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INOTIFY(7) Linux Programmer's Manual INOTIFY(7)
NAME
inotify - monitoring file system events
DESCRIPTION
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 direc-
tory 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 ino-
tify_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 re-use by the kernel; all associ-
ated 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.
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 direc-
tory. 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(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.
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 call-
ing 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 ino-
tify_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 ino-
tify_add_watch(2):
IN_DONT_FOLLOW (since Linux 2.6.15)
Don't dereference pathname if it is a symbolic link.
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 auto-
matically (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.
/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 corre-
sponding 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 only added in 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 sigac-
tion(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.
BUGS
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.
SEE ALSO
inotify_add_watch(2), inotify_init(2), inotify_init1(2), inotify_rm_watch(2),
read(2), stat(2), Documentation/filesystems/inotify.txt.
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of
the project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2008-11-18 INOTIFY(7)
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