inotify (7) - Linux Manuals
inotify: monitoring filesystem events
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 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 {
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 filename within the watched directory.
This filename 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
will be sufficient to read at least one event.
Inotify monitoring is inode-based: when monitoring a file
(but not when monitoring the directory containing a file),
an event can be generated for activity on any link to the file
(in the same or a different directory).
When monitoring a directory:
Note:
when monitoring a directory,
events are not generated for the files inside the directory
when the events are performed via a pathname (i.e., a link)
that lies outside the monitored directory.
When events are generated for objects inside a watched directory, 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:
The following further bits can be specified in
mask
when calling
inotify_add_watch(2):
The following bits may be set in the
mask
field returned by
read(2):
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.
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:
Suppose an application is watching the directory
dir
and (the empty) directory
dir/subdir.
The following examples show some events that may be generated.
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 set of watch descriptors that is being monitored via
an inotify file descriptor can be viewed via the entry for
the inotify file descriptor in the process's
/proc/[pid]/fdinfo
directory.
See
proc(5)
for further details.
The
FIONREAD
ioctl(2)
returns the number of bytes available to read from an
inotify file descriptor.
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),
msync(2),
and
munmap(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.)
If a filesystem is mounted on top of a monitored directory,
no event is generated, and no events are generated
for objects immediately under the new mount point.
If the filesystem is subsequently unmounted,
events will subsequently be generated for the directory and
the objects it contains.
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.
Furthermore, it is not guaranteed that the event pair is atomically
inserted into the queue: there may be a brief interval where the
IN_MOVED_FROM
has appeared, but the
IN_MOVED_TO
has not.
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),
which should be done with a (small) timeout to allow for the fact that
insertion of the
IN_MOVED_FROM-IN_MOVED_TO
event pair is not atomic,
and also the possibility that there may not be any
IN_MOVED_TO
event.
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.
When a watch descriptor is removed by calling
inotify_rm_watch(2)
(or because a watch file is deleted or the filesystem
that contains it is unmounted),
any pending unread events for that watch descriptor remain available to read.
As watch descriptors are subsequently allocated with
inotify_add_watch(2),
the kernel cycles through the range of possible watch descriptors (0 to
INT_MAX)
incrementally.
When allocating a free watch descriptor, no check is made to see whether that
watch descriptor number has any pending unread events in the inotify queue.
Thus, it can happen that a watch descriptor is reallocated even
when pending unread events exist for a previous incarnation of
that watch descriptor number, with the result that the application
might then read those events and interpret them as belonging to
the file associated with the newly recycled watch descriptor.
In practice, the likelihood of hitting this bug may be extremely low,
since it requires that an application cycle through
INT_MAX
watch descriptors,
release a watch descriptor while leaving unread events for that
watch descriptor in the queue,
and then recycle that watch descriptor.
For this reason, and because there have been no reports
of the bug occurring in real-world applications,
as of Linux 3.15,
no kernel changes have yet been made to eliminate this possible bug.
The following output was recorded while editing the file
/home/user/temp/foo
and listing directory
/tmp.
Before the file and the directory were opened,
IN_OPEN
events occurred.
After the file was closed, an
IN_CLOSE_WRITE
event occurred.
After the directory was closed, an
IN_CLOSE_NOWRITE
event occurred.
Execution of the program ended when the user pressed the ENTER key.
/* Read all available inotify events from the file descriptor 'fd'.
static void
handle_events(int fd, int *wd, int argc, char* argv[])
{
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):
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.
/proc interfaces
The following interfaces can be used to limit the amount of
kernel memory consumed by inotify:
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.
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.
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.
BUGS
Before Linux 3.19,
fallocate(2)
did not create any inotify events.
Since Linux 3.19,
calls to
fallocate(2)
generate
IN_MODIFY
events.
EXAMPLES
The following program demonstrates the usage of the inotify API.
It marks the directories passed as a command-line arguments
and waits for events of type
IN_OPEN,
IN_CLOSE_NOWRITE,
and
IN_CLOSE_WRITE.
Example output
$ ./a.out /tmp /home/user/temp
Press enter key to terminate.
Listening for events.
IN_OPEN: /home/user/temp/foo [file]
IN_CLOSE_WRITE: /home/user/temp/foo [file]
IN_OPEN: /tmp/ [directory]
IN_CLOSE_NOWRITE: /tmp/ [directory]
Program source
#include <errno.h>
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/inotify.h>
#include <unistd.h>
#include <string.h>