inotify

Sommaire

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 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.

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 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

/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 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 a limit on the number of watches that can be associated with each inotify instance.

NOTES

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.

VERSIONS

Inotify was merged into the 2.6.13 Linux kernel. The required library interfaces were added to glibc in version 2.4.

BUGS

In kernels before 2.6.16, the IN_ONESHOT mask flag does not work.

As at glibc 2.4, the definitions for IN_DONT_FOLLOW, IN_MASK_ADD, and IN_ONLYDIR are missing from <sys/inotify.h>.

CONFORMING TO

The inotify API is Linux specific.

SEE ALSO

inotify_add_watch(2), inotify_init(2), inotify_rm_watch(2), read(2), stat(2), Documentation/filesystems/inotify.txt.