man-pages/man2/fcntl.2

'\" t
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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"                               1993 Michael Haardt, Ian Jackson;
.\"                               1998 Jamie Lokier;
.\"                               2002 Michael Kerrisk.
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.\" manual page may be incorrect or out-of-date.  The author(s) assume no
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.\" the use of the information contained herein.  The author(s) may not
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.\" which is licensed free of charge, as they might when working
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.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1995-09-26 by Andries Brouwer <aeb@cwi.nl>
.\" and again on 960413 and 980804 and 981223.
.\" Modified 1998-12-11 by Jamie Lokier <jamie@imbolc.ucc.ie>
.\" Applied correction by Christian Ehrhardt - aeb, 990712
.\" Modified 2002-04-23 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Added note on F_SETFL and O_DIRECT
.\"	Complete rewrite + expansion of material on file locking
.\"	Incorporated description of F_NOTIFY, drawing on
.\"		Stephen Rothwell's notes in Documentation/dnotify.txt.
.\"	Added description of F_SETLEASE and F_GETLEASE
.\" Corrected and polished, aeb, 020527.
.\" Modified 2004-03-03 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Modified description of file leases: fixed some errors of detail
.\"     Replaced the term "lease contestant" by "lease breaker"
.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on capability requirements
.\"
.TH FCNTL 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual"
.SH NAME
fcntl \- manipulate file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.B #include <fcntl.h>
.sp
.BI "int fcntl(int " fd ", int " cmd );
.BI "int fcntl(int " fd ", int " cmd ", long " arg );
.BI "int fcntl(int " fd ", int " cmd ", struct flock *" lock );
.fi
.SH DESCRIPTION
.B fcntl
performs one of various miscellaneous operations on
.IR fd .
The operation in question is determined by
.IR cmd .
.SS "Handling close-on-exec"
.TP
.B F_DUPFD
Find the lowest numbered available file descriptor
greater than or equal to
.I arg
and make it be a copy of
.IR fd .
This is different form
.BR dup2 (2)
which uses exactly the descriptor specified.
.sp
The old and new descriptors may be used interchangeably. They share locks,
file position pointers and flags; for example, if the file position is
modified by using
.B lseek
on one of the descriptors, the position is also changed for the other.
.sp
The two descriptors do not share the close-on-exec flag, however.
The close-on-exec flag of the copy is off, meaning that it will
not be closed on exec.
.sp
On success, the new descriptor is returned.
.TP
.B F_GETFD
Read the close-on-exec flag.  If the
.B FD_CLOEXEC
bit is 0, the file will remain open across
.BR exec ,
otherwise it will be closed.
.TP
.B F_SETFD
Set the close-on-exec flag to the value specified by the
.B FD_CLOEXEC
bit of
.IR arg .
.SS "The file status flags"
A file descriptor has certain associated flags, initialized by
.BR open (2)
.\" or
.\" .BR creat (2),
and possibly modified by
.BR fcntl (2).
The flags are shared between copies (made with
.BR dup (2),
.BR fork (2),
etc.) of the same file descriptor.
.sp
The flags and their semantics are described in
.BR open (2).
.TP
.B F_GETFL
Read the file descriptor's flags.
.TP
.B F_SETFL
Set the file status flags part of the descriptor's flags to the value
specified by
.IR arg .
Remaining bits (access mode, file creation flags) in
.I arg
are ignored.
On Linux this command can only change the O_APPEND, O_NONBLOCK, O_ASYNC,
and O_DIRECT flags.
.P
.SS "Advisory locking"
.BR F_GETLK ", " F_SETLK " and " F_SETLKW
are used to acquire, release, and test for the existence of record
locks (also known as file-segment or file-region locks).
The third argument
.I lock
is a pointer to a structure that has at least the following fields
(in unspecified order).
.in +2n
.nf
.sp
struct flock {
    ...
    short l_type;    /* Type of lock: F_RDLCK,
                        F_WRLCK, F_UNLCK */
    short l_whence;  /* How to interpret l_start:
                        SEEK_SET, SEEK_CUR, SEEK_END */
    off_t l_start;   /* Starting offset for lock */
    off_t l_len;     /* Number of bytes to lock */
    pid_t l_pid;     /* PID of process blocking our lock
                        (F_GETLK only) */
    ...
};
.fi
.in -2n
.P
The
.IR l_whence ", " l_start ", and " l_len
fields of this structure specify the range of bytes we wish to lock.
.I l_start
is the starting offset for the lock, and is interpreted
relative to either:
the start of the file (if
.I l_whence
is
.BR SEEK_SET );
the current file offset (if
.I l_whence
is
.BR SEEK_CUR );
or the end of the file (if
.I l_whence
is
.BR SEEK_END ).
In the final two cases,
.I l_start
can be a negative number provided the
offset does not lie before the start of the file.
.I l_len
is a non-negative integer (but see the NOTES below) specifying
the number of bytes to be locked.
Bytes past the end of the file may be locked,
but not bytes before the start of the file.
Specifying 0 for
.I l_len
has the special meaning: lock all bytes starting at the
location specified by
.IR l_whence " and " l_start
through to the end of file, no matter how large the file grows.
.P
The
.I l_type
field can be used to place a read
.RB ( F_RDLCK )
or a write
.RB ( F_WDLCK )
lock on a file.
Any number of processes may hold a read lock (shared lock)
on a file region, but only one process may hold a write lock
(exclusive lock). An exclusive lock excludes all other locks,
both shared and exclusive.
A single process can hold only one type of lock on a file region;
if a new lock is applied to an already-locked region,
then the existing lock is converted to the the new lock type.
(Such conversions may involve splitting, shrinking, or coalescing with
an existing lock if the byte range specified by the new lock does not
precisely coincide with the range of the existing lock.)
.TP
.B F_SETLK
Acquire a lock (when
.I l_type
is
.B F_RDLCK
or
.BR F_WRLCK )
or release a lock (when
.I l_type
is
.BR F_UNLCK )
on the bytes specified by the
.IR l_whence ", " l_start ", and " l_len
fields of
.IR lock .
If a conflicting lock is held by another process,
this call returns \-1 and sets
.I errno
to
.B EACCES
or
.BR EAGAIN .
.TP
.B F_SETLKW
As for
.BR F_SETLK ,
but if a conflicting lock is held on the file, then wait for that
lock to be released.
If a signal is caught while waiting, then the call is interrupted
and (after the signal handler has returned)
returns immediately (with return value \-1 and
.I errno
set to
.BR EINTR ).
.TP
.B F_GETLK
On input to this call,
.I lock
describes a lock we would like to place on the file.
If the lock could be placed,
.BR fcntl ()
does not actually place it, but returns
.B F_UNLCK
in the
.I l_type
field of
.I lock
and leaves the other fields of the structure unchanged.
If one or more incompatible locks would prevent
this lock being placed, then
.BR fcntl ()
returns details about one of these locks in the
.IR l_type ", " l_whence ", " l_start ", and " l_len
fields of
.I lock
and sets
.I l_pid
to be the PID of the process holding that lock.
.P
In order to place a read lock,
.I fd
must be open for reading.
In order to place a write lock,
.I fd
must be open for writing.
To place both types of lock, open a file read-write.
.P
As well as being removed by an explicit
.BR F_UNLCK ,
record locks are automatically released when the process
terminates or if it closes
.I any
file descriptor referring to a file on which locks are held.
.\" (Additional file descriptors referring to the same file
.\" may have been obtained by calls to
.\" .BR open "(2), " dup "(2), " dup2 "(2), or " fcntl (2).)
This is bad: it means that a process can lose the locks on
a file like
.I /etc/passwd
or
.I /etc/mtab
when for some reason a library function decides to open, read
and close it.
.P
Record locks are not inherited by a child created via
.BR fork (2),
but are preserved across an
.BR execve (2).
.P
Because of the buffering performed by the
.BR stdio (3)
library, the use of record locking with routines in that package
should be avoided; use
.BR read "(2) and " write (2)
instead.
.P
.SS "Mandatory locking"
(Non-POSIX.)
The above record locks may be either advisory or mandatory,
and are advisory by default.
To make use of mandatory locks, mandatory locking must be enabled
(using the "-o mand" option to
.BR mount (8))
for the file system containing the
file to be locked and enabled on the file itself (by disabling
group execute permission on the file and enabling the set-GID
permission bit).

Advisory locks are not enforced and are useful only between
cooperating processes. Mandatory locks are enforced for all
processes.
.P
.SS "Managing signals"
.BR F_GETOWN ", " F_SETOWN ", " F_GETSIG " and " F_SETSIG
are used to manage I/O availability signals:
.TP
.B F_GETOWN
Get the process ID or process group currently receiving SIGIO
and SIGURG signals for events on file descriptor
.IR fd .
Process groups are returned as negative values.
.TP
.B F_SETOWN
Set the process ID or process group that will receive SIGIO
and SIGURG signals for events on file descriptor
.IR fd .
Process groups are specified using negative values.
.RB ( F_SETSIG
can be used to specify a different signal instead of SIGIO).

.\" From glibc.info:
If you set the
.B O_ASYNC
status flag on a file descriptor (either by providing this flag with the
.IR open (2)
call, or by using the
.B F_SETFL
command of
.BR fcntl ),
a SIGIO signal is sent whenever input or output becomes possible
on that file descriptor.
.sp
The process or process group to receive the signal can be selected by
using the
.B F_SETOWN
command to the
.B fcntl
function.  If the file descriptor is a socket, this also selects
the recipient of SIGURG signals that are delivered when out-of-band
data arrives on that socket.  (SIGURG is sent in any situation where
.BR select (2)
would report the socket as having an "exceptional condition".)
If the file descriptor corresponds to a terminal device, then SIGIO
signals are sent to the foreground process group of the terminal.
.TP
.B F_GETSIG
Get the signal sent when input or output becomes possible.  A value of
zero means SIGIO is sent.  Any other value (including SIGIO) is the
signal sent instead, and in this case additional info is available to
the signal handler if installed with SA_SIGINFO.
.TP
.B F_SETSIG
Sets the signal sent when input or output becomes possible.  A value of
zero means to send the default SIGIO signal.  Any other value (including
SIGIO) is the signal to send instead, and in this case additional info
is available to the signal handler if installed with SA_SIGINFO.
.sp
By using F_SETSIG with a non-zero value, and setting SA_SIGINFO for the
signal handler (see
.BR sigaction (2)),
extra information about I/O events is passed to
the handler in a
.I siginfo_t
structure.
If the
.I si_code
field indicates the source is SI_SIGIO, the
.I si_fd
field gives the file descriptor associated with the event.  Otherwise,
there is no indication which file descriptors are pending, and you
should use the usual mechanisms
.RB ( select (2),
.BR poll (2),
.BR read (2)
with
.B O_NONBLOCK
set etc.) to determine which file descriptors are available for I/O.
.sp
By selecting a POSIX.1b real time signal (value >= SIGRTMIN), multiple
I/O events may be queued using the same signal numbers.  (Queuing is
dependent on available memory).  Extra information is available
if SA_SIGINFO is set for the signal handler, as above.
.PP
Using these mechanisms, a program can implement fully asynchronous I/O
without using
.BR select (2)
or
.BR poll (2)
most of the time.
.PP
The use of
.BR O_ASYNC ,
.BR F_GETOWN ,
.B F_SETOWN
is specific to BSD and Linux.
.B F_GETSIG
and
.B F_SETSIG
are Linux-specific.  POSIX has asynchronous I/O and the
.I aio_sigevent
structure to achieve similar things; these are also available
in Linux as part of the GNU C Library (Glibc).
.P
.SS Leases
.B F_SETLEASE
and
.B F_GETLEASE
(Linux 2.4 onwards) are used (respectively) to establish and
retrieve the current setting of the calling process's lease on
the file referred to by
.IR fd .
A file lease provides a mechanism whereby the process holding
the lease (the "lease holder") is notified (via delivery of a signal)
when another process (the "lease breaker") tries to
.BR open (2)
or
.BR truncate (2)
that file.
.TP
.B F_SETLEASE
Set or remove a file lease according to which of the following
values is specified in the integer
.IR arg :

.RS
.TP
.B F_RDLCK
Take out a read lease.
This will cause us to be notified when
another process opens the file for writing or truncates it.
.TP
.B F_WRLCK
Take out a write lease.
This will cause us to be notified when
another process opens the file (for reading or writing) or truncates it.
A write lease may be placed on a file only if no other process
currently has the file open.
.TP
.B F_UNLCK
Remove our lease from the file.
.RE
.P
A process may hold only one type of lease on a file.
.P
Leases may only be taken out on regular files.
An unprivileged process may only take out a lease on a file whose
UID matches the file system UID of the process.
A process with the
.B CAP_LEASE
capability may take out leases on arbitrary files.
.TP
.B F_GETLEASE
Indicates what type of lease we hold on the file
referred to by
.I fd
by returning either
.BR F_RDLCK ", " F_WRLCK ", or " F_UNLCK,
indicating, respectively, that the calling process holds a
read, a write, or no lease on the file.
(The third argument to
.BR fcntl ()
is omitted.)
.PP
When a process (the "lease breaker") performs an
.BR open ()
or
.BR truncate ()
that conflicts with a lease established via
.BR F_SETLEASE ,
the system call is blocked by the kernel, unless the
.B O_NONBLOCK
flag was specified to
.BR open (),
in which case the system call will return with the error
.BR EWOULDBLOCK .
The kernel notifies the lease holder by sending it a signal
(SIGIO by default).
The lease holder should respond to receipt of this signal by doing
whatever cleanup is required in preparation for the file to be
accessed by another process (e.g., flushing cached buffers) and
then either remove or downgrade its lease.
A lease is removed by performing an
.B F_SETLEASE
command specifying
.I arg
as
.BR F_UNLCK .
If we currently hold a write lease on the file,
and the lease breaker is opening the file for reading,
then it is sufficient to downgrade the lease to a read lease.
This is done by performing an
.B F_SETLEASE
command specifying
.I arg
as
.BR F_RDLCK .

If the lease holder fails to downgrade or remove the lease within
the number of seconds specified in
.I /proc/sys/fs/lease-break-time
then the kernel forcibly removes or downgrades the lease holder's lease.

Once the lease has been voluntarily or forcibly removed or downgraded,
and assuming the lease breaker has not unblocked its system call,
the kernel permits the lease breaker's system call to proceed.

The default signal used to notify the lease holder is SIGIO,
but this can be changed using the
.B F_SETSIG
command to
.B fcntl ().
If a
.B F_SETSIG
command is performed (even one specifying SIGIO), and the signal
handler is established using SA_SIGINFO, then the handler will
receive a
.I siginfo_t
sructure as its second argument, and the
.I si_fd
field of this argument will hold the descriptor of the leased file
that has been accessed by another process.
(This is useful if the caller holds leases against multiple files).
.SS "File and directory change notification"
.TP
.B F_NOTIFY
(Linux 2.4 onwards)
Provide notification when the directory referred to by
.I fd
or any of the files that it contains is changed.
The events to be notified are specified in
.IR arg ,
which is a bit mask specified by ORing together zero or more of
the following bits:

.TS
l l
----
lB l.
Bit	Description (event in directory)
DN_ACCESS	A file was accessed (read, pread, readv)
DN_MODIFY	A file was modified (write, pwrite,
	writev, truncate, ftruncate)
DN_CREATE	A file was created (open, creat, mknod,
	mkdir, link, symlink, rename)
DN_DELETE	A file was unlinked (unlink, rename to
	another directory, rmdir)
DN_RENAME	A file was renamed within this
	directory (rename)
DN_ATTRIB	The attributes of a file were changed
	(chown, chmod, utime[s])
.TE
.sp
(In order to obtain these definitions, the _GNU_SOURCE macro must be
defined before including <fcntl.h>.)
.sp
Directory notifications are normally "one-shot", and the application
must re-register to receive further notifications.
Alternatively, if
.B DN_MULTISHOT
is included in
.IR arg ,
then notification will remain in effect until explicitly removed.

.\" The following does seem a poor API-design choice...
A series of
.B F_NOTIFY
requests is cumulative, with the events in
.I arg
being added to the set already monitored.
To disable notification of all events, make an
.B F_NOTIFY
call specifying
.I arg
as 0.
.sp
Notification occurs via delivery of a signal.
The default signal is SIGIO, but this can be changed using the
.B F_SETSIG
command to
.BR fcntl ().
In the latter case, the signal handler receives a
.I siginfo_t
structure as its second argument (if the handler was
established using SA_SIGINFO) and the
.I si_fd
field of this structure contains the file descriptor which
generated the notification (useful when establishing notification
on multiple directories).
.sp
Especially when using
.BR DN_MULTISHOT ,
a POSIX.1b real time signal should be used for notication,
so that multiple notifications can be queued.
.SH "RETURN VALUE"
For a successful call, the return value depends on the operation:
.TP 0.9i
.B F_DUPFD
The new descriptor.
.TP
.B F_GETFD
Value of flag.
.TP
.B F_GETFL
Value of flags.
.TP
.B F_GETOWN
Value of descriptor owner.
.TP
.B F_GETSIG
Value of signal sent when read or write becomes possible, or zero
for traditional SIGIO behaviour.
.TP
All other commands
Zero.
.PP
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.BR EACCES " or " EAGAIN
Operation is prohibited by locks held by other processes.
Or, operation is prohibited because the file has been memory-mapped by
another process.
.TP
.B EBADF
.I fd
is not an open file descriptor, or the command was
.B F_SETLK
or
.B F_SETLKW
and the file descriptor open mode doesn't match with the
type of lock requested.
.TP
.B EDEADLK
It was detected that the specified
.B F_SETLKW
command would cause a deadlock.
.TP
.B EFAULT
.I lock
is outside your accessible address space.
.TP
.B EINTR
For
.BR F_SETLKW ,
the command was interrupted by a signal.
For
.BR F_GETLK " and " F_SETLK ,
the command was interrupted by a signal before the lock was checked or
acquired.  Most likely when locking a remote file (e.g. locking over
NFS), but can sometimes happen locally.
.TP
.B EINVAL
For
.BR F_DUPFD ,
.I arg
is negative or is greater than the maximum allowable value.  For
.BR F_SETSIG ,
.I arg
is not an allowable signal number.
.TP
.B EMFILE
For
.BR F_DUPFD ,
the process already has the maximum number of file descriptors open.
.TP
.B ENOLCK
Too many segment locks open, lock table is full, or a remote locking
protocol failed (e.g. locking over NFS).
.TP
.B EPERM
Attempted to clear the
.B O_APPEND
flag on a file that has the append-only attribute set.
.SH NOTES
The errors returned by
.B dup2
are different from those returned by
.BR F_DUPFD .

Since kernel 2.0, there is no interaction between the types of lock
placed by
.BR flock (2)
and
.BR fcntl (2).

POSIX 1003.1-2001 allows
.I l_len
to be negative. (And if it is, the interval described by the lock
covers bytes
.IR l_start + l_len
up to and including
.IR l_start -1.)
This is supported by Linux since Linux 2.4.21 and 2.5.49.

Several systems have more fields in
.I "struct flock"
such as e.g.
.IR l_sysid .
Clearly,
.I l_pid
alone is not going to be very useful if the process holding the lock
may live on a different machine.

.SH "CONFORMING TO"
SVr4, SVID, POSIX, X/OPEN, BSD 4.3.  Only the operations F_DUPFD,
F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK and F_SETLKW are
specified in POSIX.1.  F_GETOWN and F_SETOWN are BSDisms not supported
in SVr4; F_GETSIG and F_SETSIG are specific to Linux.
.BR F_NOTIFY ", " F_GETLEASE ", and " F_SETLEASE
are Linux specific.
(Define the _GNU_SOURCE macro before including <fcntl.h> to
obtain these definitions.)
The flags legal for F_GETFL/F_SETFL are those supported by
.BR open (2)
and vary between these systems; O_APPEND, O_NONBLOCK, O_RDONLY,
and O_RDWR are specified in POSIX.1.  SVr4 supports several other
options and flags not documented here.
.PP
SVr4 documents additional EIO, ENOLINK and EOVERFLOW error conditions.
.SH "SEE ALSO"
.BR dup2 (2),
.BR flock (2),
.BR open (2),
.BR socket (2),
.BR lockf (3),
.BR capabilities (7)
.P
See also locks.txt, mandatory.txt, and dnotify.txt in
/usr/src/linux/Documentation.