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(),
inotify_add_watch(),
inotify_rm_watch(),
read(), and
close().
inotify_init(2)
creates an inotify instance and returns a file descriptor
referring to the inotify instance.
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() 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() 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 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() will block until at least one event occurs.
Each successful
read() 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().
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 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(inotify_event)+len.
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() and may be returned in the
mask field returned by
read():
| Bit | Description |
| IN_ACCESS | File was accessed (read) (*) |
| IN_ATTRIB | Metadata changed (permissions, timestamps, |
| | extended attributes, 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().
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():
| Bit | Description |
| IN_DONT_FOLLOW | 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 | Only watch pathname if it is a directory |
The following bits may be set in the
mask field returned by
read():
| Bit | Description |
| IN_IGNORED | Watch was removed explicitly (inotify_rm_watch()) |
| | 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 |
Inotify file descriptors can be monitored using
select(2),
poll(2),
and
epoll(7).
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.
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() 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.
