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NAME

       fanotify - monitoring filesystem events

DESCRIPTION

       The  fanotify  API provides notification and interception of filesystem events.  Use cases
       include virus scanning and hierarchical storage management.  Currently, only a limited set
       of  events  is supported.  In particular, there is no support for create, delete, and move
       events.  (See inotify(7) for details of an API that does notify those events.)

       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 mount points 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  point,  or  directory  to  be
       marked  for  receiving events, while at the same time ignoring events for specific objects
       under a mount point or directory.

       The fanotify_mark(2) system call adds a file, directory, filesystem or mount  point  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 point 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 updated.

       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 the exception being that the file descriptor provided within a
       generic event must be closed.  The closing of file descriptors for each event applies only
       to  applications  that have initialized fanotify without using FAN_REPORT_FID (see below).
       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 the FAN_REPORT_FID flag in fanotify_init(2) influences what data structures are
       returned to the event listener for each event.   After  a  successful  read(2),  the  read
       buffer contains one or more of the following structures:

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

       In  the case where FAN_REPORT_FID is supplied as one of the flags to fanotify_init(2), you
       should also  expect  to  receive  the  structure  detailed  below  following  the  generic
       fanotify_event_metadata structure within the read buffer:

           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:

       event_len
              This  is  the  length  of the data for the current event and the offset to the next
              event in the buffer.  Without FAN_REPORT_FID, the  value  of  event_len  is  always
              FAN_EVENT_METADATA_LEN.   With FAN_REPORT_FID, event_len also includes the variable
              length file identifier.

       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.

       reserved
              This field is not used.

       metadata_len
              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.  If the fanotify  file  descriptor  has  been  initialized
              using  FAN_REPORT_FID,  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
              event.

       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:

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

       FAN_OPEN
              A file or a directory was opened.

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

       FAN_ATTRIB
              A file or directory metadata was changed.

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

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

       FAN_DELETE_SELF
              A watched file or directory was deleted.

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

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

       FAN_MOVE_SELF
              A watched file or directory was moved.

       FAN_MODIFY
              A file was modified.

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

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

       FAN_Q_OVERFLOW
              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).

       FAN_ACCESS_PERM
              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.

       FAN_OPEN_PERM
              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
              granted.

       FAN_OPEN_EXEC_PERM
              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:

       FAN_CLOSE
              A file was closed.  This is a synonym for:

                  FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE

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

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

                  FAN_MOVED_FROM | FAN_MOVED_TO

       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 additional information attached to the
              event.   For  example,  when  an  fanotify  file  descriptor   is   created   using
              FAN_REPORT_FID,    the    info_type    field    of    this   header   is   set   to
              FAN_EVENT_INFO_TYPE_FID.  Event listeners can use this  field  to  check  that  the
              additional information received for an event is of the correct type.  Additionally,
              the  fanotify_event_info_header  also  contains  a  len  field.   In  the   current
              implementation, the value of len is always (event_len - FAN_EVENT_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).

       file_handle
              This  is  a  variable length structure of type 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  directory
              entry  events,  such  as  FAN_CREATE,  FAN_DELETE,  and  FAN_MOVE,  the file_handle
              describes the modified directory and not the  created/deleted/moved  child  object.
              The   events   FAN_ATTRIB,   FAN_DELETE_SELF,  and  FAN_MOVE_SELF  will  carry  the
              file_handle information for the child object if the child object is being watched.

       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:

       FAN_EVENT_METADATA_LEN
              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.

       response
              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
              operation.

       If access is denied, the requesting application call will receive an EPERM error.

   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.

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

ERRORS

       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).

       ETXTBSY
              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.

VERSIONS

       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.

CONFORMING TO

       The fanotify API is Linux-specific.

NOTES

       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.  (But note that the fanotify API provides  no
       way  of  detecting  when  a  subdirectory has been created under a marked directory, which
       makes recursive monitoring difficult.)  Monitoring mounts offers the capability to monitor
       a  whole  directory tree.  Monitoring filesystems offers the capability to monitor changes
       made from any mount of a filesystem instance.

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

BUGS

       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.

EXAMPLE

       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 point 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, (void *) &buf, sizeof(buf));
               if (len == -1 && errno != EAGAIN) {
                   perror("read");
                   exit(EXIT_FAILURE);
               }

               /* Check if end of available data reached */

               if (len <= 0)
                   break;

               /* 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) {
                       fprintf(stderr,
                               "Mismatch of fanotify metadata version.\n");
                       exit(EXIT_FAILURE);
                   }

                   /* 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(struct fanotify_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) {
                           perror("readlink");
                           exit(EXIT_FAILURE);
                       }

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

                       /* Close the file descriptor of the event */

                       close(metadata->fd);
                   }

                   /* Advance to next event */

                   metadata = FAN_EVENT_NEXT(metadata, len);
               }
           }
       }

       int
       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]);
               exit(EXIT_FAILURE);
           }

           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) {
               perror("fanotify_init");
               exit(EXIT_FAILURE);
           }

           /* 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) {
               perror("fanotify_mark");
               exit(EXIT_FAILURE);
           }

           /* Prepare for polling */

           nfds = 2;

           /* Console input */

           fds[0].fd = STDIN_FILENO;
           fds[0].events = POLLIN;

           /* Fanotify input */

           fds[1].fd = fd;
           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 */
                   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) {

                       /* Fanotify events are available */

                       handle_events(fd);
                   }
               }
           }

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

   Example program: fanotify_fid.c
       The second program is an example of fanotify being used with FAN_REPORT_FID enabled.   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  created  and  reported against the file's parent watched directory object.  Program
       execution ends once all events captured within the buffer have  been  processed.   Program
       execution ends once all events captured within the buffer are processed.

           # ./fanotify_fid /home/user
           Listening for events.
           FAN_CREATE (file created): Directory /home/user has been modified.
           All events processed successfully. Program exiting.

           $ touch /home/user/testing              # 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 the  program
       producing a FAN_CREATE and FAN_ONDIR event.

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

           $ mkdir -p /home/user/testing          # 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

       int
       main(int argc, char **argv)
       {
           int fd, ret, event_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;

           if (argc != 2) {
               fprintf(stderr, "Invalid number of command line arguments.\0);
               exit(EXIT_FAILURE);
           }

           /* Create an fanotify file descriptor with FAN_REPORT_FID as a flag
              so that program can receive fid events. */

           fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_FID, 0);
           if (fd == -1) {
               perror("fanotify_init");
               exit(EXIT_FAILURE);
           }

           /* 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) {
               perror("fanotify_mark");
               exit(EXIT_FAILURE);
           }

           printf("Listening for events.\0);

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

           len = read(fd, (void *) &events_buf, sizeof(events_buf));
           if (len == -1 && errno != EAGAIN) {
               perror("read");
               exit(EXIT_FAILURE);
           }

           /* 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) {
                   fprintf(stderr, "Received unexpected event info type.\0);
                   exit(EXIT_FAILURE);
               }

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

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

               /* metadata->fd is set to FAN_NOFD when FAN_REPORT_FID is enabled.
                  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(AT_FDCWD, file_handle, O_RDONLY);
               if (ret == -1) {
                   if (errno == ESTALE) {
                       printf("File handle is no longer valid. "
                               "File has been deleted\0);
                       continue;
                   } else {
                       perror("open_by_handle_at");
                       exit(EXIT_FAILURE);
                }
               }

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

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

               path_len = readlink(procfd_path, path, sizeof(path) - 1);
               if (path_len == -1) {
                   perror("readlink");
                   exit(EXIT_FAILURE);
               }

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

               /* Close associated file descriptor for this event */

               close(event_fd);
           }

           printf("All events processed successfully. Program exiting.\0);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

       This page is part of release 5.02 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.