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-.TH INOTIFY 7 2011-12-07 "Linux" "Linux Programmer's Manual"
+.TH INOTIFY 7 2014-04-01 "Linux" "Linux Programmer's Manual"
.SH NAME
-inotify \- monitoring file system events
+inotify \- monitoring filesystem events
.SH DESCRIPTION
The
.I inotify
-API provides a mechanism for monitoring file system events.
+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:
-.BR inotify_init (2)
-(or
-.BR inotify_init1 (2)),
-.BR inotify_add_watch (2),
-.BR inotify_rm_watch (2),
-.BR read (2),
-and
-.BR close (2).
-
+.IP * 3
.BR inotify_init (2)
creates an inotify instance and returns a file descriptor
referring to the inotify instance.
.BR inotify_init1 (2)
is like
.BR inotify_init (2),
-but provides some extra functionality.
-
+but has a
+.IR flags
+argument that provides access to some extra functionality.
+.IP *
.BR inotify_add_watch (2)
manipulates the "watch list" associated with an inotify instance.
Each item ("watch") in the watch list specifies the pathname of
returned by
.BR inotify_add_watch (2)
when the watch is created.
-
+.IP *
+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
+.BR read (2)
+(see below).
+.IP *
.BR inotify_rm_watch (2)
removes an item from an inotify watch list.
-
+.IP *
When all file descriptors referring to an inotify
-instance have been closed,
+instance have been closed (using
+.BR 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 to do some consistency checking,
+and rebuild the cache when inconsistencies are detected.
+.SS Reading events from an inotify file descriptor
To determine what events have occurred, an application
.BR read (2)s
from the inotify file descriptor.
.I cookie
is a unique integer that connects related events.
-Currently this is only used for rename events, and
+Currently this is used only for rename events, and
allows the resulting pair of
-.B IN_MOVE_FROM
+.B IN_MOVED_FROM
and
-.B IN_MOVE_TO
+.B IN_MOVED_TO
events to be connected by the application.
For all other event types,
.I cookie
The
.I name
-field is only present when an event is returned
+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
+and may include further null bytes (\(aq\\0\(aq) to align subsequent reads to a
suitable address boundary.
The
the length of each
.I inotify_event
structure is thus
-.IR "sizeof(inotify_event)+len" .
+.IR "sizeof(struct inotify_event)+len" .
The behavior when the buffer given to
.BR read (2)
.BR read (2)
fails with the error
.BR EINVAL .
+Specifying a buffer of size
+
+ sizeof(struct inotify_event) + NAME_MAX + 1
+
+will be sufficient to read at least one event.
.SS inotify events
The
.BR inotify_add_watch (2)
field returned by
.BR read (2):
.RS 4
-.sp
-.PD 0
-.TP 18
-.B IN_ACCESS
-File was accessed (read) (*).
.TP
-.B IN_ATTRIB
-Metadata changed, e.g., permissions, timestamps, extended attributes,
-link count (since Linux 2.6.25), UID, GID, etc. (*).
+.BR IN_ACCESS " (*)"
+File was accessed (e.g.,
+.BR read (2),
+.BR execve (2)).
.TP
-.B IN_CLOSE_WRITE
-File opened for writing was closed (*).
+.BR IN_ATTRIB " (*)"
+Metadata changed\(emfor example, permissions (e.g.,
+.BR chmod (2)),
+timestamps (e.g.,
+.BR utimensat (2)),
+extended attributes
+.RB ( setxattr (2)),
+link count (since Linux 2.6.25; e.g.,
+for the target of
+.BR link (2)
+and for
+.BR unlink (2)),
+and user/group ID (e.g.,
+.BR chown (2)).
.TP
-.B IN_CLOSE_NOWRITE
-File not opened for writing was closed (*).
+.BR IN_CLOSE_WRITE " (*)"
+File opened for writing was closed.
.TP
-.B IN_CREATE
-File/directory created in watched directory (*).
+.BR IN_CLOSE_NOWRITE " (*)"
+File not opened for writing was closed.
.TP
-.B IN_DELETE
-File/directory deleted from watched directory (*).
+.BR IN_CREATE " (*)"
+File/directory created in watched directory (e.g.,
+.BR open (2)
+.BR O_CREAT ,
+.BR mkdir (2),
+.BR link (2),
+.BR symlink (2),
+.BR bind (2)
+on a UNIX domain socket).
+.TP
+.BR IN_DELETE " (*)"
+File/directory deleted from watched directory.
.TP
.B IN_DELETE_SELF
Watched file/directory was itself deleted.
+(This event also occurs if an object is moved to another filesystem,
+since
+.BR mv (1)
+in effect copies the file to the other filesystem and
+then deletes it from the original filesystem.)
+In addition, an
+.B IN_IGNORED
+event will subsequently be generated for the watch descriptor.
.TP
-.B IN_MODIFY
-File was modified (*).
+.BR IN_MODIFY " (*)"
+File was modified (e.g.,
+.BR write (2),
+.BR truncate (2)).
.TP
.B IN_MOVE_SELF
Watched file/directory was itself moved.
.TP
-.B IN_MOVED_FROM
-File moved out of watched directory (*).
+.BR IN_MOVED_FROM " (*)"
+Generated for the directory containing the old filename
+when a file is renamed.
.TP
-.B IN_MOVED_TO
-File moved into watched directory (*).
+.BR IN_MOVED_TO " (*)"
+Generated for the directory containing the new filename
+when a file is renamed.
.TP
-.B IN_OPEN
-File was opened (*).
-.PD
+.BR IN_OPEN " (*)"
+File was opened.
.RE
.PP
When monitoring a directory,
argument when calling
.BR inotify_add_watch (2).
-Two additional convenience macros are
-.BR IN_MOVE ,
-which equates to
-IN_MOVED_FROM|IN_MOVED_TO,
-and
-.BR IN_CLOSE ,
-which equates to
-IN_CLOSE_WRITE|IN_CLOSE_NOWRITE.
+Two additional convenience macros are defined:
+.RS 4
+.TP
+.BR IN_MOVE
+Equates to
+.BR "IN_MOVED_FROM | IN_MOVED_TO" .
+.TP
+.BR IN_CLOSE
+Equates to
+.BR "IN_CLOSE_WRITE | IN_CLOSE_NOWRITE" .
+.RE
.PP
The following further bits can be specified in
.I mask
when calling
.BR inotify_add_watch (2):
.RS 4
-.sp
-.PD 0
-.TP 18
+.TP
.BR IN_DONT_FOLLOW " (since Linux 2.6.15)"
-Don't dereference \fIpathname\fP if it is a symbolic link.
+Don't dereference
+.I pathname
+if it is a symbolic link.
.TP
.BR IN_EXCL_UNLINK " (since Linux 2.6.36)"
.\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6
it already exists (instead of replacing mask).
.TP
.B IN_ONESHOT
-Monitor \fIpathname\fP for one event, then remove from
+Monitor
+.I pathname
+for one event, then remove from
watch list.
.TP
.BR IN_ONLYDIR " (since Linux 2.6.15)"
-Only watch \fIpathname\fP if it is a directory.
-.PD
+Only watch
+.I pathname
+if it is a directory.
.RE
.PP
The following bits may be set in the
field returned by
.BR read (2):
.RS 4
-.sp
-.PD 0
-.TP 18
+.TP
.B IN_IGNORED
-Watch was removed explicitly (\fBinotify_rm_watch\fP(2))
-or automatically (file was deleted, or file system was unmounted).
+Watch was removed explicitly
+.RB ( inotify_rm_watch (2))
+or automatically (file was deleted, or filesystem was unmounted).
+See also BUGS.
.TP
.B IN_ISDIR
Subject of this event is a directory.
.TP
.B IN_Q_OVERFLOW
-Event queue overflowed (\fIwd\fP is \-1 for this event).
+Event queue overflowed
+.RI ( wd
+is \-1 for this event).
.TP
.B IN_UNMOUNT
-File system containing watched object was unmounted.
-.PD
+Filesystem containing watched object was unmounted.
+In addition, an
+.B IN_IGNORED
+event will subsequently be generated for the watch descriptor.
+.RE
+.SS Examples
+Suppose an application is watching the directory
+.I dir
+and the file
+.IR dir/myfile
+for all events.
+The examples below show some events that will be generated
+for these two objects.
+.RS 4
+.TP
+fd = open("dir/myfile", O_RDWR);
+Generates
+.B IN_OPEN
+events for both
+.I dir
+and
+.IR dir/myfile .
+.TP
+read(fd, buf, count);
+Generates
+.B IN_ACCESS
+events for both
+.I dir
+and
+.IR dir/myfile .
+.TP
+write(fd, buf, count);
+Generates
+.B IN_MODIFY
+events for both
+.I dir
+and
+.IR dir/myfile .
+.TP
+fchmod(fd, mode);
+Generates
+.B IN_ATTRIB
+events for both
+.I dir
+and
+.IR dir/myfile .
+.TP
+close(fd);
+Generates
+.B IN_CLOSE_WRITE
+events for both
+.I dir
+and
+.IR dir/myfile .
+.RE
+.PP
+Suppose an application is watching the directories
+.I dir1
+and
+.IR dir2 ,
+and the file
+.IR dir1/myfile .
+The following examples show some events that may be generated.
+.RS 4
+.TP
+link("dir1/myfile", "dir2/new");
+Generates an
+.B IN_ATTRIB
+event for
+.IR myfile
+and an
+.B IN_CREATE
+event for
+.IR dir2 .
+.TP
+rename("dir1/myfile", "dir2/myfile");
+Generates an
+.B IN_MOVED_FROM
+event for
+.IR dir1 ,
+an
+.B IN_MOVED_TO
+event for
+.IR dir2 ,
+and an
+.B IN_MOVE_SELF
+event for
+.IR myfile .
+The
+.B IN_MOVED_FROM
+and
+.B IN_MOVED_TO
+events will have the same
+.I cookie
+value.
+.RE
+.PP
+Suppose that
+.IR dir1/xx
+and
+.IR dir2/yy
+are (the only) links to the same file, and an application is watching
+.IR dir1 ,
+.IR dir2 ,
+.IR dir1/xx ,
+and
+.IR dir2/yy .
+Executing the following calls in the order given below will generate
+the following events:
+.RS 4
+.TP
+unlink("dir2/yy");
+Generates an
+.BR IN_ATTRIB
+event for
+.IR xx
+(because its link count changes)
+and an
+.B IN_DELETE
+event for
+.IR dir2 .
+.TP
+unlink("dir1/xx");
+Generates
+.BR IN_ATTRIB ,
+.BR IN_DELETE_SELF ,
+and
+.BR IN_IGNORED
+events for
+.IR xx ,
+and an
+.BR IN_DELETE
+event for
+.IR dir1 .
+.RE
+.PP
+Suppose an application is watching the directory
+.IR dir
+and (the empty) directory
+.IR dir/subdir .
+The following examples show some events that may be generated.
+.RS 4
+.TP
+mkdir("dir/new", mode);
+Generates an
+.B "IN_CREATE | IN_ISDIR"
+event for
+.IR dir .
+.TP
+rmdir("dir/subdir");
+Generates
+.B IN_DELETE_SELF
+and
+.B IN_IGNORED
+events for
+.IR subdir ,
+and an
+.B "IN_DELETE | IN_ISDIR"
+event for
+.IR dir .
.RE
.SS /proc interfaces
The following interfaces can be used to limit the amount of
.I /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.
-.SH "VERSIONS"
+.SH VERSIONS
Inotify was merged into the 2.6.13 Linux kernel.
The required library interfaces were added to glibc in version 2.4.
.RB ( IN_DONT_FOLLOW ,
.BR IN_MASK_ADD ,
and
.B IN_ONLYDIR
-were only added in version 2.5.)
-.SH "CONFORMING TO"
+were added in glibc version 2.5.)
+.SH CONFORMING TO
The inotify API is Linux-specific.
-.SH "NOTES"
+.SH NOTES
Inotify file descriptors can be monitored using
.BR select (2),
.BR poll (2),
.IR mask ,
.IR cookie ,
and
-.IR name )
+.IR 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.
returns the number of bytes available to read from an
inotify file descriptor.
.SS 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
+.IR /proc ,
+.IR /sys ,
+and
+.IR /dev/pts
+are not monitorable with inotify.
+
+The inotify API does not report file accesses and modifications that
+may occur because of
+.BR mmap (2)
+and
+.BR msync (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.
-The inotify API provides no information about the user or process that
-triggered the inotify event.
+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.)
+.SS Dealing with rename() events
+As noted above, the
+.B IN_MOVED_FROM
+and
+.B IN_MOVED_TO
+event pair that is generated by
+.BR rename (2)
+can be matched up via their shared cookie value.
+However, the task of matching has some challenges.
-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.
+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
+.B IN_MOVED_FROM
+and
+.B IN_MOVED_TO
+events.
-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.
-.SH "BUGS"
+Matching up the
+.B IN_MOVED_FROM
+and
+.B IN_MOVED_TO
+event pair generated by
+.BR 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
+.BR 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
+.B IN_MOVED_FROM
+and
+.B 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
+.B IN_MOVED_FROM
+event was the last event that could fit in the buffer
+returned by the current call to
+.BR read (2),
+and the accompanying
+.B IN_MOVED_TO
+event might be fetched only on the next
+.BR read (2).
+.SH BUGS
+.\" FIXME kernel commit 611da04f7a31b2208e838be55a42c7a1310ae321
+.\" implies that unmount events were buggy 2.6.11 to 2.6.36
+.\"
In kernels before 2.6.16, the
.B IN_ONESHOT
.I mask
flag does not work.
+As originally designed and implemented, the
+.B IN_ONESHOT
+flag did not cause an
+.B 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
+.B IN_IGNORED
+event is generated in this case.
+
Before kernel 2.6.25,
+.\" commit 1c17d18e3775485bf1e0ce79575eb637a94494a2
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
.I oldest
unread event.
-.SH "SEE ALSO"
+.SH SEE ALSO
+.BR inotifywait (1),
+.BR inotifywatch (1),
.BR inotify_add_watch (2),
.BR inotify_init (2),
.BR inotify_init1 (2),
.BR inotify_rm_watch (2),
.BR read (2),
-.BR stat (2),
-.IR Documentation/filesystems/inotify.txt .
+.BR stat (2)
+
+.IR Documentation/filesystems/inotify.txt
+in the Linux kernel source tree
+.SH COLOPHON
+This page is part of release 3.65 of the Linux
+.I man-pages
+project.
+A description of the project,
+and information about reporting bugs,
+can be found at
+\%http://www.kernel.org/doc/man\-pages/.
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