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
;/* 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 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 behaviour 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):
Bit
Description
IN_ACCESS
File was accessed (read) (*) IN_ATTRIB
Metadata changed (permissions, timestamps, extended attributes, etc.) (*) 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 (*) IN_DELETE
File/directory deleted from watched directory (*) IN_DELETE_SELF
Watched file/directory was itself deleted IN_MODIFY
File was modified (*) IN_MOVE_SELF
Watched file/directory was itself moved IN_MOVED_FROM
File moved out of watched directory (*) IN_MOVED_TO
File moved into watched directory (*) 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 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):
Bit
Description
IN_DONT_FOLLOW
Don't dereference pathname
if it is a symbolic linkIN_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 listIN_ONLYDIR
Only watch pathname
if it is a directory
The following bits may be set in the mask
field returned by
read(2):
Bit
Description
IN_IGNORED
Watch was removed explicitly ( inotify_rm_watch(2)) or automatically (file was deleted, or file system was unmounted) IN_ISDIR
Subject of this event is a directory IN_Q_OVERFLOW
Event queue overflowed ( wd
is −1 for this event)IN_UNMOUNT
File 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_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 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
.
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