK_ADDAdd (OR) events to watch mask for this pathname if it already exists (instead of replacing mask).
IN_ONESHOTMonitor
pathnamefor one event, then remove from watch list.IN_ONLYDIROnly watch
pathnameif it is a directory (since Linux 2.6.15).
The following bits may be set in the mask field returned by
read(2):
IN_IGNOREDWatch was removed explicitly ( inotify_rm_watch(2)) or automatically (file was deleted, or file system was unmounted).
IN_ISDIRSubject of this event is a directory.
IN_Q_OVERFLOWEvent queue overflowed (
wdis −1 for this event).IN_UNMOUNTFile system containing watched object was unmounted.
The following interfaces can be used to limit the amount of kernel memory consumed by inotify:
/proc/sys/fs/inotify/max_queued_eventsThe 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_instancesThis specifies an upper limit on the number of inotify instances that can be created per real user ID.
/proc/sys/fs/inotify/max_user_watchesThis specifies a limit on the number of watches that can be associated with each inotify instance.
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 only added in version
2.5.)
Inotify file descriptors can be monitored using select(2), poll(2), and epoll(7).
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.
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.
Inotify monitoring of directories is not recursive: to monitor subdirectories under a directory, additional watches must be created.
inotify_add_watch(2),
inotify_init(2), inotify_rm_watch(2),
read(2), stat(2), Documentation/filesystems/inotify.txt.
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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t Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |