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[thirdparty/man-pages.git] / man2 / fcntl.2

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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"                 and Copyright (C) 1993 Michael Haardt, Ian Jackson;
.\"                 and Copyright (C) 1998 Jamie Lokier;
.\"                 and Copyright (C) 2002 Michael Kerrisk.
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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@gmail.com>
.\"	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@gmail.com>
.\"     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@gmail.com>
.\"     Added notes on capability requirements
.\" Modified 2004-12-08, added O_NOATIME after note from Martin Pool
.\" 2004-12-10, mtk, noted F_GETOWN bug after suggestion from aeb.
.\" 2005-04-08 Jamie Lokier <jamie@shareable.org>, mtk
.\"	Described behavior of F_SETOWN/F_SETSIG in
.\"	multi-threaded processes, and generally cleaned
.\"	up the discussion of F_SETOWN.
.\" 2005-05-20, Johannes Nicolai <johannes.nicolai@hpi.uni-potsdam.de>,
.\"	mtk: Noted F_SETOWN bug for socket file descriptor in Linux 2.4
.\"	and earlier.  Added text on permissions required to send signal.
.\"
.TH FCNTL 2 2007-12-12 "Linux" "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
.BR fcntl ()
performs one of the operations described below on the open file descriptor
.IR fd .
The operation is determined by
.IR cmd .
.SS "Duplicating a file descriptor"
.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 from
.BR dup2 (2)
which uses exactly the descriptor specified.
.sp
On success, the new descriptor is returned.
.sp
See
.BR dup (2)
for further details.
.TP
.BR F_DUPFD_CLOEXEC " (since Linux 2.6.24)"
As for a
.BR F_DUPFD ,
but additionally set the
close-on-exec flag for the duplicate descriptor.
Specifying this flag permits a program to avoid an additional
.BR fcntl ()
.B F_SETFD
operation to set the
.B FD_CLOEXEC
flag.
For an explanation of why this flag is useful,
see the explanation of
.B O_CLOEXEC
in
.BR open (2).
.SS "File descriptor flags"
The following commands manipulate the flags associated with
a file descriptor.
Currently, only one such flag is defined:
.BR FD_CLOEXEC ,
the close-on-exec flag.
If the
.B FD_CLOEXEC
bit is 0, the file descriptor will remain open across an
.BR execve (2),
otherwise it will be closed.
.TP
.B F_GETFD
Read the file descriptor flags.
.TP
.B F_SETFD
Set the file descriptor flags to the value specified by
.IR arg .
.SS "File status flags"
Each open file description has certain associated status flags,
initialized by
.BR open (2)
.\" or
.\" .BR creat (2),
and possibly modified by
.BR fcntl ().
Duplicated file descriptors
(made with
.BR dup (2),
.BR fcntl (F_DUPFD),
.BR fork (2),
etc.) refer to the same open file description, and thus
share the same file status flags.
.sp
The file status flags and their semantics are described in
.BR open (2).
.TP
.B F_GETFL
Read the file status flags.
.TP
.B F_SETFL
Set the file status flags to the value specified by
.IR arg .
File access mode
.RB ( O_RDONLY ", " O_WRONLY ", " O_RDWR )
and file creation flags
(i.e.,
.BR O_CREAT ", " O_EXCL ", " O_NOCTTY ", " O_TRUNC )
in
.I arg
are ignored.
On Linux this command can only change the
.BR O_APPEND ,
.BR O_ASYNC ,
.BR O_DIRECT ,
.BR O_NOATIME ,
and
.B O_NONBLOCK
flags.
.\" FIXME . According to POSIX.1-2001, O_SYNC should also be modifiable
.\" via fcntl(2), but currently Linux does not permit this
.\" See http://bugzilla.kernel.org/show_bug.cgi?id=5994
.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_WRLCK )
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 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 ().)
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
.BR write (2)
instead.
.SS "Mandatory locking"
(Non-POSIX.)
The above record locks may be either advisory or mandatory,
and are advisory by default.

Advisory locks are not enforced and are useful only between
cooperating processes.

Mandatory locks are enforced for all processes.
If a process tries to perform an incompatible access (e.g.,
.BR read (2)
or
.BR write (2))
on a file region that has an incompatible mandatory lock,
then the result depends upon whether the
.B O_NONBLOCK
flag is enabled for its open file description.
If the
.B O_NONBLOCK
flag is not enabled, then
system call is blocked until the lock is removed
or converted to a mode that is compatible with the access.
If the
.B O_NONBLOCK
flag is enabled, then the system call fails with the error
.B EAGAIN
or
.BR EWOULDBLOCK .

To make use of mandatory locks, mandatory locking must be enabled
both on the file system that contains the file to be locked,
and on the file itself.
Mandatory locking is enabled on a file system
using the "\-o mand" option to
.BR mount (8),
or the
.B MS_MANDLOCK
flag for
.BR mount (2).
Mandatory locking is enabled on a file by disabling
group execute permission on the file and enabling the set-group-ID
permission bit (see
.BR chmod (1)
and
.BR chmod (2)).

The Linux implementation of mandatory locking is unreliable.  See BUGS
below.
.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
.B SIGIO
and
.B SIGURG
signals for events on file descriptor
.IR fd .
Process IDs are returned as positive values;
process group IDs are returned as negative values (but see BUGS below).
.TP
.B F_SETOWN
Set the process ID or process group ID that will receive
.B SIGIO
and
.B SIGURG
signals for events on file descriptor
.IR fd .
A process ID is specified as a positive value;
a process group ID is specified as a negative value.
Most commonly, the calling process specifies itself as the owner
(that is,
.I arg
is specified as
.BR getpid (2)).

.\" From glibc.info:
If you set the
.B O_ASYNC
status flag on a file descriptor by using the
.B F_SETFL
command of
.BR fcntl (),
a
.B SIGIO
signal is sent whenever input or output becomes possible
on that file descriptor.
.B F_SETSIG
can be used to obtain delivery of a signal other than
.BR SIGIO .
If this permission check fails, then the signal is
silently discarded.

Sending a signal to the owner process (group) specified by
.B F_SETOWN
is subject to the same permissions checks as are described for
.BR kill (2),
where the sending process is the one that employs
.B F_SETOWN
(but see BUGS below).
.sp
If the file descriptor
.I fd
refers to a socket,
.B F_SETOWN
also selects
the recipient of
.B SIGURG
signals that are delivered when out-of-band
data arrives on that socket.
.RB ( SIGURG
is sent in any situation where
.BR select (2)
would report the socket as having an "exceptional condition".)
.\" The following appears to be rubbish.  It doesn't seem to
.\" be true according to the kernel source, and I can write
.\" a program that gets a terminal-generated SIGIO even though
.\" it is not the foreground process group of the terminal.
.\" -- MTK, 8 Apr 05
.\"
.\" If the file descriptor
.\" .I fd
.\" refers to a terminal device, then SIGIO
.\" signals are sent to the foreground process group of the terminal.

If a non-zero value is given to
.B F_SETSIG
in a multi-threaded process running with a threading library
that supports thread groups (e.g., NPTL),
then a positive value given to
.B F_SETOWN
has a different meaning:
.\" The relevant place in the (2.6) kernel source is the
.\" 'switch' in fs/fcntl.c::send_sigio_to_task() -- MTK, Apr 2005
instead of being a process ID identifying a whole process,
it is a thread ID identifying a specific thread within a process.
Consequently, it may be necessary to pass
.B F_SETOWN
the result of
.BR gettid (2)
instead of
.BR getpid (2)
to get sensible results when
.B F_SETSIG
is used.
(In current Linux threading implementations,
a main thread's thread ID is the same as its process ID.
This means that a single-threaded program can equally use
.BR gettid (2)
or
.BR getpid (2)
in this scenario.)
Note, however, that the statements in this paragraph do not apply
to the
.B SIGURG
signal generated for out-of-band data on a socket:
this signal is always sent to either a process or a process group,
depending on the value given to
.BR F_SETOWN .
.\" send_sigurg()/send_sigurg_to_task() bypasses
.\" kill_fasync()/send_sigio()/send_sigio_to_task()
.\" to directly call send_group_sig_info()
.\"	-- MTK, Apr 2005 (kernel 2.6.11)
Note also that Linux imposes a limit on the
number of real-time signals that may be queued to a
process (see
.BR getrlimit (2)
and
.BR signal (7))
and if this limit is reached, then the kernel reverts to
delivering
.BR SIGIO ,
and this signal is delivered to the entire
process rather than to a specific thread.
.\" See fs/fcntl.c::send_sigio_to_task() (2.4/2.6) sources -- MTK, Apr 05
.TP
.B F_GETSIG
Get the signal sent when input or output becomes possible.
A value of zero means
.B SIGIO
is sent.
Any other value (including
.BR SIGIO )
is the
signal sent instead, and in this case additional info is available to
the signal handler if installed with
.BR 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
.B SIGIO
signal.
Any other value (including
.BR SIGIO )
is the signal to send instead, and in this case additional info
is available to the signal handler if installed with
.BR SA_SIGINFO .
.sp
Additionally, passing a non-zero value to
.B F_SETSIG
changes the signal recipient from a whole process to a specific thread
within a process.
See the description of
.B F_SETOWN
for more details.
.sp
By using
.B F_SETSIG
with a non-zero value, and setting
.B 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
.BR 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 real time signal (value >=
.BR SIGRTMIN ),
multiple I/O events may be queued using the same signal numbers.
(Queuing is dependent on available memory).
Extra information is available
if
.B 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).
.SS Leases
.B F_SETLEASE
and
.B F_GETLEASE
(Linux 2.4 onwards) are used (respectively) to establish a new lease,
and retrieve the current lease, on the open file description
referred to by the file descriptor
.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 a process (the "lease breaker") tries to
.BR open (2)
or
.BR truncate (2)
the file referred to by that file descriptor.
.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 the calling process to be notified when
the file is opened for writing or is truncated.
.\" The following became true in kernel 2.6.10:
.\" See the man-pages-2.09 Changelog for further info.
A read lease can only be placed on a file descriptor that
is opened read-only.
.TP
.B F_WRLCK
Take out a write lease.
This will cause the caller to be notified when
the file is opened for reading or writing or is truncated.
A write lease may be placed on a file only if there are no
other open file descriptors for the file.
.TP
.B F_UNLCK
Remove our lease from the file.
.RE
.P
Leases are associated with an open file description (see
.BR open (2)).
This means that duplicate file descriptors (created by, for example,
.BR fork (2)
or
.BR dup (2))
refer to the same lease, and this lease may be modified
or released using any of these descriptors.
Furthermore, the lease is released by either an explicit
.B F_UNLCK
operation on any of these duplicate descriptors, or when all
such descriptors have been closed.
.P
Leases may only be taken out on regular files.
An unprivileged process may only take out a lease on a file whose
UID (owner) 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 is associated with the file descriptor
.I fd
by returning either
.BR F_RDLCK ", " F_WRLCK ", or " F_UNLCK ,
indicating, respectively, a read lease , a write lease, or no lease.
(The third argument to
.BR fcntl ()
is omitted.)
.PP
When a process (the "lease breaker") performs an
.BR open (2)
or
.BR truncate (2)
that conflicts with a lease established via
.BR F_SETLEASE ,
the system call is blocked by the kernel and
the kernel notifies the lease holder by sending it a signal
.RB ( 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 the lease holder currently holds a write lease on the file,
and the lease breaker is opening the file for reading,
then it is sufficient for the lease holder 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.

If the lease breaker's blocked
.BR open (2)
or
.BR truncate (2)
is interrupted by a signal handler,
then the system call fails with the error
.BR EINTR ,
but the other steps still occur as described above.
If the lease breaker is killed by a signal while blocked in
.BR open (2)
or
.BR truncate (2),
then the other steps still occur as described above.
If the lease breaker specifies the
.B O_NONBLOCK
flag when calling
.BR open (2),
then the call immediately fails with the error
.BR EWOULDBLOCK ,
but the other steps still occur as described above.

The default signal used to notify the lease holder is
.BR SIGIO ,
but this can be changed using the
.B F_SETSIG
command to
.BR fcntl ().
If a
.B F_SETSIG
command is performed (even one specifying
.BR SIGIO ),
and the signal
handler is established using
.BR SA_SIGINFO ,
then the handler will receive a
.I siginfo_t
structure 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 (dnotify)"
.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
.B _GNU_SOURCE
feature test macro must be defined.)
.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
.BR 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
.BR 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 real time signal should be used for notification,
so that multiple notifications can be queued.

.B NOTE:
New applications should consider using the
.I inotify
interface (available since kernel 2.6.13),
which provides a superior interface for obtaining notifications of
file system events.
See
.BR inotify (7).
.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 flags.
.TP
.B F_GETFL
Value of flags.
.TP
.B F_GETLEASE
Type of lease held on file descriptor.
.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
.B SIGIO
behavior.
.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.
.TP
.B EAGAIN
The 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 "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
Only the operations
.BR F_DUPFD ,
.BR F_GETFD ,
.BR F_SETFD ,
.BR F_GETFL ,
.BR F_SETFL ,
.BR F_GETLK ,
.BR F_SETLK ,
.BR F_SETLKW ,
.BR F_GETOWN ,
and
.B F_SETOWN
are specified in POSIX.1-2001.

.BR F_GETSIG ,
.BR F_SETSIG ,
.BR F_NOTIFY ,
.BR F_GETLEASE ,
and
.B F_SETLEASE
are Linux specific.
(Define the
.B _GNU_SOURCE
macro to obtain these definitions.)
.\" .PP
.\" SVr4 documents additional EIO, ENOLINK and EOVERFLOW error conditions.
.SH NOTES
The errors returned by
.BR dup2 (2)
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 ().

POSIX.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, for example,
.IR l_sysid .
.\" e.g., Solaris 8 documents this field in fcntl(2), and Irix 6.5
.\" documents it in fcntl(5).  mtk, May 2007
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 BUGS
A limitation of the Linux system call conventions on some
architectures (notably x86) means that if a (negative)
process group ID to be returned by
.B F_GETOWN
falls in the range \-1 to \-4095, then the return value is wrongly
interpreted by glibc as an error in the system call;
.\" glibc source: sysdeps/unix/sysv/linux/i386/sysdep.h
that is, the return value of
.BR fcntl ()
will be \-1, and
.I errno
will contain the (positive) process group ID.
.\" mtk, Dec 04: some limited testing on alpha and ia64 seems to
.\" indicate that ANY negative PGID value will cause F_GETOWN
.\" to misinterpret the return as an error. Some other architectures
.\" seem to have the same range check as x86.

In Linux 2.4 and earlier, there is bug that can occur
when an unprivileged process uses
.B F_SETOWN
to specify the owner
of a socket file descriptor
as a process (group) other than the caller.
In this case,
.BR fcntl ()
can return \-1 with
.I errno
set to
.BR EPERM ,
even when the owner process (group) is one that the caller
has permission to send signals to.
Despite this error return, the file descriptor owner is set,
and signals will be sent to the owner.

The implementation of mandatory locking in all known versions of Linux
is subject to races which render it unreliable:
.\" http://marc.info/?l=linux-kernel&m=119013491707153&w=2
a
.BR write (2)
call that overlaps with a lock may modify data after the mandatory lock is
acquired;
a
.BR read (2)
call that overlaps with a lock may detect changes to data that were made
only after a write lock was acquired.
Similar races exist between mandatory locks and
.BR mmap (2).
It is therefore inadvisable to rely on mandatory locking.
.SH "SEE ALSO"
.BR dup2 (2),
.BR flock (2),
.BR open (2),
.BR socket (2),
.BR lockf (3),
.BR capabilities (7),
.BR feature_test_macros (7)
.P
See also
.IR Documentation/locks.txt ,
.IR Documentation/mandatory.txt ,
and
.I Documentation/dnotify.txt
in the kernel source.