| NAME | DESCRIPTION | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
INOTIFY(7) Linux Programmer's Manual INOTIFY(7)
inotify - monitoring file system events
The inotify API provides a mechanism for monitoring file system
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)
(or inotify_init1(2)), inotify_add_watch(2), inotify_rm_watch(2),
read(2), and close(2).
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 provides 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.
inotify_rm_watch(2) removes an item from an inotify watch list.
When all file descriptors referring to an inotify instance have been
closed, the underlying object and its resources are freed for reuse
by the kernel; all associated watches are automatically freed.
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 of events */
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 file pathname relative
to the watched directory. This pathname is null-terminated, and may
include further null bytes 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.
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 (read) (*).
IN_ATTRIB Metadata changed, e.g., permissions,
timestamps, extended attributes, link count
(since Linux 2.6.25), UID, GID, etc. (*).
IN_CLOSE_WRITE File opened for writing was closed (*).
IN_CLOSE_NOWRITE File not opened for writing was closed (*).
IN_CREATE File/directory created in watched directory
(*).
IN_DELETE File/directory deleted from watched directory
(*).
IN_DELETE_SELF Watched file/directory was itself deleted.
IN_MODIFY File was modified (*).
IN_MOVE_SELF Watched file/directory was itself moved.
IN_MOVED_FROM File moved out of watched directory (*).
IN_MOVED_TO File moved into watched directory (*).
IN_OPEN File was opened (*).
When monitoring a directory, the events marked with an asterisk (*)
above can occur for files in the directory, in which case 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 IN_MOVE, which equates to
IN_MOVED_FROM|IN_MOVED_TO, and IN_CLOSE, which 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 Add (OR) events to watch mask for this pathname
if it already exists (instead of replacing
mask).
IN_ONESHOT Monitor pathname for one event, then remove
from watch list.
IN_ONLYDIR (since Linux 2.6.15)
Only watch pathname if it 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 file system was unmounted).
IN_ISDIR Subject of this event is a directory.
IN_Q_OVERFLOW Event queue overflowed (wd is -1 for this
event).
IN_UNMOUNT File system containing watched object was
unmounted.
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 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).
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.
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.
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.
Note that the event queue can overflow. In this case, events are
lost. Robust applications should handle the possibility of lost
events gracefully.
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.
If monitoring an entire directory subtree, and a new subdirectory is
created in that tree, be aware that by the time you create a watch
for the new subdirectory, new files may already have been created in
the subdirectory. Therefore, you may want to scan the contents of
the subdirectory immediately after adding the watch.
In kernels before 2.6.16, the IN_ONESHOT mask flag does not work.
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.
inotify_add_watch(2), inotify_init(2), inotify_init1(2),
inotify_rm_watch(2), read(2), stat(2)
Documentation/filesystems/inotify.txt in the Linux kernel source tree
This page is part of release 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-02-25 INOTIFY(7)
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