FANOTIFY_INIT(2)           Linux Programmer's Manual          FANOTIFY_INIT(2)

       fanotify_init - create and initialize fanotify group

       #include <fcntl.h>
       #include <sys/fanotify.h>

       int fanotify_init(unsigned int flags, unsigned int event_f_flags);

       For an overview of the fanotify API, see fanotify(7).

       fanotify_init() initializes a new fanotify group and returns a file
       descriptor for the event queue associated with the group.

       The file descriptor is used in calls to fanotify_mark(2) to specify the
       files, directories, mounts or filesystems for which fanotify events
       shall be created.  These events are received by reading from the file
       descriptor.  Some events are only informative, indicating that a file
       has been accessed.  Other events can be used to determine whether
       another application is permitted to access a file or directory.
       Permission to access filesystem objects is granted by writing to the
       file descriptor.

       Multiple programs may be using the fanotify interface at the same time
       to monitor the same files.

       In the current implementation, the number of fanotify groups per user
       is limited to 128.  This limit cannot be overridden.

       Calling fanotify_init() requires the CAP_SYS_ADMIN capability.  This
       constraint might be relaxed in future versions of the API.  Therefore,
       certain additional capability checks have been implemented as indicated

       The flags argument contains a multi-bit field defining the notification
       class of the listening application and further single bit fields
       specifying the behavior of the file descriptor.

       If multiple listeners for permission events exist, the notification
       class is used to establish the sequence in which the listeners receive
       the events.

       Only one of the following notification classes may be specified in

              This value allows the receipt of events notifying that a file
              has been accessed and events for permission decisions if a file
              may be accessed.  It is intended for event listeners that need
              to access files before they contain their final data.  This
              notification class might be used by hierarchical storage
              managers, for example.

              This value allows the receipt of events notifying that a file
              has been accessed and events for permission decisions if a file
              may be accessed.  It is intended for event listeners that need
              to access files when they already contain their final content.
              This notification class might be used by malware detection
              programs, for example.

              This is the default value.  It does not need to be specified.
              This value only allows the receipt of events notifying that a
              file has been accessed.  Permission decisions before the file is
              accessed are not possible.

       Listeners with different notification classes will receive events in
       The order of notification for listeners in the same notification class
       is undefined.

       The following bits can additionally be set in flags:

              Set the close-on-exec flag (FD_CLOEXEC) on the new file
              descriptor.  See the description of the O_CLOEXEC flag in

              Enable the nonblocking flag (O_NONBLOCK) for the file
              descriptor.  Reading from the file descriptor will not block.
              Instead, if no data is available, read(2) fails with the error

              Remove the limit of 16384 events for the event queue.  Use of
              this flag requires the CAP_SYS_ADMIN capability.

              Remove the limit of 8192 marks.  Use of this flag requires the
              CAP_SYS_ADMIN capability.

       FAN_REPORT_TID (since Linux 4.20)
              Report thread ID (TID) instead of process ID (PID) in the pid
              field of the struct fanotify_event_metadata supplied to read(2)
              (see fanotify(7)).

       FAN_REPORT_FID (since Linux 5.1)
              This value allows the receipt of events which contain additional
              information about the underlying filesystem object correlated to
              an event.  An additional structure encapsulates the information
              about the object and is included alongside the generic event
              metadata structure.  The file descriptor that is used to
              represent the object correlated to an event is instead
              substituted with a file handle.  It is intended for applications
              that may find the use of a file handle to identify an object
              more suitable than a file descriptor.  Additionally, it may be
              used for applications that are interested in directory entry
              events, such as FAN_CREATE, FAN_ATTRIB, FAN_MOVE, and FAN_DELETE
              for example.  Note that the use of directory modification events
              are not supported when monitoring a mount point.  The use of
              FAN_CLASS_CONTENT or FAN_CLASS_PRE_CONTENT is not permitted with
              this flag and will result in the error EINVAL.  See fanotify(7)
              for additional information.

       The event_f_flags argument defines the file status flags that will be
       set on the open file descriptions that are created for fanotify events.
       For details of these flags, see the description of the flags values in
       open(2).  event_f_flags includes a multi-bit field for the access mode.
       This field can take the following values:

              This value allows only read access.

              This value allows only write access.

       O_RDWR This value allows read and write access.

       Additional bits can be set in event_f_flags.  The most useful values

              Enable support for files exceeding 2 GB.  Failing to set this
              flag will result in an EOVERFLOW error when trying to open a
              large file which is monitored by an fanotify group on a 32-bit

       O_CLOEXEC (since Linux 3.18)
              Enable the close-on-exec flag for the file descriptor.  See the
              description of the O_CLOEXEC flag in open(2) for reasons why
              this may be useful.

       The following are also allowable: O_APPEND, O_DSYNC, O_NOATIME,
       O_NONBLOCK, and O_SYNC.  Specifying any other flag in event_f_flags
       yields the error EINVAL (but see BUGS).

       On success, fanotify_init() returns a new file descriptor.  On error,
       -1 is returned, and errno is set to indicate the error.

       EINVAL An invalid value was passed in flags or event_f_flags.
              FAN_ALL_INIT_FLAGS (deprecated since Linux kernel version 4.20)
              defines all allowable bits for flags.

       EMFILE The number of fanotify groups for this user exceeds 128.

       EMFILE The per-process limit on the number of open file descriptors has
              been reached.

       ENOMEM The allocation of memory for the notification group failed.

       ENOSYS This kernel does not implement fanotify_init().  The fanotify
              API is available only if the kernel was configured with

       EPERM  The operation is not permitted because the caller lacks the
              CAP_SYS_ADMIN capability.

       fanotify_init() was introduced in version 2.6.36 of the Linux kernel
       and enabled in version 2.6.37.

       This system call is Linux-specific.

       The following bug was present in Linux kernels before version 3.18:

       *  The O_CLOEXEC is ignored when passed in event_f_flags.

       The following bug was present in Linux kernels before version 3.14:

       *  The event_f_flags argument is not checked for invalid flags.  Flags
          that are intended only for internal use, such as FMODE_EXEC, can be
          set, and will consequently be set for the file descriptors returned
          when reading from the fanotify file descriptor.

       fanotify_mark(2), fanotify(7)

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       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                             2020-06-09                  FANOTIFY_INIT(2)