inotify(7) Miscellaneous Information Manual inotify(7)
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 directo-
ries. 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 de-
scriptor referring to the inotify instance. The more recent ino-
tify_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 descrip-
tor", 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 automati-
cally 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 monitor-
ing logic or races of the kind described below may leave the cache in-
consistent 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 applica-
tion. For all other event types, cookie is set to 0.
The name field is present only when an event is returned for a file in-
side 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 ino-
tify_event structure returned when read(2)ing an inotify file descrip-
tor 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 (setx-
attr(2)), link count (since Linux 2.6.25; e.g., for the tar-
get 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 direc-
tory).
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 ino-
tify_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 ino-
tify_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 direc-
tory, 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 tempo-
rary files whose names are immediately unlinked). Specify-
ing 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 as-
sociated with it; the error EEXIST results if pathname is
already being watched.
Using this flag provides an application with a way of ensur-
ing 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 auto-
matically (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 ad-
dition, 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. Exe-
cuting 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 mem-
ory consumed by inotify:
/proc/sys/fs/inotify/max_queued_events
The value in this file is used when an application calls ino-
tify_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.
STANDARDS
Linux.
HISTORY
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.)
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 in-
otify 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 han-
dler has the following fields set: si_fd is set to the inotify file de-
scriptor 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 de-
scriptor 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 de-
scriptor 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 al-
ready have been deleted or renamed.
The inotify API identifies events via watch descriptors. It is the ap-
plication's responsibility to cache a mapping (if one is needed) be-
tween watch descriptors and pathnames. Be aware that directory renam-
ings may affect multiple cached pathnames.
Inotify monitoring of directories is not recursive: to monitor subdi-
rectories 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 subdi-
rectory, new files (and subdirectories) may already exist inside the
subdirectory. Therefore, you may want to scan the contents of the sub-
directory immediately after adding the watch (and, if desired, recur-
sively 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 grace-
fully. 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 ino-
tify 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. Further-
more, 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 rebuild-
ing 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 re-
turned 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 possi-
bility that there may not be any IN_MOVED_TO event.
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.
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 suc-
cessive identical events (i.e., the two most recent events could poten-
tially 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 pos-
sible watch descriptors (1 to INT_MAX) incrementally. When allocating
a free watch descriptor, no check is made to see whether that watch de-
scriptor 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 ap-
plication cycle through INT_MAX watch descriptors, release a watch de-
scriptor 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.
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.
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);
}
SEE ALSO
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
Linux man-pages 6.7 2023-10-31 inotify(7)
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