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.TH FANOTIFY_INIT 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

fanotify_init \- create and initialize fanotify group

.SH SYNOPSIS

.B #include <fcntl.h>

.br

.B #include <sys/fanotify.h>

.PP

.BI "int fanotify_init(unsigned int " flags ", unsigned int " event_f_flags );

.SH DESCRIPTION

For an overview of the fanotify API, see

.BR fanotify (7).

.PP

.BR fanotify_init ()

initializes a new fanotify group and returns a file descriptor for the event

queue associated with the group.

.PP

The file descriptor is used in calls to

.BR fanotify_mark (2)

to specify the files, directories, 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.

.PP

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

monitor the same files.

.PP

In the current implementation, the number of fanotify groups per user is

limited to 128.

This limit cannot be overridden.

.PP

Calling

.BR fanotify_init ()

requires the

.B 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.

.PP

The

.I 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.

.PP

If multiple listeners for permission events exist,

the notification class is used to establish the sequence

in which the listeners receive the events.

.PP

Only one of the following notification classes may be specified in

.IR flags :

.TP

.B 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 example.

.TP

.B 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.

.TP

.B 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.

.PP

Listeners with different notification classes will receive events in the

order

.BR FAN_CLASS_PRE_CONTENT ,

.BR FAN_CLASS_CONTENT ,

.BR FAN_CLASS_NOTIF .

The order of notification for listeners in the same notification class

is undefined.

.PP

The following bits can additionally be set in

.IR flags :

.TP

.B FAN_CLOEXEC

Set the close-on-exec flag

.RB ( FD_CLOEXEC )

on the new file descriptor.

See the description of the

.B O_CLOEXEC

flag in

.BR open (2).

.TP

.B FAN_NONBLOCK

Enable the nonblocking flag

.RB ( O_NONBLOCK )

for the file descriptor.

Reading from the file descriptor will not block.

Instead, if no data is available,

.BR read (2)

fails with the error

.BR EAGAIN .

.TP

.B FAN_UNLIMITED_QUEUE

Remove the limit of 16384 events for the event queue.

Use of this flag requires the

.B CAP_SYS_ADMIN

capability.

.TP

.B FAN_UNLIMITED_MARKS

Remove the limit of 8192 marks.

Use of this flag requires the

.B CAP_SYS_ADMIN

capability.

.PP

The

.I 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

.I flags

values in

.BR open (2).

.I event_f_flags

includes a multi-bit field for the access mode.

This field can take the following values:

.TP

.B O_RDONLY

This value allows only read access.

.TP

.B O_WRONLY

This value allows only write access.

.TP

.B O_RDWR

This value allows read and write access.

.PP

Additional bits can be set in

.IR event_f_flags .

The most useful values are:

.TP

.B O_LARGEFILE

Enable support for files exceeding 2\ GB.

Failing to set this flag will result in an

.B EOVERFLOW

error when trying to open a large file which is monitored by

an fanotify group on a 32-bit system.

.TP

.BR O_CLOEXEC " (since Linux 3.18)"

.\" commit 0b37e097a648aa71d4db1ad108001e95b69a2da4

Enable the close-on-exec flag for the file descriptor.

See the description of the

.B O_CLOEXEC

flag in

.BR open (2)

for reasons why this may be useful.

.PP

The following are also allowable:

.BR O_APPEND ,

.BR O_DSYNC ,

.BR O_NOATIME ,

.BR O_NONBLOCK ,

and

.BR O_SYNC .

Specifying any other flag in

.I event_f_flags

yields the error

.B EINVAL

(but see BUGS).

.SH RETURN VALUE

On success,

.BR fanotify_init ()

returns a new file descriptor.

On error, \-1 is returned, and

.I errno

is set to indicate the error.

.SH ERRORS

.TP

.B EINVAL

An invalid value was passed in

.I flags

or

.IR event_f_flags .

.B FAN_ALL_INIT_FLAGS

defines all allowable bits for

.IR flags .

.TP

.B EMFILE

The number of fanotify groups for this user exceeds 128.

.TP

.B EMFILE

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

.TP

.B ENOMEM

The allocation of memory for the notification group failed.

.TP

.B ENOSYS

This kernel does not implement

.BR fanotify_init ().

The fanotify API is available only if the kernel was configured with

.BR CONFIG_FANOTIFY .

.TP

.B EPERM

The operation is not permitted because the caller lacks the

.B CAP_SYS_ADMIN

capability.

.SH VERSIONS

.BR fanotify_init ()

was introduced in version 2.6.36 of the Linux kernel and enabled in version

2.6.37.

.SH CONFORMING TO

This system call is Linux-specific.

.SH BUGS

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

.IP * 3

.\" Fixed by commit 0b37e097a648aa71d4db1ad108001e95b69a2da4

The

.B O_CLOEXEC

is ignored when passed in

.IR event_f_flags .

.PP

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

.IP * 3

.\" Fixed by commit 48149e9d3a7e924010a0daab30a6197b7d7b6580

The

.I event_f_flags

argument is not checked for invalid flags.

Flags that are intended only for internal use,

such as

.BR FMODE_EXEC ,

can be set, and will consequently be set for the file descriptors

returned when reading from the fanotify file descriptor.

.SH SEE ALSO

.BR fanotify_mark (2),

.BR fanotify (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.