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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
only used for rename events, and allows the resulting pair of IN_MOVE_FROM
and IN_MOVE_TO events to be connected by the application. For all other
event types, cookie is set to 0.
The name field is only present 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 only 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.
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.
This page is part of release 3.41 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 2012-04-26 INOTIFY(7)
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