man7/fanotify.7

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.TH FANOTIFY 7 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
fanotify \- monitoring filesystem events
.SH DESCRIPTION
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
.BR inotify (7)
for details of an API that did notify those events pre Linux 5.1.)
.PP
Additional capabilities compared to the
.BR 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.
.PP
The following system calls are used with this API:
.BR fanotify_init (2),
.BR fanotify_mark (2),
.BR read (2),
.BR write (2),
and
.BR close (2).
.SS fanotify_init(), fanotify_mark(), and notification groups
The
.BR fanotify_init (2)
system call creates and initializes an fanotify notification group
and returns a file descriptor referring to it.
.PP
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.
.PP
For each entry in an fanotify notification group, two bit masks exist: the
.I mark
mask and the
.I 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.
.PP
The
.BR 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.
.PP
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.
.PP
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.
.SS 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
.BR read (2)
or similar)
from the fanotify file descriptor
returned by
.BR fanotify_init (2).
.PP
Two types of events are generated:
.I notification
events and
.I 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.
.PP
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.
.SS Reading fanotify events
Calling
.BR read (2)
for the file descriptor returned by
.BR fanotify_init (2)
blocks (if the flag
.B FAN_NONBLOCK
is not specified in the call to
.BR fanotify_init (2))
until either a file event occurs or the call is interrupted by a signal
(see
.BR signal (7)).
.PP
The use of one of the flags
.BR FAN_REPORT_FID ,
.BR FAN_REPORT_DIR_FID
in
.BR 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.
.TP
After a successful
.BR read (2),
the read buffer contains one or more of the following structures:
.PP
.in +4n
.EX
struct fanotify_event_metadata {
    __u32 event_len;
    __u8 vers;
    __u8 reserved;
    __u16 metadata_len;
    __aligned_u64 mask;
    __s32 fd;
    __s32 pid;
};
.EE
.in
.PP
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
.I fanotify_event_metadata
structure within the read buffer:
.PP
.in +4n
.EX
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];
};
.EE
.in
.PP
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
.BR read (2).
.PP
The return value of
.BR read (2)
is the number of bytes placed in the buffer,
or \-1 in case of an error (but see BUGS).
.PP
The fields of the
.I fanotify_event_metadata
structure are as follows:
.TP
.I event_len
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
.I event_len
is always
.BR FAN_EVENT_METADATA_LEN .
For a group that identifies filesystem objects by file handles,
.I event_len
also includes the variable length file identifier records.
.TP
.I vers
This field holds a version number for the structure.
It must be compared to
.B 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.
.TP
.I reserved
This field is not used.
.TP
.I 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.
.TP
.I mask
This is a bit mask describing the event (see below).
.TP
.I fd
This is an open file descriptor for the object being accessed, or
.B 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
.B 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.
.IP
When calling
.BR fanotify_init (2),
the caller may specify (via the
.I 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)
.B 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.
.TP
.I pid
If flag
.B FAN_REPORT_TID
was set in
.BR 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.
.PP
A program listening to fanotify events can compare this PID
to the PID returned by
.BR getpid (2),
to determine whether the event is caused by the listener itself,
or is due to a file access by another process.
.PP
The bit mask in
.I 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.
.PP
The bits that may appear in
.I mask
are as follows:
.TP
.B FAN_ACCESS
A file or a directory (but see BUGS) was accessed (read).
.TP
.B FAN_OPEN
A file or a directory was opened.
.TP
.B FAN_OPEN_EXEC
A file was opened with the intent to be executed.
See NOTES in
.BR fanotify_mark (2)
for additional details.
.TP
.B FAN_ATTRIB
A file or directory metadata was changed.
.TP
.B FAN_CREATE
A child file or directory was created in a watched parent.
.TP
.B FAN_DELETE
A child file or directory was deleted in a watched parent.
.TP
.B FAN_DELETE_SELF
A watched file or directory was deleted.
.TP
.B FAN_MOVED_FROM
A file or directory has been moved from a watched parent directory.
.TP
.B FAN_MOVED_TO
A file or directory has been moved to a watched parent directory.
.TP
.B FAN_MOVE_SELF
A watched file or directory was moved.
.TP
.B FAN_MODIFY
A file was modified.
.TP
.B FAN_CLOSE_WRITE
A file that was opened for writing
.RB ( O_WRONLY
or
.BR O_RDWR )
was closed.
.TP
.B FAN_CLOSE_NOWRITE
A file or directory that was opened read-only
.RB ( O_RDONLY )
was closed.
.TP
.B FAN_Q_OVERFLOW
The event queue exceeded the limit of 16384 entries.
This limit can be overridden by specifying the
.BR FAN_UNLIMITED_QUEUE
flag when calling
.BR fanotify_init (2).
.TP
.B FAN_ACCESS_PERM
An application wants to read a file or directory, for example using
.BR read (2)
or
.BR readdir (2).
The reader must write a response (as described below)
that determines whether the permission to
access the filesystem object shall be granted.
.TP
.B 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.
.TP
.B 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
.BR fanotify_mark (2)
for additional details.
.PP
To check for any close event, the following bit mask may be used:
.TP
.B FAN_CLOSE
A file was closed.
This is a synonym for:
.IP
    FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE
.PP
To check for any move event, the following bit mask may be used:
.TP
.B FAN_MOVE
A file or directory was moved.
This is a synonym for:
.IP
    FAN_MOVED_FROM | FAN_MOVED_TO
.PP
The following bits may appear in
.I mask
only in conjunction with other event type bits:
.TP
.B FAN_ONDIR
The events described in the
.I mask
have occurred on a directory object.
Reporting events on directories requires setting this flag in the mark mask.
See
.BR fanotify_mark (2)
for additional details.
The
.BR FAN_ONDIR
flag is reported in an event mask only if the fanotify group identifies
filesystem objects by file handles.
.PP
The fields of the
.I fanotify_event_info_fid
structure are as follows:
.TP
.I hdr
This is a structure of type
.IR 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
.BR FAN_REPORT_FID ,
a single information record is expected to be attached to the event with
.I info_type
field value of
.BR FAN_EVENT_INFO_TYPE_FID .
When an fanotify file descriptor is created using the combination of
.BR FAN_REPORT_FID
and
.BR FAN_REPORT_DIR_FID ,
there may be two information records attached to the event:
one with
.I info_type
field value of
.BR FAN_EVENT_INFO_TYPE_DFID ,
identifying a parent directory object, and one with
.I info_type
field value of
.BR FAN_EVENT_INFO_TYPE_FID ,
identifying a non-directory object.
The
.I fanotify_event_info_header
contains a
.I len
field.
The value of
.I len
is the size of the additional information record including the
.IR fanotify_event_info_header
itself.
The total size of all additional information records is not expected
to be bigger than
(
.IR event_len
\-
.IR metadata_len
).
.TP
.I fsid
This is a unique identifier of the filesystem containing the object
associated with the event.
It is a structure of type
.I __kernel_fsid_t
and contains the same value as
.I f_fsid
when calling
.BR statfs (2).
.TP
.I file_handle
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
.BR 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
.BR open_by_handle_at (2).
Note that for the directory entry modification events
.BR FAN_CREATE ,
.BR FAN_DELETE ,
and
.BR FAN_MOVE ,
the
.IR file_handle
identifies the modified directory and not the created/deleted/moved child
object.
If the value of
.I info_type
field is
.BR 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
.BR FAN_OPEN ,
.BR FAN_ATTRIB ,
.BR FAN_DELETE_SELF ,
and
.BR FAN_MOVE_SELF ,
if the value of
.I info_type
field is
.BR FAN_EVENT_INFO_TYPE_FID ,
the
.IR file_handle
identifies the object correlated to the event.
If the value of
.I info_type
field is
.BR FAN_EVENT_INFO_TYPE_DFID ,
the
.IR 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
.I info_type
field is
.BR FAN_EVENT_INFO_TYPE_DFID_NAME ,
the
.IR file_handle
identifies the same directory object that would be reported with
.BR 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.
.PP
The following macros are provided to iterate over a buffer containing
fanotify event metadata returned by a
.BR read (2)
from an fanotify file descriptor:
.TP
.B FAN_EVENT_OK(meta, len)
This macro checks the remaining length
.I len
of the buffer
.I meta
against the length of the metadata structure and the
.I event_len
field of the first metadata structure in the buffer.
.TP
.B FAN_EVENT_NEXT(meta, len)
This macro uses the length indicated in the
.I event_len
field of the metadata structure pointed to by
.IR meta
to calculate the address of the next metadata structure that follows
.IR meta .
.I 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
.IR meta ,
and reduces
.I len
by the number of bytes in the metadata structure that
has been skipped over (i.e., it subtracts
.IR meta\->event_len
from
.IR len ).
.PP
In addition, there is:
.TP
.B FAN_EVENT_METADATA_LEN
This macro returns the size (in bytes) of the structure
.IR fanotify_event_metadata .
This is the minimum size (and currently the only size) of any event metadata.
.\"
.SS Monitoring an fanotify file descriptor for events
When an fanotify event occurs, the fanotify file descriptor indicates as
readable when passed to
.BR epoll (7),
.BR poll (2),
or
.BR select (2).
.SS Dealing with permission events
For permission events, the application must
.BR write (2)
a structure of the following form to the
fanotify file descriptor:
.PP
.in +4n
.EX
struct fanotify_response {
    __s32 fd;
    __u32 response;
};
.EE
.in
.PP
The fields of this structure are as follows:
.TP
.I fd
This is the file descriptor from the structure
.IR fanotify_event_metadata .
.TP
.I response
This field indicates whether or not the permission is to be granted.
Its value must be either
.B FAN_ALLOW
to allow the file operation or
.B FAN_DENY
to deny the file operation.
.PP
If access is denied, the requesting application call will receive an
.BR EPERM
error.
Additionally, if the notification group has been created with the
.B FAN_ENABLE_AUDIT
flag, then the
.B FAN_AUDIT
flag can be set in the
.I response
field.
In that case, the audit subsystem will log information about the access
decision to the audit logs.
.\"
.SS 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
.BR close (2),
outstanding permission events will be set to allowed.
.SS /proc/[pid]/fdinfo
The file
.I /proc/[pid]/fdinfo/[fd]
contains information about fanotify marks for file descriptor
.I fd
of process
.IR pid .
See
.BR proc (5)
for details.
.SH ERRORS
In addition to the usual errors for
.BR read (2),
the following errors can occur when reading from the
fanotify file descriptor:
.TP
.B EINVAL
The buffer is too small to hold the event.
.TP
.B EMFILE
The per-process limit on the number of open files has been reached.
See the description of
.B RLIMIT_NOFILE
in
.BR getrlimit (2).
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
See
.I /proc/sys/fs/file\-max
in
.BR proc (5).
.TP
.B ETXTBSY
This error is returned by
.BR read (2)
if
.B O_RDWR
or
.B O_WRONLY
was specified in the
.I event_f_flags
argument when calling
.BR fanotify_init (2)
and an event occurred for a monitored file that is currently being executed.
.PP
In addition to the usual errors for
.BR write (2),
the following errors can occur when writing to the fanotify file descriptor:
.TP
.B EINVAL
Fanotify access permissions are not enabled in the kernel configuration
or the value of
.I response
in the response structure is not valid.
.TP
.B ENOENT
The file descriptor
.I fd
in the response structure is not valid.
This may occur when a response for the permission event has already been
written.
.SH 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.
.SH CONFORMING TO
The fanotify API is Linux-specific.
.SH NOTES
The fanotify API is available only if the kernel was built with the
.B CONFIG_FANOTIFY
configuration option enabled.
In addition, fanotify permission handling is available only if the
.B CONFIG_FANOTIFY_ACCESS_PERMISSIONS
configuration option is enabled.
.SS 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.
.PP
The fanotify API does not report file accesses and modifications that
may occur because of
.BR mmap (2),
.BR msync (2),
and
.BR munmap (2).
.PP
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.
.PP
Fanotify monitoring of directories is not recursive:
to monitor subdirectories under a directory,
additional marks must be created.
The
.B 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.
.PP
The event queue can overflow.
In this case, events are lost.
.SH BUGS
Before Linux 3.19,
.BR fallocate (2)
did not generate fanotify events.
Since Linux 3.19,
.\" commit 820c12d5d6c0890bc93dd63893924a13041fdc35
calls to
.BR fallocate (2)
generate
.B FAN_MODIFY
events.
.PP
As of Linux 3.17,
the following bugs exist:
.IP * 3
On Linux, a filesystem object may be accessible through multiple paths,
for example, a part of a filesystem may be remounted using the
.IR \-\-bind
option of
.BR 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.
.IP *
.\" FIXME . A patch was proposed.
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
.B CAP_SYS_ADMIN
capability is set for programs executed by unprivileged users.
.IP *
If a call to
.BR 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
.I errno
will not be set.
Thus, the reading application has no way to detect the error.
.SH EXAMPLES
The two example programs below demonstrate the usage of the fanotify API.
.SS 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
.B FAN_OPEN_PERM
and
.BR FAN_CLOSE_WRITE .
When a permission event occurs, a
.B FAN_ALLOW
response is given.
.PP
The following shell session shows an example of
running this program.
This session involved editing the file
.IR /home/user/temp/notes .
Before the file was opened, a
.B FAN_OPEN_PERM
event occurred.
After the file was closed, a
.B FAN_CLOSE_WRITE
event occurred.
Execution of the program ends when the user presses the ENTER key.
.PP
.in +4n
.EX
# \fB./fanotify_example /home\fP
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.
.EE
.in
.SS Program source: fanotify_example.c
\&
.EX
#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 \(aqfd\(aq. */

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) {
            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.\en");
                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(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] = \(aq\e0\(aq;
                printf("File %s\en", 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\en", argv[0]);
        exit(EXIT_FAILURE);
    }

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

    /* 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;

    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.\en");

    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 != \(aq\en\(aq)
                    continue;
                break;
            }

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

                /* Fanotify events are available. */

                handle_events(fd);
            }
        }
    }

    printf("Listening for events stopped.\en");
    exit(EXIT_SUCCESS);
}
.EE
.\"
.SS 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
.B FAN_CREATE
has occurred.
The event mask indicates which type of filesystem object\(emeither
a file or a directory\(emwas created.
Once all events have been read from the buffer and processed accordingly,
the program simply terminates.
.PP
The following shell sessions show two different invocations of
this program, with different actions performed on a watched object.
.PP
The first session shows a mark being placed on
.IR /home/user .
This is followed by the creation of a regular file,
.IR /home/user/testfile.txt .
This results in a
.B 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.
.PP
.in +4n
.EX
# \fB./fanotify_fid /home/user\fP
Listening for events.
FAN_CREATE (file created):
        Directory /home/user has been modified.
        Entry \(aqtestfile.txt\(aq is not a subdirectory.
All events processed successfully. Program exiting.

$ \fBtouch /home/user/testfile.txt\fP              # In another terminal
.EE
.in
.PP
The second session shows a mark being placed on
.IR /home/user .
This is followed by the creation of a directory,
.IR /home/user/testdir .
This specific action results in a
.B FAN_CREATE
event being generated and is reported with the
.B FAN_ONDIR
flag set and with the created directory name.
.PP
.in +4n
.EX
# \fB./fanotify_fid /home/user\fP
Listening for events.
FAN_CREATE | FAN_ONDIR (subdirectory created):
        Directory /home/user has been modified.
        Entry \(aqtestdir\(aq is a subdirectory.
All events processed successfully. Program exiting.

$ \fBmkdir \-p /home/user/testdir\fP          # In another terminal
.EE
.in
.SS Program source: fanotify_fid.c
\&
.EX
#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, 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.\en");
        exit(EXIT_FAILURE);
    }

    mount_fd = open(argv[1], O_DIRECTORY | O_RDONLY);
    if (mount_fd == \-1) {
        perror(argv[1]);
        exit(EXIT_FAILURE);
    }


    /* 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) {
        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.\en");

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

    len = read(fd, 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 ||
            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 +
                        file_handle\->handle_bytes;
        } else {
            fprintf(stderr, "Received unexpected event info type.\en");
            exit(EXIT_FAILURE);
        }

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

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

	/* 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\(aqs 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\en");
                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] = \(aq\e0\(aq;
        printf("\etDirectory \(aq%s\(aq has been modified.\en", path);

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

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

        close(event_fd);
    }

    printf("All events processed successfully. Program exiting.\en");
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.ad l
.BR fanotify_init (2),
.BR fanotify_mark (2),
.BR inotify (7)