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

NAME
fanotify_init - create and initialize fanotify group

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

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

DESCRIPTION
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, directo‐
ries, and mounts 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 lis‐
tening 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 flags:

FAN_CLASS_PRE_CONTENT
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 exam‐
ple.

FAN_CLASS_CONTENT
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.

FAN_CLASS_NOTIF
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
FAN_CLASS_PRE_CONTENT, FAN_CLASS_CONTENT, FAN_CLASS_NOTIF. The order of notification for
listeners in the same notification class is undefined.

The following bits can additionally be set in flags:

FAN_CLOEXEC
Set the close-on-exec flag (FD_CLOEXEC) on the new file descriptor. See the
description of the O_CLOEXEC flag in open(2).

FAN_NONBLOCK
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) will
fail with the error EAGAIN.

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

FAN_UNLIMITED_MARKS
Remove the limit of 8192 marks. Use of this flag requires the CAP_SYS_ADMIN capa‐
bility.

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:

O_RDONLY
This value allows only read access.

O_WRONLY
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 are:

O_LARGEFILE
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 fan‐
otify group on a 32-bit system.

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

ERRORS
EINVAL An invalid value was passed in flags or event_f_flags. FAN_ALL_INIT_FLAGS defines
all allowable bits for flags.

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

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 CONFIG_FANOTIFY.

EPERM The operation is not permitted because the caller lacks the CAP_SYS_ADMIN capabil‐
ity.

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

CONFORMING TO
This system call is Linux-specific.

BUGS
As of Linux 3.17, the following bug exists:

* 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.