'\" t .\" Copyright (C) 2006, 2014 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .TH INOTIFY 7 2014-04-01 "Linux" "Linux Programmer's Manual" .SH NAME inotify \- monitoring filesystem events .SH DESCRIPTION The .I inotify 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: .IP * 3 .BR inotify_init (2) creates an inotify instance and returns a file descriptor referring to the inotify instance. The more recent .BR inotify_init1 (2) is like .BR inotify_init (2), 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 a file or directory, along with some set of events that the kernel should monitor for the file referred to by that pathname. .BR inotify_add_watch (2) either creates a new watch item, or modifies an existing watch. Each watch has a unique "watch descriptor", an integer 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 (using .BR close (2)), the underlying object and its resources are freed for reuse by the kernel; all associated watches are automatically freed. .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. If no events have so far occurred, then, assuming a blocking file descriptor, .BR read (2) will block until at least one event occurs (unless interrupted by a signal, in which case the call fails with the error .BR EINTR ; see .BR signal (7)). Each successful .BR read (2) returns a buffer containing one or more of the following structures: .in +4n .nf struct inotify_event { int wd; /* Watch descriptor */ .\" FIXME . The type of the 'wd' field should probably be "int32_t". .\" I submitted a patch to fix this. See the LKML thread .\" "[patch] Fix type errors in inotify interfaces", 18 Nov 2008 .\" Glibc bug filed: http://sources.redhat.com/bugzilla/show_bug.cgi?id=7040 uint32_t mask; /* Mask of events */ uint32_t cookie; /* Unique cookie associating related events (for rename(2)) */ uint32_t len; /* Size of \fIname\fP field */ char name[]; /* Optional null-terminated name */ }; .fi .in .I wd identifies the watch for which this event occurs. It is one of the watch descriptors returned by a previous call to .BR inotify_add_watch (2). .I mask contains bits that describe the event that occurred (see below). .I cookie is a unique integer that connects related events. Currently this is used only for rename events, and allows the resulting pair of .B IN_MOVED_FROM and .B IN_MOVED_TO events to be connected by the application. For all other event types, .I cookie is set to 0. The .I name 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 (\(aq\\0\(aq) to align subsequent reads to a suitable address boundary. The .I len field counts all of the bytes in .IR name , including the null bytes; the length of each .I inotify_event structure is thus .IR "sizeof(struct inotify_event)+len" . The behavior when the buffer given to .BR read (2) is too small to return information about the next event depends on the kernel version: in kernels before 2.6.21, .BR read (2) returns 0; since kernel 2.6.21, .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) .I mask argument and the .I mask field of the .I inotify_event structure returned when .BR read (2)ing an inotify file descriptor are both bit masks identifying inotify events. The following bits can be specified in .I mask when calling .BR inotify_add_watch (2) and may be returned in the .I mask field returned by .BR read (2): .RS 4 .TP .BR IN_ACCESS " (*)" File was accessed (e.g., .BR read (2), .BR execve (2)). .TP .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 .BR IN_CLOSE_WRITE " (*)" File opened for writing was closed. .TP .BR IN_CLOSE_NOWRITE " (*)" File not opened for writing was closed. .TP .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 .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 .BR IN_MOVED_FROM " (*)" Generated for the directory containing the old filename when a file is renamed. .TP .BR IN_MOVED_TO " (*)" Generated for the directory containing the new filename when a file is renamed. .TP .BR IN_OPEN " (*)" File was opened. .RE .PP When monitoring a directory, the events marked with an asterisk (*) above can occur for files in the directory, in which case the .I name field in the returned .I inotify_event structure identifies the name of the file within the directory. .PP The .B IN_ALL_EVENTS macro is defined as a bit mask of all of the above events. This macro can be used as the .I mask argument when calling .BR inotify_add_watch (2). 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 .TP .BR IN_DONT_FOLLOW " (since Linux 2.6.15)" Don't dereference .I pathname if it is a symbolic link. .TP .BR IN_EXCL_UNLINK " (since Linux 2.6.36)" .\" commit 8c1934c8d70b22ca8333b216aec6c7d09fdbd6a6 By default, when watching events on the children of a directory, events are generated for children even after they have been unlinked from the directory. This can result in large numbers of uninteresting events for some applications (e.g., if watching .IR /tmp , in which many applications create temporary files whose names are immediately unlinked). Specifying .B IN_EXCL_UNLINK changes the default behavior, so that events are not generated for children after they have been unlinked from the watched directory. .TP .B IN_MASK_ADD Add (OR) events to watch mask for this pathname if it already exists (instead of replacing mask). .TP .B IN_ONESHOT Monitor .I pathname for one event, then remove from watch list. .TP .BR IN_ONLYDIR " (since Linux 2.6.15)" Only watch .I pathname if it is a directory. .RE .PP The following bits may be set in the .I mask field returned by .BR read (2): .RS 4 .TP .B IN_IGNORED 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 .RI ( wd is \-1 for this event). .TP .B IN_UNMOUNT 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 dir , and the file .IR dir1/myfile . The following examples show some events that may be generated. .RS 4 .TP link("dir/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 .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 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/sub"); 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 kernel memory consumed by inotify: .TP .I /proc/sys/fs/inotify/max_queued_events The value in this file is used when an application calls .BR 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 .B IN_Q_OVERFLOW event is always generated. .TP .I /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. .TP .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 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 added in glibc version 2.5.) .SH CONFORMING TO The inotify API is Linux-specific. .SH NOTES Inotify file descriptors can be monitored using .BR select (2), .BR poll (2), and .BR epoll (7). When an event is available, the file descriptor indicates as readable. Since Linux 2.6.25, signal-driven I/O notification is available for inotify file descriptors; see the discussion of .B F_SETFL (for setting the .B O_ASYNC flag), .BR F_SETOWN , and .B F_SETSIG in .BR fcntl (2). The .I siginfo_t structure (described in .BR sigaction (2)) that is passed to the signal handler has the following fields set: .IR si_fd is set to the inotify file descriptor number; .IR si_signo is set to the signal number; .IR si_code is set to .BR POLL_IN ; and .B POLLIN is set in .IR si_band . If successive output inotify events produced on the inotify file descriptor are identical (same .IR wd , .IR mask , .IR cookie , and .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. 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 .B FIONREAD .BR ioctl (2) returns the number of bytes available to read from an inotify file descriptor. .SS Limitations and caveats The inotify applications 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. 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. 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 descriuptor, and then re-create watches and cache entries for the objects to be monitored.) 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. 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). .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 .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 in the Linux kernel source tree