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NAME | DESCRIPTION | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
INOTIFY(7) Linux Programmer's Manual INOTIFY(7)
inotify - monitoring filesystem events
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 {
int wd; /* Watch descriptor */
uint32_t mask; /* Mask describing event */
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 filename within to 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
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 or directory 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 or directory was opened.
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:
* the events marked above with an asterisk (*) can occur both for
the directory itself and for objects inside the directory; and
* the events marked with a plus sign (+) occur only for objects
inside the directory (not for the directory itself).
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:
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
If a watch instance already exists for the filesystem
object corresponding to pathname, add (OR) the events in
mask to the watch mask (instead of replacing the mask).
IN_ONESHOT
Monitor the filesystem object corresponding to pathname
for one event, then remove from watch list.
IN_ONLYDIR (since Linux 2.6.15)
Watch pathname only if it is a directory. Using this flag
provides an application with a race-free way of ensuring
that the monitored object 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.
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.)
The inotify API is Linux-specific.
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 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.
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), 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.
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. 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.
Before Linux 3.19, fallocate(2) did not create any inotify events.
Since Linux 3.19, calls to fallocate(2) generate IN_MODIFY events.
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 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.
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.
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]
Listening for events stopped.
Program source
#include <errno.h>
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/inotify.h>
#include <unistd.h>
/* Read all available inotify events from the file descriptor 'fd'.
wd is the table of watch descriptors for the directories in argv.
argc is the length of wd and argv.
argv is the list of watched directories.
Entry 0 of wd and argv is unused. */
static void
handle_events(int fd, int *wd, int argc, char* argv[])
{
/* Some systems cannot read integer variables if they are not
properly aligned. On other systems, incorrect alignment may
decrease performance. Hence, the buffer used for reading from
the inotify file descriptor should have the same alignment as
struct inotify_event. */
char buf[4096]
__attribute__ ((aligned(__alignof__(struct inotify_event))));
const struct inotify_event *event;
int i;
ssize_t len;
char *ptr;
/* Loop while events can be read from inotify file descriptor. */
for (;;) {
/* Read some events. */
len = read(fd, buf, sizeof buf);
if (len == -1 && errno != EAGAIN) {
perror("read");
exit(EXIT_FAILURE);
}
/* If the nonblocking read() found no events to read, then
it returns -1 with errno set to EAGAIN. In that case,
we exit the loop. */
if (len <= 0)
break;
/* Loop over all events in the buffer */
for (ptr = buf; ptr < buf + len;
ptr += sizeof(struct inotify_event) + event->len) {
event = (const struct inotify_event *) ptr;
/* Print event type */
if (event->mask & IN_OPEN)
printf("IN_OPEN: ");
if (event->mask & IN_CLOSE_NOWRITE)
printf("IN_CLOSE_NOWRITE: ");
if (event->mask & IN_CLOSE_WRITE)
printf("IN_CLOSE_WRITE: ");
/* Print the name of the watched directory */
for (i = 1; i < argc; ++i) {
if (wd[i] == event->wd) {
printf("%s/", argv[i]);
break;
}
}
/* Print the name of the file */
if (event->len)
printf("%s", event->name);
/* Print type of filesystem object */
if (event->mask & IN_ISDIR)
printf(" [directory]\n");
else
printf(" [file]\n");
}
}
}
int
main(int argc, char* argv[])
{
char buf;
int fd, i, poll_num;
int *wd;
nfds_t nfds;
struct pollfd fds[2];
if (argc < 2) {
printf("Usage: %s PATH [PATH ...]\n", argv[0]);
exit(EXIT_FAILURE);
}
printf("Press ENTER key to terminate.\n");
/* Create the file descriptor for accessing the inotify API */
fd = inotify_init1(IN_NONBLOCK);
if (fd == -1) {
perror("inotify_init1");
exit(EXIT_FAILURE);
}
/* Allocate memory for watch descriptors */
wd = calloc(argc, sizeof(int));
if (wd == NULL) {
perror("calloc");
exit(EXIT_FAILURE);
}
/* Mark directories for events
- file was opened
- file was closed */
for (i = 1; i < argc; i++) {
wd[i] = inotify_add_watch(fd, argv[i],
IN_OPEN | IN_CLOSE);
if (wd[i] == -1) {
fprintf(stderr, "Cannot watch '%s'\n", argv[i]);
perror("inotify_add_watch");
exit(EXIT_FAILURE);
}
}
/* Prepare for polling */
nfds = 2;
/* Console input */
fds[0].fd = STDIN_FILENO;
fds[0].events = POLLIN;
/* Inotify input */
fds[1].fd = fd;
fds[1].events = POLLIN;
/* Wait for events and/or terminal input */
printf("Listening for events.\n");
while (1) {
poll_num = poll(fds, nfds, -1);
if (poll_num == -1) {
if (errno == EINTR)
continue;
perror("poll");
exit(EXIT_FAILURE);
}
if (poll_num > 0) {
if (fds[0].revents & POLLIN) {
/* Console input is available. Empty stdin and quit */
while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\n')
continue;
break;
}
if (fds[1].revents & POLLIN) {
/* Inotify events are available */
handle_events(fd, wd, argc, argv);
}
}
}
printf("Listening for events stopped.\n");
/* Close inotify file descriptor */
close(fd);
free(wd);
exit(EXIT_SUCCESS);
}
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
This page is part of release 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 INOTIFY(7)
Pages that refer to this page: inotifywait(1), inotifywatch(1), fcntl(2), inotify_add_watch(2), inotify_init(2), inotify_rm_watch(2), ptrace(2), proc(5), systemd.path(5), cgroups(7), credentials(7), fanotify(7), signal(7), agetty(8), cron(8), nscd(8)