man-pages/man2/fanotify_init.2

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.TH FANOTIFY_INIT 2 2014-04-24 "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>
.sp
.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 of the same value is undefined.
.PP
The following bit mask values can be set additionally in
.IR flags :
.TP
.B FAN_CLOEXEC
This flag sets 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
This flag enables 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)
will fail with the error
.BR EAGAIN .
.TP
.B FAN_UNLIMITED_QUEUE
This flag removes the limit of 16384 events for the event queue.
It requires the
.B CAP_SYS_ADMIN
capability.
.TP
.B FAN_UNLIMITED_MARKS
This flag removes the limit of 8192 marks.
It requires the
.B CAP_SYS_ADMIN
capability.
.PP
The argument
.I event_f_flags
defines the file flags with which file descriptors for fanotify events
shall be created.
For explanations of possible values, see the argument
.I flags
of the
.BR open (2)
system call.
Useful values are:
.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.
.TP
.B O_CLOEXEC
This flag enables the close-on-exec flag for the file descriptor.
.TP
.B O_LARGEFILE
This flag enables 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.
.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
.IR flags .
.B FAN_ALL_INIT_FLAGS
defines all allowable bits.
.TP
.B EMFILE
The number of fanotify groups of the user exceeds 128.
.TP
.B ENOMEM
The allocation of memory for the notification group failed.
.TP
.B ENOSYS
This kernel does not implement
.BR fanotify_init ().
To make the fanotify API available,
.B CONFIG_FANOTIFY
has to be enabled in the Linux configuration.
.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
As of Linux 3.15,
the following bug exists:
.IP * 3
.\" FIXME: Patch proposed.
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)