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       fanotify - monitoring filesystem events


       The  fanotify  API provides notification and interception of filesystem events.  Use cases
       include virus scanning and hierarchical storage management.  In the original fanotify API,
       only  a  limited  set  of  events  was supported.  In particular, there was no support for
       create, delete, and move events.  The support for those events was  added  in  Linux  5.1.
       (See inotify(7) for details of an API that did notify those events pre Linux 5.1.)

       Additional  capabilities compared to the inotify(7) API include the ability to monitor all
       of the objects in a mounted filesystem, the ability to make access  permission  decisions,
       and the possibility to read or modify files before access by other applications.

       The  following  system  calls  are used with this API: fanotify_init(2), fanotify_mark(2),
       read(2), write(2), and close(2).

   fanotify_init(), fanotify_mark(), and notification groups
       The fanotify_init(2) system call creates and initializes an  fanotify  notification  group
       and returns a file descriptor referring to it.

       An  fanotify  notification  group  is a kernel-internal object that holds a list of files,
       directories, filesystems, and mounts for which events shall be created.

       For each entry in an fanotify notification group, two bit masks exist: the mark  mask  and
       the  ignore  mask.   The  mark  mask  defines  file activities for which an event shall be
       created.  The ignore mask defines activities  for  which  no  event  shall  be  generated.
       Having these two types of masks permits a filesystem, mount, or directory to be marked for
       receiving events, while at the same time ignoring events  for  specific  objects  under  a
       mount or directory.

       The  fanotify_mark(2)  system  call  adds  a  file,  directory,  filesystem, or mount to a
       notification group and specifies which events shall be reported (or ignored),  or  removes
       or modifies such an entry.

       A  possible  usage  of the ignore mask is for a file cache.  Events of interest for a file
       cache are modification of a file and closing of the same.  Hence, the cached directory  or
       mount  is to be marked to receive these events.  After receiving the first event informing
       that a file has been modified, the corresponding cache  entry  will  be  invalidated.   No
       further  modification  events  for  this  file  are  of interest until the file is closed.
       Hence, the modify event can be added to the ignore mask.  Upon receiving the close  event,
       the  modify  event  can  be  removed  from the ignore mask and the file cache entry can be

       The entries in the fanotify notification groups refer to files and directories  via  their
       inode  number  and  to  mounts via their mount ID.  If files or directories are renamed or
       moved within the same mount, the respective entries survive.  If files or directories  are
       deleted  or  moved  to  another  mount  or  if  filesystems  or  mounts are unmounted, the
       corresponding entries are deleted.

   The event queue
       As events occur on the filesystem objects monitored by a notification group, the  fanotify
       system  generates  events  that  are  collected in a queue.  These events can then be read
       (using read(2) or similar) from the fanotify file descriptor returned by fanotify_init(2).

       Two  types  of  events  are  generated:  notification  events   and   permission   events.
       Notification  events  are  merely  informative  and  require  no action to be taken by the
       receiving application with one exception: if a valid file descriptor is provided within  a
       generic  event, the file descriptor must be closed.  Permission events are requests to the
       receiving application to decide whether permission for a file  access  shall  be  granted.
       For  these  events,  the  recipient  must write a response which decides whether access is
       granted or not.

       An event is removed from the event queue of the fanotify group  when  it  has  been  read.
       Permission  events  that have been read are kept in an internal list of the fanotify group
       until either a permission decision  has  been  taken  by  writing  to  the  fanotify  file
       descriptor or the fanotify file descriptor is closed.

   Reading fanotify events
       Calling  read(2)  for the file descriptor returned by fanotify_init(2) blocks (if the flag
       FAN_NONBLOCK is not specified in the call to fanotify_init(2)) until either a  file  event
       occurs or the call is interrupted by a signal (see signal(7)).

       The  use  of  one  of  the  flags  FAN_REPORT_FID,  FAN_REPORT_DIR_FID in fanotify_init(2)
       influences what data structures are returned to the event listener for each event.  Events
       reported  to a group initialized with one of these flags will use file handles to identify
       filesystem objects instead of file descriptors.

       After a successful read(2), the  read  buffer  contains  one  or  more  of  the  following

           struct fanotify_event_metadata {
               __u32 event_len;
               __u8 vers;
               __u8 reserved;
               __u16 metadata_len;
               __aligned_u64 mask;
               __s32 fd;
               __s32 pid;

       In  case  of  an  fanotify  group  that identifies filesystem objects by file handles, you
       should also expect to receive one or more additional information records of the  structure
       detailed  below  following  the  generic fanotify_event_metadata structure within the read

           struct fanotify_event_info_header {
               __u8 info_type;
               __u8 pad;
               __u16 len;

           struct fanotify_event_info_fid {
               struct fanotify_event_info_header hdr;
               __kernel_fsid_t fsid;
               unsigned char file_handle[0];

       For performance reasons, it is recommended to use a large buffer size (for  example,  4096
       bytes), so that multiple events can be retrieved by a single read(2).

       The  return value of read(2) is the number of bytes placed in the buffer, or -1 in case of
       an error (but see BUGS).

       The fields of the fanotify_event_metadata structure are as follows:

              This is the length of the data for the current event and the  offset  to  the  next
              event  in  the  buffer.   Unless  the  group  identifies filesystem objects by file
              handles, the value of event_len is always FAN_EVENT_METADATA_LEN.  For a group that
              identifies filesystem objects by file handles, event_len also includes the variable
              length file identifier records.

       vers   This field holds a version number for  the  structure.   It  must  be  compared  to
              FANOTIFY_METADATA_VERSION  to verify that the structures returned at run time match
              the structures defined at compile time.  In case of  a  mismatch,  the  application
              should abandon trying to use the fanotify file descriptor.

              This field is not used.

              This  is  the  length of the structure.  The field was introduced to facilitate the
              implementation of optional headers per event type.  No such optional headers  exist
              in the current implementation.

       mask   This is a bit mask describing the event (see below).

       fd     This  is  an  open  file descriptor for the object being accessed, or FAN_NOFD if a
              queue overflow occurred.  With an fanotify group that identifies filesystem objects
              by  file  handles,  applications should expect this value to be set to FAN_NOFD for
              each event that is received.  The  file  descriptor  can  be  used  to  access  the
              contents   of  the  monitored  file  or  directory.   The  reading  application  is
              responsible for closing this file descriptor.

              When calling fanotify_init(2),  the  caller  may  specify  (via  the  event_f_flags
              argument) various file status flags that are to be set on the open file description
              that corresponds to this  file  descriptor.   In  addition,  the  (kernel-internal)
              FMODE_NONOTIFY  file  status  flag  is set on the open file description.  This flag
              suppresses fanotify event generation.  Hence, when the  receiver  of  the  fanotify
              event  accesses  the  notified  file  or  directory  using this file descriptor, no
              additional events will be created.

       pid    If flag FAN_REPORT_TID was set in fanotify_init(2), this is the TID of  the  thread
              that  caused  the  event.   Otherwise,  this the PID of the process that caused the

       A program listening to fanotify events can  compare  this  PID  to  the  PID  returned  by
       getpid(2), to determine whether the event is caused by the listener itself, or is due to a
       file access by another process.

       The bit mask in mask indicates which events have occurred for a single filesystem  object.
       Multiple  bits  may be set in this mask, if more than one event occurred for the monitored
       filesystem object.  In particular, consecutive events for the same filesystem  object  and
       originating  from  the  same process may be merged into a single event, with the exception
       that two permission events are never merged into one queue entry.

       The bits that may appear in mask are as follows:

              A file or a directory (but see BUGS) was accessed (read).

              A file or a directory was opened.

              A file was opened with the intent to be executed.  See  NOTES  in  fanotify_mark(2)
              for additional details.

              A file or directory metadata was changed.

              A child file or directory was created in a watched parent.

              A child file or directory was deleted in a watched parent.

              A watched file or directory was deleted.

              A file or directory has been moved from a watched parent directory.

              A file or directory has been moved to a watched parent directory.

              A watched file or directory was moved.

              A file was modified.

              A file that was opened for writing (O_WRONLY or O_RDWR) was closed.

              A file or directory that was opened read-only (O_RDONLY) was closed.

              The  event queue exceeded the limit of 16384 entries.  This limit can be overridden
              by specifying the FAN_UNLIMITED_QUEUE flag when calling fanotify_init(2).

              An application wants to read a file or directory,  for  example  using  read(2)  or
              readdir(2).   The reader must write a response (as described below) that determines
              whether the permission to access the filesystem object shall be granted.

              An application wants to open a file or directory.  The reader must write a response
              that  determines  whether  the  permission  to  open the filesystem object shall be

              An application wants to open a  file  for  execution.   The  reader  must  write  a
              response  that  determines whether the permission to open the filesystem object for
              execution shall be granted.  See NOTES in fanotify_mark(2) for additional details.

       To check for any close event, the following bit mask may be used:

              A file was closed.  This is a synonym for:


       To check for any move event, the following bit mask may be used:

              A file or directory was moved.  This is a synonym for:

                  FAN_MOVED_FROM | FAN_MOVED_TO

       The following bits may appear in mask only in conjunction with other event type bits:

              The events described in the mask have occurred on a  directory  object.   Reporting
              events   on  directories  requires  setting  this  flag  in  the  mark  mask.   See
              fanotify_mark(2) for additional details.  The FAN_ONDIR  flag  is  reported  in  an
              event  mask  only  if  the  fanotify  group  identifies  filesystem objects by file

       The fields of the fanotify_event_info_fid structure are as follows:

       hdr    This is a structure of type fanotify_event_info_header.  It  is  a  generic  header
              that  contains  information  used  to  describe  an  additional  information record
              attached to the event.  For example, when an fanotify file  descriptor  is  created
              using FAN_REPORT_FID, a single information record is expected to be attached to the
              event with info_type field value of FAN_EVENT_INFO_TYPE_FID.  When an fanotify file
              descriptor    is    created   using   the   combination   of   FAN_REPORT_FID   and
              FAN_REPORT_DIR_FID, there may be two information records attached to the event: one
              with  info_type  field  value  of  FAN_EVENT_INFO_TYPE_DFID,  identifying  a parent
              directory object, and one with info_type field  value  of  FAN_EVENT_INFO_TYPE_FID,
              identifying  a non-directory object.  The fanotify_event_info_header contains a len
              field.  The value of len is the size of the additional information record including
              the   fanotify_event_info_header   itself.    The  total  size  of  all  additional
              information records is not expected to be bigger than ( event_len - metadata_len ).

       fsid   This is a unique identifier of the filesystem containing the object associated with
              the  event.   It is a structure of type __kernel_fsid_t and contains the same value
              as f_fsid when calling statfs(2).

              This is a variable length structure of type struct file_handle.  It  is  an  opaque
              handle  that  corresponds  to  a  specified  object  on a filesystem as returned by
              name_to_handle_at(2).  It can be used to uniquely identify a file on  a  filesystem
              and  can  be  passed  as  an  argument  to open_by_handle_at(2).  Note that for the
              directory entry modification  events  FAN_CREATE,  FAN_DELETE,  and  FAN_MOVE,  the
              file_handle  identifies  the  modified  directory and not the created/deleted/moved
              child object.  If the value of info_type  field  is  FAN_EVENT_INFO_TYPE_DFID_NAME,
              the  file  handle  is  followed  by  a  null  terminated string that identifies the
              created/deleted/moved directory entry name.  For other  events  such  as  FAN_OPEN,
              FAN_ATTRIB,  FAN_DELETE_SELF, and FAN_MOVE_SELF, if the value of info_type field is
              FAN_EVENT_INFO_TYPE_FID, the file_handle identifies the object  correlated  to  the
              event.    If   the  value  of  info_type  field  is  FAN_EVENT_INFO_TYPE_DFID,  the
              file_handle identifies the directory object correlated to the event or  the  parent
              directory  of  a  non-directory  object  correlated  to the event.  If the value of
              info_type field is FAN_EVENT_INFO_TYPE_DFID_NAME, the  file_handle  identifies  the
              same  directory object that would be reported with FAN_EVENT_INFO_TYPE_DFID and the
              file handle is followed by a null terminated string that identifies the name  of  a
              directory entry in that directory, or '.' to identify the directory object itself.

       The  following  macros  are  provided  to  iterate over a buffer containing fanotify event
       metadata returned by a read(2) from an fanotify file descriptor:

       FAN_EVENT_OK(meta, len)
              This macro checks the remaining length len of the buffer meta against the length of
              the  metadata  structure and the event_len field of the first metadata structure in
              the buffer.

       FAN_EVENT_NEXT(meta, len)
              This macro uses the length  indicated  in  the  event_len  field  of  the  metadata
              structure  pointed  to  by  meta  to  calculate  the  address  of the next metadata
              structure that follows meta.  len is the number of bytes of metadata that currently
              remain  in  the buffer.  The macro returns a pointer to the next metadata structure
              that follows meta, and reduces len by the number of bytes in the metadata structure
              that has been skipped over (i.e., it subtracts meta->event_len from len).

       In addition, there is:

              This  macro  returns  the size (in bytes) of the structure fanotify_event_metadata.
              This is the minimum size (and currently the only size) of any event metadata.

   Monitoring an fanotify file descriptor for events
       When an fanotify event occurs, the fanotify file descriptor  indicates  as  readable  when
       passed to epoll(7), poll(2), or select(2).

   Dealing with permission events
       For  permission events, the application must write(2) a structure of the following form to
       the fanotify file descriptor:

           struct fanotify_response {
               __s32 fd;
               __u32 response;

       The fields of this structure are as follows:

       fd     This is the file descriptor from the structure fanotify_event_metadata.

              This field indicates whether or not the permission is to  be  granted.   Its  value
              must  be  either FAN_ALLOW to allow the file operation or FAN_DENY to deny the file

       If access is denied,  the  requesting  application  call  will  receive  an  EPERM  error.
       Additionally,  if  the notification group has been created with the FAN_ENABLE_AUDIT flag,
       then the FAN_AUDIT flag can be set in  the  response  field.   In  that  case,  the  audit
       subsystem will log information about the access decision to the audit logs.

   Closing the fanotify file descriptor
       When  all  file  descriptors  referring to the fanotify notification group are closed, the
       fanotify group is released and its resources are freed for  reuse  by  the  kernel.   Upon
       close(2), outstanding permission events will be set to allowed.

       The  file  /proc/[pid]/fdinfo/[fd]  contains  information  about  fanotify  marks for file
       descriptor fd of process pid.  See proc(5) for details.


       In addition to the usual errors for read(2), the following errors can occur  when  reading
       from the fanotify file descriptor:

       EINVAL The buffer is too small to hold the event.

       EMFILE The  per-process  limit  on  the  number  of  open files has been reached.  See the
              description of RLIMIT_NOFILE in getrlimit(2).

       ENFILE The system-wide limit on the total number of open  files  has  been  reached.   See
              /proc/sys/fs/file-max in proc(5).

              This  error  is  returned  by  read(2)  if  O_RDWR or O_WRONLY was specified in the
              event_f_flags argument when calling fanotify_init(2) and an event  occurred  for  a
              monitored file that is currently being executed.

       In  addition to the usual errors for write(2), the following errors can occur when writing
       to the fanotify file descriptor:

       EINVAL Fanotify access permissions are not enabled in  the  kernel  configuration  or  the
              value of response in the response structure is not valid.

       ENOENT The file descriptor fd in the response structure is not valid.  This may occur when
              a response for the permission event has already been written.


       The fanotify API was introduced in version 2.6.36 of  the  Linux  kernel  and  enabled  in
       version 2.6.37.  Fdinfo support was added in version 3.8.


       The fanotify API is Linux-specific.


       The  fanotify  API  is  available  only  if  the kernel was built with the CONFIG_FANOTIFY
       configuration option enabled.  In addition, fanotify permission handling is available only
       if the CONFIG_FANOTIFY_ACCESS_PERMISSIONS configuration option is enabled.

   Limitations and caveats
       Fanotify  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.

       The fanotify API does not report file accesses and modifications that may occur because of
       mmap(2), msync(2), and munmap(2).

       Events  for  directories  are  created  only  if the directory itself is opened, read, and
       closed.  Adding, removing, or changing children of a  marked  directory  does  not  create
       events for the monitored directory itself.

       Fanotify  monitoring  of  directories  is not recursive: to monitor subdirectories under a
       directory, additional marks must be  created.   The  FAN_CREATE  event  can  be  used  for
       detecting  when  a  subdirectory has been created under a marked directory.  An additional
       mark must then be set on the newly created subdirectory.  This approach is  racy,  because
       it  can  lose events that occurred inside the newly created subdirectory, before a mark is
       added on that subdirectory.  Monitoring mounts offers the capability to  monitor  a  whole
       directory  tree  in  a  race-free manner.  Monitoring filesystems offers the capability to
       monitor changes made from any mount of a filesystem instance in a race-free manner.

       The event queue can overflow.  In this case, events are lost.


       Before Linux 3.19, fallocate(2) did not generate fanotify events.  Since Linux 3.19, calls
       to fallocate(2) generate FAN_MODIFY events.

       As of Linux 3.17, the following bugs exist:

       *  On  Linux, a filesystem object may be accessible through multiple paths, for example, a
          part of a filesystem may be remounted using the --bind option of mount(8).  A  listener
          that  marked  a  mount  will  be  notified  only  of  events  that were triggered for a
          filesystem object using the same mount.  Any other event will pass unnoticed.

       *  When an event is generated, no check is  made  to  see  whether  the  user  ID  of  the
          receiving  process  has  authorization  to read or write the file before passing a file
          descriptor for  that  file.   This  poses  a  security  risk,  when  the  CAP_SYS_ADMIN
          capability is set for programs executed by unprivileged users.

       *  If  a  call  to  read(2) processes multiple events from the fanotify queue and an error
          occurs, the return value will be the total length of the events successfully copied  to
          the  user-space buffer before the error occurred.  The return value will not be -1, and
          errno will not be set.  Thus, the reading application has no way to detect the error.


       The two example programs below demonstrate the usage of the fanotify API.

   Example program: fanotify_example.c
       The first program is an example of fanotify being used with its event  object  information
       passed in the form of a file descriptor.  The program marks the mount passed as a command-
       line argument and waits for events of type  FAN_OPEN_PERM  and  FAN_CLOSE_WRITE.   When  a
       permission event occurs, a FAN_ALLOW response is given.

       The  following  shell  session  shows  an  example  of running this program.  This session
       involved  editing  the  file  /home/user/temp/notes.   Before  the  file  was  opened,   a
       FAN_OPEN_PERM  event  occurred.   After  the  file  was  closed,  a  FAN_CLOSE_WRITE event
       occurred.  Execution of the program ends when the user presses the ENTER key.

           # ./fanotify_example /home
           Press enter key to terminate.
           Listening for events.
           FAN_OPEN_PERM: File /home/user/temp/notes
           FAN_CLOSE_WRITE: File /home/user/temp/notes

           Listening for events stopped.

   Program source: fanotify_example.c

       #define _GNU_SOURCE     /* Needed to get O_LARGEFILE definition */
       #include <errno.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <poll.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/fanotify.h>
       #include <unistd.h>

       /* Read all available fanotify events from the file descriptor 'fd'. */

       static void
       handle_events(int fd)
           const struct fanotify_event_metadata *metadata;
           struct fanotify_event_metadata buf[200];
           ssize_t len;
           char path[PATH_MAX];
           ssize_t path_len;
           char procfd_path[PATH_MAX];
           struct fanotify_response response;

           /* Loop while events can be read from fanotify file descriptor. */

           for (;;) {

               /* Read some events. */

               len = read(fd, buf, sizeof(buf));
               if (len == -1 && errno != EAGAIN) {

               /* Check if end of available data reached. */

               if (len <= 0)

               /* Point to the first event in the buffer. */

               metadata = buf;

               /* Loop over all events in the buffer. */

               while (FAN_EVENT_OK(metadata, len)) {

                   /* Check that run-time and compile-time structures match. */

                   if (metadata->vers != FANOTIFY_METADATA_VERSION) {
                               "Mismatch of fanotify metadata version.\n");

                   /* metadata->fd contains either FAN_NOFD, indicating a
                      queue overflow, or a file descriptor (a nonnegative
                      integer). Here, we simply ignore queue overflow. */

                   if (metadata->fd >= 0) {

                       /* Handle open permission event. */

                       if (metadata->mask & FAN_OPEN_PERM) {
                           printf("FAN_OPEN_PERM: ");

                           /* Allow file to be opened. */

                           response.fd = metadata->fd;
                           response.response = FAN_ALLOW;
                           write(fd, &response, sizeof(response));

                       /* Handle closing of writable file event. */

                       if (metadata->mask & FAN_CLOSE_WRITE)
                           printf("FAN_CLOSE_WRITE: ");

                       /* Retrieve and print pathname of the accessed file. */

                       snprintf(procfd_path, sizeof(procfd_path),
                                "/proc/self/fd/%d", metadata->fd);
                       path_len = readlink(procfd_path, path,
                                           sizeof(path) - 1);
                       if (path_len == -1) {

                       path[path_len] = '\0';
                       printf("File %s\n", path);

                       /* Close the file descriptor of the event. */


                   /* Advance to next event. */

                   metadata = FAN_EVENT_NEXT(metadata, len);

       main(int argc, char *argv[])
           char buf;
           int fd, poll_num;
           nfds_t nfds;
           struct pollfd fds[2];

           /* Check mount point is supplied. */

           if (argc != 2) {
               fprintf(stderr, "Usage: %s MOUNT\n", argv[0]);

           printf("Press enter key to terminate.\n");

           /* Create the file descriptor for accessing the fanotify API. */

           fd = fanotify_init(FAN_CLOEXEC | FAN_CLASS_CONTENT | FAN_NONBLOCK,
                              O_RDONLY | O_LARGEFILE);
           if (fd == -1) {

           /* Mark the mount for:
              - permission events before opening files
              - notification events after closing a write-enabled
                file descriptor. */

           if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_MOUNT,
                             FAN_OPEN_PERM | FAN_CLOSE_WRITE, AT_FDCWD,
                             argv[1]) == -1) {

           /* Prepare for polling. */

           nfds = 2;

           fds[0].fd = STDIN_FILENO;       /* Console input */
           fds[0].events = POLLIN;

           fds[1].fd = fd;                 /* Fanotify input */
           fds[1].events = POLLIN;

           /* This is the loop to wait for incoming events. */

           printf("Listening for events.\n");

           while (1) {
               poll_num = poll(fds, nfds, -1);
               if (poll_num == -1) {
                   if (errno == EINTR)     /* Interrupted by a signal */
                       continue;           /* Restart poll() */

                   perror("poll");         /* Unexpected error */

               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')

                   if (fds[1].revents & POLLIN) {

                       /* Fanotify events are available. */


           printf("Listening for events stopped.\n");

   Example program: fanotify_fid.c
       The second program is an example of fanotify being  used  with  a  group  that  identifies
       objects  by  file  handles.   The  program marks the filesystem object that is passed as a
       command-line argument and waits until an event of type FAN_CREATE has occurred.  The event
       mask  indicates  which type of filesystem object—either a file or a directory—was created.
       Once all events have been read from the buffer  and  processed  accordingly,  the  program
       simply terminates.

       The  following  shell  sessions  show  two  different  invocations  of  this program, with
       different actions performed on a watched object.

       The first session shows a mark being placed  on  /home/user.   This  is  followed  by  the
       creation  of  a regular file, /home/user/testfile.txt.  This results in a FAN_CREATE event
       being generated and reported against the file's parent watched directory object  and  with
       the  created file name.  Program execution ends once all events captured within the buffer
       have been processed.

           # ./fanotify_fid /home/user
           Listening for events.
           FAN_CREATE (file created):
                   Directory /home/user has been modified.
                   Entry 'testfile.txt' is not a subdirectory.
           All events processed successfully. Program exiting.

           $ touch /home/user/testfile.txt              # In another terminal

       The second session shows a mark being placed on  /home/user.   This  is  followed  by  the
       creation of a directory, /home/user/testdir.  This specific action results in a FAN_CREATE
       event being generated and is reported with the FAN_ONDIR flag set  and  with  the  created
       directory name.

           # ./fanotify_fid /home/user
           Listening for events.
           FAN_CREATE | FAN_ONDIR (subdirectory created):
                   Directory /home/user has been modified.
                   Entry 'testdir' is a subdirectory.
           All events processed successfully. Program exiting.

           $ mkdir -p /home/user/testdir          # In another terminal

   Program source: fanotify_fid.c

       #define _GNU_SOURCE
       #include <errno.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <sys/fanotify.h>
       #include <unistd.h>

       #define BUF_SIZE 256

       main(int argc, char *argv[])
           int fd, ret, event_fd, mount_fd;
           ssize_t len, path_len;
           char path[PATH_MAX];
           char procfd_path[PATH_MAX];
           char events_buf[BUF_SIZE];
           struct file_handle *file_handle;
           struct fanotify_event_metadata *metadata;
           struct fanotify_event_info_fid *fid;
           const char *file_name;
           struct stat sb;

           if (argc != 2) {
               fprintf(stderr, "Invalid number of command line arguments.\n");

           mount_fd = open(argv[1], O_DIRECTORY | O_RDONLY);
           if (mount_fd == -1) {

           /* Create an fanotify file descriptor with FAN_REPORT_DFID_NAME as
              a flag so that program can receive fid events with directory
              entry name. */

           fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_DFID_NAME, 0);
           if (fd == -1) {

           /* Place a mark on the filesystem object supplied in argv[1]. */

           ret = fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_ONLYDIR,
                               FAN_CREATE | FAN_ONDIR,
                               AT_FDCWD, argv[1]);
           if (ret == -1) {

           printf("Listening for events.\n");

           /* Read events from the event queue into a buffer. */

           len = read(fd, events_buf, sizeof(events_buf));
           if (len == -1 && errno != EAGAIN) {

           /* Process all events within the buffer. */

           for (metadata = (struct fanotify_event_metadata *) events_buf;
                   FAN_EVENT_OK(metadata, len);
                   metadata = FAN_EVENT_NEXT(metadata, len)) {
               fid = (struct fanotify_event_info_fid *) (metadata + 1);
               file_handle = (struct file_handle *) fid->handle;

               /* Ensure that the event info is of the correct type. */

               if (fid->hdr.info_type == FAN_EVENT_INFO_TYPE_FID ||
                   fid->hdr.info_type == FAN_EVENT_INFO_TYPE_DFID) {
                   file_name = NULL;
               } else if (fid->hdr.info_type == FAN_EVENT_INFO_TYPE_DFID_NAME) {
                   file_name = file_handle->f_handle +
               } else {
                   fprintf(stderr, "Received unexpected event info type.\n");

               if (metadata->mask == FAN_CREATE)
                   printf("FAN_CREATE (file created):\n");

               if (metadata->mask == (FAN_CREATE | FAN_ONDIR))
                   printf("FAN_CREATE | FAN_ONDIR (subdirectory created):\n");

            /* metadata->fd is set to FAN_NOFD when the group identifies
               objects by file handles.  To obtain a file descriptor for
               the file object corresponding to an event you can use the
               struct file_handle that's provided within the
               fanotify_event_info_fid in conjunction with the
               open_by_handle_at(2) system call.  A check for ESTALE is
               done to accommodate for the situation where the file handle
               for the object was deleted prior to this system call. */

               event_fd = open_by_handle_at(mount_fd, file_handle, O_RDONLY);
               if (event_fd == -1) {
                   if (errno == ESTALE) {
                       printf("File handle is no longer valid. "
                               "File has been deleted\n");
                   } else {

               snprintf(procfd_path, sizeof(procfd_path), "/proc/self/fd/%d",

               /* Retrieve and print the path of the modified dentry. */

               path_len = readlink(procfd_path, path, sizeof(path) - 1);
               if (path_len == -1) {

               path[path_len] = '\0';
               printf("\tDirectory '%s' has been modified.\n", path);

               if (file_name) {
                   ret = fstatat(event_fd, file_name, &sb, 0);
                   if (ret == -1) {
                       if (errno != ENOENT) {
                       printf("\tEntry '%s' does not exist.\n", file_name);
                   } else if ((sb.st_mode & S_IFMT) == S_IFDIR) {
                       printf("\tEntry '%s' is a subdirectory.\n", file_name);
                   } else {
                       printf("\tEntry '%s' is not a subdirectory.\n",

               /* Close associated file descriptor for this event. */


           printf("All events processed successfully. Program exiting.\n");


       fanotify_init(2), fanotify_mark(2), inotify(7)


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