inotify使用说明

NAME         top

       inotify - monitoring filesystem events

DESCRIPTION         top

       The 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:       *  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 has a flags argument          that provides access to 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.       *  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 read(2)          (see below).       *  inotify_rm_watch(2) removes an item from an inotify watch list.       *  When all file descriptors referring to an inotify instance have          been closed (using close(2)), the underlying object and its          resources are freed for reuse by the kernel; all associated          watches are automatically freed.       With careful programming, an application can use inotify to       efficiently monitor and cache the state of a set of filesystem       objects.  However, robust applications should allow for the fact that       bugs in the monitoring logic or races of the kind described below may       leave the cache inconsistent with the filesystem state.  It is       probably wise to do some consistency checking, and rebuild the cache       when inconsistencies are detected.   Reading events from an inotify file descriptor       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 describing event */               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 used only for rename events, and allows the resulting pair of       IN_MOVED_FROM and IN_MOVED_TO events to be connected by the       application.  For all other event types, cookie is set to 0.       The name field is present only when an event is returned for a file       inside a watched directory; it identifies the filename within to the       watched directory.  This filename is null-terminated, and may include       further null bytes ('\0') 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(struct 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.  Specifying a buffer of size           sizeof(struct inotify_event) + NAME_MAX + 1       will be sufficient to read at least one event.   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(2) and may be returned in the mask field returned       by read(2):           IN_ACCESS (+)                  File was accessed (e.g., read(2), execve(2)).           IN_ATTRIB (*)                  Metadata changed—for example, permissions (e.g.,                  chmod(2)), timestamps (e.g., utimensat(2)), extended                  attributes (setxattr(2)), link count (since Linux 2.6.25;                  e.g., for the target of link(2) and for unlink(2)), and                  user/group ID (e.g., chown(2)).           IN_CLOSE_WRITE (+)                  File opened for writing was closed.           IN_CLOSE_NOWRITE (*)                  File or directory not opened for writing was closed.           IN_CREATE (+)                  File/directory created in watched directory (e.g., open(2)                  O_CREAT, mkdir(2), link(2), symlink(2), bind(2) on a UNIX                  domain socket).           IN_DELETE (+)                  File/directory deleted from watched directory.           IN_DELETE_SELF                  Watched file/directory was itself deleted.  (This event                  also occurs if an object is moved to another filesystem,                  since mv(1) in effect copies the file to the other                  filesystem and then deletes it from the original                  filesystem.)  In addition, an IN_IGNORED event will                  subsequently be generated for the watch descriptor.           IN_MODIFY (+)                  File was modified (e.g., write(2), truncate(2)).           IN_MOVE_SELF                  Watched file/directory was itself moved.           IN_MOVED_FROM (+)                  Generated for the directory containing the old filename                  when a file is renamed.           IN_MOVED_TO (+)                  Generated for the directory containing the new filename                  when a file is renamed.           IN_OPEN (*)                  File or directory was opened.       When monitoring a directory:       *  the events marked above with an asterisk (*) can occur both for          the directory itself and for objects inside the directory; and       *  the events marked with a plus sign (+) occur only for objects          inside the directory (not for the directory itself).       When events are generated for objects inside a watched directory, 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 defined:           IN_MOVE                  Equates to IN_MOVED_FROM | IN_MOVED_TO.           IN_CLOSE                  Equates to IN_CLOSE_WRITE | IN_CLOSE_NOWRITE.       The following further bits can be specified in mask when calling       inotify_add_watch(2):           IN_DONT_FOLLOW (since Linux 2.6.15)                  Don't dereference pathname if it is a symbolic link.           IN_EXCL_UNLINK (since Linux 2.6.36)                  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 /tmp, in which many                  applications create temporary files whose names are                  immediately unlinked).  Specifying IN_EXCL_UNLINK changes                  the default behavior, so that events are not generated for                  children after they have been unlinked from the watched                  directory.           IN_MASK_ADD                  If a watch instance already exists for the filesystem                  object corresponding to pathname, add (OR) the events in                  mask to the watch mask (instead of replacing the mask).           IN_ONESHOT                  Monitor the filesystem object corresponding to pathname                  for one event, then remove from watch list.           IN_ONLYDIR (since Linux 2.6.15)                  Only watch pathname if it is a directory.  Using this flag                  provides an application with a race-free way of ensuring                  that the monitored object is a directory.       The following bits may be set in the mask field returned by read(2):           IN_IGNORED                  Watch was removed explicitly (inotify_rm_watch(2)) or                  automatically (file was deleted, or filesystem was                  unmounted).  See also BUGS.           IN_ISDIR                  Subject of this event is a directory.           IN_Q_OVERFLOW                  Event queue overflowed (wd is -1 for this event).           IN_UNMOUNT                  Filesystem containing watched object was unmounted.  In                  addition, an IN_IGNORED event will subsequently be                  generated for the watch descriptor.   Examples       Suppose an application is watching the directory dir and the file       dir/myfile for all events.  The examples below show some events that       will be generated for these two objects.           fd = open("dir/myfile", O_RDWR);                  Generates IN_OPEN events for both dir and dir/myfile.           read(fd, buf, count);                  Generates IN_ACCESS events for both dir and dir/myfile.           write(fd, buf, count);                  Generates IN_MODIFY events for both dir and dir/myfile.           fchmod(fd, mode);                  Generates IN_ATTRIB events for both dir and dir/myfile.           close(fd);                  Generates IN_CLOSE_WRITE events for both dir and                  dir/myfile.       Suppose an application is watching the directories dir1 and dir2, and       the file dir1/myfile.  The following examples show some events that       may be generated.           link("dir1/myfile", "dir2/new");                  Generates an IN_ATTRIB event for myfile and an IN_CREATE                  event for dir2.           rename("dir1/myfile", "dir2/myfile");                  Generates an IN_MOVED_FROM event for dir1, an IN_MOVED_TO                  event for dir2, and an IN_MOVE_SELF event for myfile.  The                  IN_MOVED_FROM and IN_MOVED_TO events will have the same                  cookie value.       Suppose that dir1/xx and dir2/yy are (the only) links to the same       file, and an application is watching dir1, dir2, dir1/xx, and       dir2/yy.  Executing the following calls in the order given below will       generate the following events:           unlink("dir2/yy");                  Generates an IN_ATTRIB event for xx (because its link                  count changes) and an IN_DELETE event for dir2.           unlink("dir1/xx");                  Generates IN_ATTRIB, IN_DELETE_SELF, and IN_IGNORED events                  for xx, and an IN_DELETE event for dir1.       Suppose an application is watching the directory dir and (the empty)       directory dir/subdir.  The following examples show some events that       may be generated.           mkdir("dir/new", mode);                  Generates an IN_CREATE | IN_ISDIR event for dir.           rmdir("dir/subdir");                  Generates IN_DELETE_SELF and IN_IGNORED events for subdir,                  and an IN_DELETE | IN_ISDIR event for dir.   /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 an upper limit on the number of watches that              can be created per real user ID.

VERSIONS         top

       Inotify was merged into the 2.6.13 Linux kernel.  The required       library interfaces were added to glibc in version 2.4.       (IN_DONT_FOLLOW, IN_MASK_ADD, and IN_ONLYDIR were added in glibc       version 2.5.)

CONFORMING TO         top

       The inotify API is Linux-specific.

NOTES         top

       Inotify file descriptors can be monitored using select(2), poll(2),       and 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 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).  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 set of watch descriptors that is being monitored via an inotify       file descriptor can be viewed via the entry for the inotify file       descriptor in the process's /proc/[pid]/fdinfo directory.  See       proc(5) for further details.  The FIONREAD ioctl(2) returns the       number of bytes available to read from an inotify file descriptor.   Limitations and caveats       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.       Inotify reports only events that a user-space program triggers       through the filesystem API.  As a result, it does not catch remote       events that occur on network filesystems.  (Applications must fall       back to polling the filesystem to catch such events.)  Furthermore,       various pseudo-filesystems such as /proc, /sys, and /dev/pts are not       monitorable with inotify.       The inotify API does not report file accesses and modifications that       may occur because of mmap(2), msync(2), and munmap(2).       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.       The inotify API 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.       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).       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 descriptor, and then re-create       watches and cache entries for the objects to be monitored.)   Dealing with rename() events       As noted above, the IN_MOVED_FROM and IN_MOVED_TO event pair that is       generated by rename(2) can be matched up via their shared cookie       value.  However, the task of matching has some challenges.       These two events are usually consecutive in the event stream       available when reading from the inotify file descriptor.  However,       this is not guaranteed.  If multiple processes are triggering events       for monitored objects, then (on rare occasions) an arbitrary number       of other events may appear between the IN_MOVED_FROM and IN_MOVED_TO       events.  Furthermore, it is not guaranteed that the event pair is       atomically inserted into the queue: there may be a brief interval       where the IN_MOVED_FROM has appeared, but the IN_MOVED_TO has not.       Matching up the IN_MOVED_FROM and IN_MOVED_TO event pair generated by       rename(2) is thus inherently racy.  (Don't forget that if an object       is renamed outside of a monitored directory, there may not even be an       IN_MOVED_TO event.)  Heuristic approaches (e.g., assume the events       are always consecutive) can be used to ensure a match in most cases,       but will inevitably miss some cases, causing the application to       perceive the IN_MOVED_FROM and IN_MOVED_TO events as being unrelated.       If watch descriptors are destroyed and re-created as a result, then       those watch descriptors will be inconsistent with the watch       descriptors in any pending events.  (Re-creating the inotify file       descriptor and rebuilding the cache may be useful to deal with this       scenario.)       Applications should also allow for the possibility that the       IN_MOVED_FROM event was the last event that could fit in the buffer       returned by the current call to read(2), and the accompanying       IN_MOVED_TO event might be fetched only on the next read(2), which       should be done with a (small) timeout to allow for the fact that       insertion of the IN_MOVED_FROM-IN_MOVED_TO event pair is not atomic,       and also the possibility that there may not be any IN_MOVED_TO event.

BUGS         top

       Before Linux 3.19, fallocate(2) did not create any inotify events.       Since Linux 3.19, calls to fallocate(2) generate IN_MODIFY events.       In kernels before 2.6.16, the IN_ONESHOT mask flag does not work.       As originally designed and implemented, the IN_ONESHOT flag did not       cause an 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 IN_IGNORED event is generated in this case.       Before kernel 2.6.25, 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.       When a watch descriptor is removed by calling inotify_rm_watch(2) (or       because a watch file is deleted or the filesystem that contains it is       unmounted), any pending unread events for that watch descriptor       remain available to read.  As watch descriptors are subsequently       allocated with inotify_add_watch(2), the kernel cycles through the       range of possible watch descriptors (0 to INT_MAX) incrementally.       When allocating a free watch descriptor, no check is made to see       whether that watch descriptor number has any pending unread events in       the inotify queue.  Thus, it can happen that a watch descriptor is       reallocated even when pending unread events exist for a previous       incarnation of that watch descriptor number, with the result that the       application might then read those events and interpret them as       belonging to the file associated with the newly recycled watch       descriptor.  In practice, the likelihood of hitting this bug may be       extremely low, since it requires that an application cycle through       INT_MAX watch descriptors, release a watch descriptor while leaving       unread events for that watch descriptor in the queue, and then       recycle that watch descriptor.  For this reason, and because there       have been no reports of the bug occurring in real-world applications,       as of Linux 3.15, no kernel changes have yet been made to eliminate       this possible bug.

EXAMPLE         top

       The following program demonstrates the usage of the inotify API.  It       marks the directories passed as a command-line arguments and waits       for events of type IN_OPEN, IN_CLOSE_NOWRITE and IN_CLOSE_WRITE.       The following output was recorded while editing the file       /home/user/temp/foo and listing directory /tmp.  Before the file and       the directory were opened, IN_OPEN events occurred.  After the file       was closed, an IN_CLOSE_WRITE event occurred.  After the directory       was closed, an IN_CLOSE_NOWRITE event occurred.  Execution of the       program ended when the user pressed the ENTER key.   Example output           $ ./a.out /tmp /home/user/temp           Press enter key to terminate.           Listening for events.           IN_OPEN: /home/user/temp/foo [file]           IN_CLOSE_WRITE: /home/user/temp/foo [file]           IN_OPEN: /tmp/ [directory]           IN_CLOSE_NOWRITE: /tmp/ [directory]           Listening for events stopped.   Program source       #include <errno.h>       #include <poll.h>       #include <stdio.h>       #include <stdlib.h>       #include <sys/inotify.h>       #include <unistd.h>       /* Read all available inotify events from the file descriptor 'fd'.          wd is the table of watch descriptors for the directories in argv.          argc is the length of wd and argv.          argv is the list of watched directories.          Entry 0 of wd and argv is unused. */       static void       handle_events(int fd, int *wd, int argc, char* argv[])       {           /* Some systems cannot read integer variables if they are not              properly aligned. On other systems, incorrect alignment may              decrease performance. Hence, the buffer used for reading from              the inotify file descriptor should have the same alignment as              struct inotify_event. */           char buf[4096]               __attribute__ ((aligned(__alignof__(struct inotify_event))));           const struct inotify_event *event;           int i;           ssize_t len;           char *ptr;           /* Loop while events can be read from inotify file descriptor. */           for (;;) {               /* Read some events. */               len = read(fd, buf, sizeof buf);               if (len == -1 && errno != EAGAIN) {                   perror("read");                   exit(EXIT_FAILURE);               }               /* If the nonblocking read() found no events to read, then                  it returns -1 with errno set to EAGAIN. In that case,                  we exit the loop. */               if (len <= 0)                   break;               /* Loop over all events in the buffer */               for (ptr = buf; ptr < buf + len;                       ptr += sizeof(struct inotify_event) + event->len) {                   event = (const struct inotify_event *) ptr;                   /* Print event type */                   if (event->mask & IN_OPEN)                       printf("IN_OPEN: ");                   if (event->mask & IN_CLOSE_NOWRITE)                       printf("IN_CLOSE_NOWRITE: ");                   if (event->mask & IN_CLOSE_WRITE)                       printf("IN_CLOSE_WRITE: ");                   /* Print the name of the watched directory */                   for (i = 1; i < argc; ++i) {                       if (wd[i] == event->wd) {                           printf("%s/", argv[i]);                           break;                       }                   }                   /* Print the name of the file */                   if (event->len)                       printf("%s", event->name);                   /* Print type of filesystem object */                   if (event->mask & IN_ISDIR)                       printf(" [directory]\n");                   else                       printf(" [file]\n");               }           }       }       int       main(int argc, char* argv[])       {           char buf;           int fd, i, poll_num;           int *wd;           nfds_t nfds;           struct pollfd fds[2];           if (argc < 2) {               printf("Usage: %s PATH [PATH ...]\n", argv[0]);               exit(EXIT_FAILURE);           }           printf("Press ENTER key to terminate.\n");           /* Create the file descriptor for accessing the inotify API */           fd = inotify_init1(IN_NONBLOCK);           if (fd == -1) {               perror("inotify_init1");               exit(EXIT_FAILURE);           }           /* Allocate memory for watch descriptors */           wd = calloc(argc, sizeof(int));           if (wd == NULL) {               perror("calloc");               exit(EXIT_FAILURE);           }           /* Mark directories for events              - file was opened              - file was closed */           for (i = 1; i < argc; i++) {               wd[i] = inotify_add_watch(fd, argv[i],                                         IN_OPEN | IN_CLOSE);               if (wd[i] == -1) {                   fprintf(stderr, "Cannot watch '%s'\n", argv[i]);                   perror("inotify_add_watch");                   exit(EXIT_FAILURE);               }           }           /* Prepare for polling */           nfds = 2;           /* Console input */           fds[0].fd = STDIN_FILENO;           fds[0].events = POLLIN;           /* Inotify input */           fds[1].fd = fd;           fds[1].events = POLLIN;           /* Wait for events and/or terminal input */           printf("Listening for events.\n");           while (1) {               poll_num = poll(fds, nfds, -1);               if (poll_num == -1) {                   if (errno == EINTR)                       continue;                   perror("poll");                   exit(EXIT_FAILURE);               }               if (poll_num > 0) {                   if (fds[0].revents & POLLIN) {                       /* Console input is available. Empty stdin and quit */                       while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\n')                           continue;                       break;                   }                   if (fds[1].revents & POLLIN) {                       /* Inotify events are available */                       handle_events(fd, wd, argc, argv);                   }               }           }           printf("Listening for events stopped.\n");           /* Close inotify file descriptor */           close(fd);           free(wd);           exit(EXIT_SUCCESS);       }

SEE ALSO         top

       inotifywait(1), inotifywatch(1), inotify_add_watch(2),       inotify_init(2), inotify_init1(2), inotify_rm_watch(2), read(2),       stat(2), fanotify(7)       Documentation/filesystems/inotify.txt in the Linux kernel source tree