NAME¶
inotify - monitoring file system events
DESCRIPTION¶
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) (or
inotify_init1(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. The more recent
inotify_init1(2) is
like
inotify_init(2), but provides 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.
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 reuse 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 (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 only used 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 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(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 (read) (*).
- IN_ATTRIB
- Metadata changed, e.g., permissions, timestamps, extended
attributes, link count (since Linux 2.6.25), UID, GID, 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):
- 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 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.
/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 only added in version 2.5.)
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).
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¶
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.
The inotify API provides no information about the user or process that triggered
the inotify event.
Note that the event queue can overflow. In this case, events are lost. Robust
applications should handle the possibility of lost events gracefully.
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.
If monitoring an entire directory subtree, and a new subdirectory is created in
that tree, be aware that by the time you create a watch for the new
subdirectory, new files may already have been created in the subdirectory.
Therefore, you may want to scan the contents of the subdirectory immediately
after adding the watch.
BUGS¶
In kernels before 2.6.16, the
IN_ONESHOT mask flag does not work.
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¶
inotify_add_watch(2),
inotify_init(2),
inotify_init1(2),
inotify_rm_watch(2),
read(2),
stat(2)
Documentation/filesystems/inotify.txt in the Linux kernel source tree
COLOPHON¶
This page is part of release 3.44 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/.