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inotify - monitoring file system events
The following system calls are used with this API: inotify_init(2) (or inotify_init1(2)), inotify_add_watch(2), inotify_rm_watch(2), read(2), and close(2).
inotify_init(2) creates an inotify instance and returns a file descriptor referring to the inotify instance. The more recent inotify_init1(2) is like inotify_init(2), but provides some extra functionality.
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(2) 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(2) 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(2) will block until at least one event occurs (unless interrupted by a signal, in which case the call fails with the error EINTR; see signal(7)).
Each successful read(2) 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(2).
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 behavior when the buffer given to read(2) is too small to return information about the next event depends on the kernel version: in kernels before 2.6.21, read(2) returns 0; since kernel 2.6.21, read(2) fails with the error EINVAL.
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(2).
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(2):
The following bits may be set in the mask field returned by read(2):
Since Linux 2.6.25, signal-driven I/O notification is available for inotify file descriptors; see the discussion of F_SETFL (for setting the O_ASYNC flag), F_SETOWN, and F_SETSIG in fcntl(2). The siginfo_t structure (described in sigaction(2)) that is passed to the signal handler has the following fields set: si_fd is set to the inotify file descriptor number; si_signo is set to the signal number; si_code is set to POLL_IN; and POLLIN is set in si_band.
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 if the older event has not yet been read (but see BUGS).
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(2) 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.
Before kernel 2.6.25, a kernel bug meant that 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 oldest unread event.
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