path: root/usr/src/man/man7/inotify.7

'\" te
.\"  Copyright (c) 2014, Joyent, Inc. All Rights Reserved.
.\"  This file and its contents are supplied under the terms of the
.\"  Common Development and Distribution License ("CDDL"), version 1.0.
.\"  You may only use this file in accordance with the terms of version
.\"  1.0 of the CDDL.
.\" 
.\"  A full copy of the text of the CDDL should have accompanied this
.\"  source.  A copy of the CDDL is also available via the Internet at
.\"  http://www.illumos.org/license/CDDL.
.TH INOTIFY 7 "Sep 17, 2014"
.SH NAME
inotify \- Linux-compatible file event notification facility
.SH SYNOPSIS

.LP
.nf
#include <sys/inotify.h>
.fi

.SH DESCRIPTION
.sp
.LP

\fBinotify\fR is a facility for receiving file system events on specified
files or directories.  When monitoring a directory, \fBinotify\fR can be
used to retrieve events not only on the directory, but also on any files
that the directory contains.  \fBinotify\fR originated with Linux, and
this facility is designed to be binary-compatible with the Linux facility,
including the following interfaces:

.RS +4
.TP
.ie t \(bu
.el o
\fBinotify_init\fR(3C) creates an \fBinotify\fR instance, returning a file
descriptor associated with the in-kernel event queue.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBinotify_init1\fR(3C) also creates an \fBinotify\fR instance, but allows
for a flags argument that controls some attributes of the returned file
descriptor.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBinotify_add_watch\fR(3C) allows a watch of a particular file or directory
to be added to a watch list associated with the specified \fBinotify\fR
instance. \fBinotify_add_watch\fR(3C) returns a watch descriptor that will
be reflected in the \fIwd\fR member of the \fIinotify_event\fR structure
returned via a \fBread\fR(2) of the instance.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBinotify_rm_watch\fR(3C) removes the watch that corresponds to the specified
watch descriptor.
.RE

When all file descriptors referring to a particular \fBinotify\fR instance
are closed, the instance and all watches associated with that instance are
freed.

To consume events on an \fBinotify\fR instance, an application should 
issue a \fBread\fR(2) to the instance.  If no events are available
(and the \fBinotify\fR instance has not been explicitly made non-blocking
via \fBinotify_init1\fR(3C)) the \fBread\fR(2) will block until a
watched event occurs. If and when events are available, \fBread\fR(2) will
return an array of the following structures:

.sp
.in +2
.nf
struct inotify_event {
        int      wd;       /* watch descriptor */
        uint32_t mask;     /* mask of event */
        uint32_t cookie;   /* cookie for associating renames */
        uint32_t len;      /* size of name field */
        char     name[];   /* optional name */
};
.fi
.in -2

\fIwd\fR contains the watch descriptor that corresponds to the event,
as returned by \fBinotify_add_watch\fR(3C).

\fImask\fR is a bitwise \fBOR\fR of event masks (see below) that
describes the event.

\fIcookie\fR is an opaque value that can be used to associate different
events into a single logical event. In particular, it allows consumers to
associate \fBIN_MOVED_FROM\fR events with subsequent \fBIN_MOVED_TO\fR
events.

\fIlen\fR denotes the length of the \fIname\fR field, including any padding
required for trailing null bytes and alignment. The size of the entire
event is therefore the size of the \fIinotify_event\fR structure plus the
value of \fIlen\fR.

\fIname\fR contains the name of the file associated with the event, if any.
This field is only present when the watched entity is a directory and
the event corresponds to a file that was contained by the watched directory
(though see \fBNOTES\fR and \fBWARNINGS\fR for details and limitations).
When present, \fIname\fR is null terminated, and may contain additional
zero bytes
to pad for alignment. (The length of this field -- including any bytes
for alignment -- is denoted by the \fIlen\fR field.)

.SS "Events"

The events that can be generated on a watched entity are as follows:

.sp
.in +2
.TS
c c
l l .
\fIEvent\fR	\fIDescription\fR
\fBIN_ACCESS\fR	File/directory was accessed
\fBIN_ATTRIB\fR	File/directory attributes were changed
\fBIN_CLOSE_WRITE\fR	File/directory opened for writing was closed 
\fBIN_CLOSE_NOWRITE\fR	File/directory not opened for writing was closed
\fBIN_CREATE\fR	File/directory created in watched directory
\fBIN_DELETE\fR	File/directory deleted from watched directory
\fBIN_DELETE_SELF\fR	Watched file/directory was deleted
\fBIN_MODIFY\fR	File/directory was modified
\fBIN_MODIFY_SELF\fR	Watched file/directory was modified
\fBIN_MOVED_FROM\fR	File was renamed from entity in watched directory
\fBIN_MOVED_TO\fR	File was renamed to entity in watched directory
\fBIN_OPEN\fR	File/directory was opened
.TE
.in -2

Of these, all events except \fBIN_MOVE_SELF\fR and \fBIN_DELETE_SELF\fR
can refer to either the watched entity or (if the watched entity
is a directory) a file or directory contained by the watched directory.
(See \fBNOTES\fR and \fBWARNINGS\fR, below for details on this
mechanism and its limitations.)
If the event corresponds to a contained entity,
\fIname\fR will be set to the name of the affected
entity.

In addition to speciyfing events of interest, watched events may
be modified by potentially setting any of the following when adding a
watch via \fBinotify_add_watch\fR(3C):

.sp
.ne 2
.na
\fBIN_DONT_FOLLOW\fR
.ad
.RS 12n
Don't follow the specified pathname if it is a symbolic link.
.RE

.sp
.ne 2
.na
\fBIN_EXCL_UNLINK\fR
.ad
.RS 12n
If watching a directory and a contained entity becomes unlinked, cease
generating events for that entity. (By default, contained entities will
continue to generate events on their former parent directory.)
.RE

.sp
.ne 2
.na
\fBIN_MASK_ADD\fR
.ad
.RS 12n
If the specified pathname is already being watched, the specified events
will be added to the watched events instead of the default behavior of
replacing them. (If one
may forgive the editorializing, this particular interface gewgaw
seems entirely superfluous, and a canonical example of
feasibility trumping wisdom.)
.RE

.sp
.ne 2
.na
\fBIN_ONESHOT\fR
.ad
.RS 12n
Once an event has been generated for the watched entity, remove the
watch from the watch list as if \fBinotify_rm_watch\fR(3C) had been called
on it (thereby inducing an \fBIN_IGNORED\fR event).
.RE

.sp
.ne 2
.na
\fBIN_ONLYDIR\fR
.ad
.RS 12n
Only watch the specified pathname if it is a directory.
.RE

In addition to the specified events, the following bits may be specified
in the \fImask\fR field as returned from \fBread\fR(2):

.sp
.ne 2
.na
\fBIN_IGNORED\fR
.ad
.RS 12n
A watch was removed explicitly (i.e, via \fBinotify_rm_watch\fR(3C)) or
implicitly (e.g., because \fBIN_ONESHOT\fR was set or because the watched
entity was deleted). 
.RE

.sp
.ne 2
.na
\fBIN_ISDIR\fR
.ad
.RS 12n
The entity inducing the event is a directory.
.RE

.sp
.ne 2
.na
\fBIN_Q_OVERFLOW\fR
.ad
.RS 12n
The event queue exceeded the maximum event queue length per instance.
(By default, this is 16384, but it can be tuned by setting 
\fBinotify_maxevents\fR via \fB/etc/system\fR.)
.RE

.sp
.ne 2
.na
\fBIN_UNMOUNT\fR
.ad
.RS 12n
The filesystem containing the watched entity was unmounted.
.RE

.sp
.SH NOTES
.sp
.LP

\fBinotify\fR instances can be monitored via \fBpoll\fR(2), 
\fBport_get\fR(3C), \fBepoll\fR(7), etc.

The event queue associated with an \fBinotify\fR instance is serialized
and ordered: events will be placed on the tail of the queue in the order
that they occur.

If at the time an event occurs the tail of the event queue is identical
to the newly received event, the newly received event will be dropped,
effectively coalescing the two events.

When watching a directory and receieving events on contained elements
(i.e., a contained file or subdirectory), note that the information
received in the \fIname\fR field may be stale:  the file may have been
renamed between the event and its processing.  If a file has been unlinked
(and if \fBIN_EXCL_UNLINK\fR has not been set),
the \fIname\fR will reflect the last name that resolved to the file.
If a new file is created in the same directory with the old name, events
on the new file and the old (unlinked) file will become undistinguishable.

The number of bytes that are available to be read on an \fBinotify\fR 
instance can be determined via a \fBFIONREAD\fR \fBioctl\fR(2).

.sp
.SH WARNINGS
.sp
.LP

While a best effort has been made to mimic the Linux semantics, there
remains a fundamental difference with respect to hard links:  on Linux,
if a file has multiple hard links to it, a notification on a watched
directory or file will be received if and only if that event was received
via the watched path.  For events that are induced by open files
(such as \fBIN_MODIFY\fR), these semantics seem peculiar:  the watched
file is in fact changing, but because it is not changing via the watched
path, no notification is received.  By contrast, the implementation here
will always yield an event in this case -- even if the event was induced
by an \fBopen\fR(2) via an unwatched path.  If an event occurs within a
watched directory on a file for which there exist multiple hard links within
the same (watched) directory, the event's \fIname\fR will correspond to one
of the links to the file.  If multiple hard links exist to the
same file in the same watched directory and one of the links is removed,
notifications may not necessarily continue to be received for the file,
despite the (remaining) link in the watched directory; users of
\fBinotify\fR should exercise extreme caution when watching directories
that contain files with multiple hard links in the same directory.

.SH SEE ALSO
.sp
.LP
\fBinotify_init\fR(3C), \fBinotify_init1\fR(3C), \fBinotify_add_watch\fR(3C),
\fBinotify_rm_watch\fR(3C), \fBport_get\fR(3C), \fBepoll\fR(7)