fanotify_init(2) — Linux manual page


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

NAME         top

       fanotify_init - create and initialize fanotify group

SYNOPSIS         top

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

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

DESCRIPTION         top

       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 below.

       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).

RETURN VALUE         top

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

ERRORS         top

       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.

VERSIONS         top

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

CONFORMING TO         top

       This system call is Linux-specific.

BUGS         top

       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.

SEE ALSO         top

       fanotify_mark(2), fanotify(7)

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09                 FANOTIFY_INIT(2)

Pages that refer to this page: fanotify_mark(2)syscalls(2)proc(5)procfs(5)capabilities(7)fanotify(7)