Documentation/filesystems/inotify.txt

   1                                    inotify
   2             a powerful yet simple file change notification system
   3
   4
   5
   6 Document started 15 Mar 2005 by Robert Love <rml@novell.com>
   7
   8
   9 (i) User Interface
  10
  11 Inotify is controlled by a set of three system calls and normal file I/O on a
  12 returned file descriptor.
  13
  14 First step in using inotify is to initialise an inotify instance:
  15
  16         int fd = inotify_init ();
  17
  18 Each instance is associated with a unique, ordered queue.
  19
  20 Change events are managed by "watches".  A watch is an (object,mask) pair where
  21 the object is a file or directory and the mask is a bit mask of one or more
  22 inotify events that the application wishes to receive.  See <linux/inotify.h>
  23 for valid events.  A watch is referenced by a watch descriptor, or wd.
  24
  25 Watches are added via a path to the file.
  26
  27 Watches on a directory will return events on any files inside of the directory.
  28
  29 Adding a watch is simple:
  30
  31         int wd = inotify_add_watch (fd, path, mask);
  32
  33 Where "fd" is the return value from inotify_init(), path is the path to the
  34 object to watch, and mask is the watch mask (see <linux/inotify.h>).
  35
  36 You can update an existing watch in the same manner, by passing in a new mask.
  37
  38 An existing watch is removed via
  39
  40         int ret = inotify_rm_watch (fd, wd);
  41
  42 Events are provided in the form of an inotify_event structure that is read(2)
  43 from a given inotify instance.  The filename is of dynamic length and follows
  44 the struct. It is of size len.  The filename is padded with null bytes to
  45 ensure proper alignment.  This padding is reflected in len.
  46
  47 You can slurp multiple events by passing a large buffer, for example
  48
  49         size_t len = read (fd, buf, BUF_LEN);
  50
  51 Where "buf" is a pointer to an array of "inotify_event" structures at least
  52 BUF_LEN bytes in size.  The above example will return as many events as are
  53 available and fit in BUF_LEN.
  54
  55 Each inotify instance fd is also select()- and poll()-able.
  56
  57 You can find the size of the current event queue via the standard FIONREAD
  58 ioctl on the fd returned by inotify_init().
  59
  60 All watches are destroyed and cleaned up on close.
  61
  62
  63 (ii)
  64
  65 Prototypes:
  66
  67         int inotify_init (void);
  68         int inotify_add_watch (int fd, const char *path, __u32 mask);
  69         int inotify_rm_watch (int fd, __u32 mask);
  70
  71
  72 (iii) Internal Kernel Implementation
  73
  74 Each inotify instance is associated with an inotify_device structure.
  75
  76 Each watch is associated with an inotify_watch structure.  Watches are chained
  77 off of each associated device and each associated inode.
  78
  79 See fs/inotify.c for the locking and lifetime rules.
  80
  81
  82 (iv) Rationale
  83
  84 Q: What is the design decision behind not tying the watch to the open fd of
  85    the watched object?
  86
  87 A: Watches are associated with an open inotify device, not an open file.
  88    This solves the primary problem with dnotify: keeping the file open pins
  89    the file and thus, worse, pins the mount.  Dnotify is therefore infeasible
  90    for use on a desktop system with removable media as the media cannot be
  91    unmounted.  Watching a file should not require that it be open.
  92
  93 Q: What is the design decision behind using an-fd-per-instance as opposed to
  94    an fd-per-watch?
  95
  96 A: An fd-per-watch quickly consumes more file descriptors than are allowed,
  97    more fd's than are feasible to manage, and more fd's than are optimally
  98    select()-able.  Yes, root can bump the per-process fd limit and yes, users
  99    can use epoll, but requiring both is a silly and extraneous requirement.
 100    A watch consumes less memory than an open file, separating the number
 101    spaces is thus sensible.  The current design is what user-space developers
 102    want: Users initialize inotify, once, and add n watches, requiring but one
 103    fd and no twiddling with fd limits.  Initializing an inotify instance two
 104    thousand times is silly.  If we can implement user-space's preferences
 105    cleanly--and we can, the idr layer makes stuff like this trivial--then we
 106    should.
 107
 108    There are other good arguments.  With a single fd, there is a single
 109    item to block on, which is mapped to a single queue of events.  The single
 110    fd returns all watch events and also any potential out-of-band data.  If
 111    every fd was a separate watch,
 112
 113    - There would be no way to get event ordering.  Events on file foo and
 114      file bar would pop poll() on both fd's, but there would be no way to tell
 115      which happened first.  A single queue trivially gives you ordering.  Such
 116      ordering is crucial to existing applications such as Beagle.  Imagine
 117      "mv a b ; mv b a" events without ordering.
 118
 119    - We'd have to maintain n fd's and n internal queues with state,
 120      versus just one.  It is a lot messier in the kernel.  A single, linear
 121      queue is the data structure that makes sense.
 122
 123    - User-space developers prefer the current API.  The Beagle guys, for
 124      example, love it.  Trust me, I asked.  It is not a surprise: Who'd want
 125      to manage and block on 1000 fd's via select?
 126
 127    - No way to get out of band data.
 128
 129    - 1024 is still too low.  ;-)
 130
 131    When you talk about designing a file change notification system that
 132    scales to 1000s of directories, juggling 1000s of fd's just does not seem
 133    the right interface.  It is too heavy.
 134
 135    Additionally, it _is_ possible to  more than one instance  and
 136    juggle more than one queue and thus more than one associated fd.  There
 137    need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
 138    process can easily want more than one queue.
 139
 140 Q: Why the system call approach?
 141
 142 A: The poor user-space interface is the second biggest problem with dnotify.
 143    Signals are a terrible, terrible interface for file notification.  Or for
 144    anything, for that matter.  The ideal solution, from all perspectives, is a
 145    file descriptor-based one that allows basic file I/O and poll/select.
 146    Obtaining the fd and managing the watches could have been done either via a
 147    device file or a family of new system calls.  We decided to implement a
 148    family of system calls because that is the preffered approach for new kernel
 149    interfaces.  The only real difference was whether we wanted to use open(2)
 150    and ioctl(2) or a couple of new system calls.  System calls beat ioctls.
 151