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NAME | DESCRIPTION | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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inotify(7) Miscellaneous Information 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 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: before Linux 2.6.21, read(2) returns 0; since Linux
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); the error EINVAL results if IN_MASK_CREATE is
also specified.
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; the error
ENOTDIR results if pathname is not a directory. Using
this flag provides an application with a race-free way
of ensuring that the monitored object is a directory.
IN_MASK_CREATE (since Linux 4.18)
Watch pathname only if it does not already have a
watch associated with it; the error EEXIST results if
pathname is already being watched.
Using this flag provides an application with a way of
ensuring that new watches do not modify existing ones.
This is useful because multiple paths may refer to the
same inode, and multiple calls to inotify_add_watch(2)
without this flag may clobber existing watch masks.
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.
Linux.
Inotify was merged into Linux 2.6.13. The required library
interfaces were added in glibc 2.4. (IN_DONT_FOLLOW,
IN_MASK_ADD, and IN_ONLYDIR were added in glibc 2.5.)
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.
Before Linux 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 Linux 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 (1 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>
#include <string.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;
ssize_t len;
/* 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 (char *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 (size_t 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': %s\n",
argv[i], strerror(errno));
exit(EXIT_FAILURE);
}
}
/* Prepare for polling. */
nfds = 2;
fds[0].fd = STDIN_FILENO; /* Console input */
fds[0].events = POLLIN;
fds[1].fd = fd; /* Inotify input */
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
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Linux man-pages 6.9.1 2024-06-15 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), sd_bus_set_watch_bind(3), sd_event_add_inotify(3), proc_pid_fdinfo(5), proc_sys_fs(5), systemd.path(5), cgroups(7), credentials(7), fanotify(7), signal(7), agetty(8), cron(8), nscd(8)
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