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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 {
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.
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).
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)
Only watch pathname 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.
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.
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 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.)
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.
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.
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.
EXAMPLE
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);
}
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
COLOPHON
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