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

       fcntl - manipulate file descriptor

       #include	<unistd.h>
       #include	<fcntl.h>

       int fcntl(int fd, int cmd, ... /* arg */	);

       fcntl() performs	one of the operations described	below on the open file
       descriptor fd.  The operation is	determined by cmd.

       fcntl() can take	an optional third argument.  Whether or	not this argu-
       ment  is	 required is determined	by cmd.	 The required argument type is
       indicated in parentheses	after  each  cmd  name	(in  most  cases,  the
       required	type is	int, and we identify the argument using	the name arg),
       or void is specified if the argument is not required.

   Duplicating a file descriptor
       F_DUPFD (int)
	      Find the lowest numbered available file descriptor greater  than
	      or  equal	to arg and make	it be a	copy of	fd.  This is different
	      from dup2(2), which uses exactly the descriptor specified.

	      On success, the new descriptor is	returned.

	      See dup(2) for further details.

       F_DUPFD_CLOEXEC (int; since Linux 2.6.24)
	      As for F_DUPFD, but additionally set the close-on-exec flag  for
	      the  duplicate  descriptor.  Specifying this flag	permits	a pro-
	      gram to avoid an additional fcntl() F_SETFD operation to set the
	      FD_CLOEXEC flag.	For an explanation of why this flag is useful,
	      see the description of O_CLOEXEC in open(2).

   File	descriptor flags
       The following commands manipulate the  flags  associated	 with  a  file
       descriptor.   Currently,	only one such flag is defined: FD_CLOEXEC, the
       close-on-exec flag.  If the FD_CLOEXEC bit is 0,	 the  file  descriptor
       will remain open	across an execve(2), otherwise it will be closed.

       F_GETFD (void)
	      Read the file descriptor flags; arg is ignored.

       F_SETFD (int)
	      Set the file descriptor flags to the value specified by arg.

   File	status flags
       Each  open  file	 description has certain associated status flags, ini-
       tialized	by open(2) and possibly	modified by fcntl().  Duplicated  file
       descriptors  (made with dup(2), fcntl(F_DUPFD), fork(2),	etc.) refer to
       the same	open file description, and thus	share  the  same  file	status

       The file	status flags and their semantics are described in open(2).

       F_GETFL (void)
	      Get  the	file  access  mode  and	 the file status flags;	arg is

       F_SETFL (int)
	      Set the file status flags	to the value specified by  arg.	  File
	      access mode (O_RDONLY, O_WRONLY, O_RDWR) and file	creation flags
	      (i.e., O_CREAT, O_EXCL, O_NOCTTY,	O_TRUNC) in arg	 are  ignored.
	      On  Linux	 this  command	can change only	the O_APPEND, O_ASYNC,
	      O_DIRECT,	O_NOATIME, and O_NONBLOCK flags.

   Advisory locking
       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, lock, is a pointer to  a  structure
       that has	at least the following fields (in unspecified order).

	   struct flock	{
	       short l_type;	/* Type	of lock: F_RDLCK,
				   F_WRLCK, F_UNLCK */
	       short l_whence;	/* How to interpret l_start:
	       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) */

       The  l_whence,  l_start,	and l_len fields of this structure specify the
       range of	bytes we wish to lock.	Bytes past the end of the file may  be
       locked, but not bytes before the	start of the file.

       l_start	is  the	starting offset	for the	lock, and is interpreted rela-
       tive to either: the start of the	file (if l_whence  is  SEEK_SET);  the
       current	file  offset (if l_whence is SEEK_CUR);	or the end of the file
       (if l_whence is SEEK_END).  In the final	two cases, l_start  can	 be  a
       negative	 number	 provided  the offset does not lie before the start of
       the file.

       l_len specifies the number of bytes to be locked.  If  l_len  is	 posi-
       tive,  then  the	 range	to  be	locked	covers bytes l_start up	to and
       including l_start+l_len-1.  Specifying 0	 for  l_len  has  the  special
       meaning:	 lock all bytes	starting at the	location specified by l_whence
       and l_start through to the end of file, no matter how  large  the  file

       POSIX.1-2001 allows (but	does not require) an implementation to support
       a negative l_len	value; if l_len	is negative, the interval described by
       lock covers bytes l_start+l_len up to and including l_start-1.  This is
       supported by Linux since	kernel versions	2.4.21 and 2.5.49.

       The l_type field	can be used to place  a	 read  (F_RDLCK)  or  a	 write
       (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.)

       F_SETLK (struct flock *)
	      Acquire  a lock (when l_type is F_RDLCK or F_WRLCK) or release a
	      lock (when l_type	is F_UNLCK) on	the  bytes  specified  by  the
	      l_whence,	 l_start,  and l_len fields of lock.  If a conflicting
	      lock is held by another process, this call returns -1  and  sets
	      errno to EACCES or EAGAIN.

       F_SETLKW	(struct	flock *)
	      As  for  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 sig-
	      nal handler has returned)	returns	immediately (with return value
	      -1 and errno set to EINTR; see signal(7)).

       F_GETLK (struct flock *)
	      On  input	 to  this call,	lock describes a lock we would like to
	      place on the file.  If the lock could be	placed,	 fcntl()  does
	      not  actually  place it, but returns F_UNLCK in the l_type field
	      of lock and leaves the other fields of the structure  unchanged.
	      If  one or more incompatible locks would prevent this lock being
	      placed, then fcntl() returns details about one of	these locks in
	      the l_type, l_whence, l_start, and l_len fields of lock and sets
	      l_pid to be the PID of the process holding that lock.

       In order	to place a read	lock, fd must be open for reading.   In	 order
       to  place  a  write  lock,  fd must be open for writing.	 To place both
       types of	lock, open a file read-write.

       As well as being	removed	by an explicit F_UNLCK,	record locks are auto-
       matically released when the process terminates or if it closes any file
       descriptor referring to a file on which locks are held.	This  is  bad:
       it  means  that a process can lose the locks on a file like /etc/passwd
       or /etc/mtab when for some reason a library function decides  to	 open,
       read and	close it.

       Record  locks are not inherited by a child created via fork(2), but are
       preserved across	an execve(2).

       Because of the buffering	performed by the stdio(3) library, the use  of
       record  locking	with  routines	in that	package	should be avoided; use
       read(2) and write(2) instead.

   Mandatory locking
       (Non-POSIX.)  The above record locks may	be either advisory  or	manda-
       tory, and are advisory by default.

       Advisory	locks are not enforced and are useful only between cooperating

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

       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 mount(8), or the	MS_MANDLOCK flag for mount(2).	Manda-
       tory locking is enabled on a file by disabling group execute permission
       on the file and enabling	the set-group-ID permission bit	(see  chmod(1)
       and chmod(2)).

       The  Linux implementation of mandatory locking is unreliable.  See BUGS

   Managing signals
       used to manage I/O availability signals:

       F_GETOWN	(void)
	      Return  (as the function result) the process ID or process group
	      currently	receiving SIGIO	and SIGURG signals for events on  file
	      descriptor  fd.	Process	 IDs  are returned as positive values;
	      process group IDs	are returned as	negative values	(but see  BUGS
	      below).  arg is ignored.

       F_SETOWN	(int)
	      Set  the	process	ID or process group ID that will receive SIGIO
	      and SIGURG signals for events on file descriptor fd  to  the  ID
	      given  in	arg.  A	process	ID is specified	as a positive value; a
	      process group ID is specified as a negative  value.   Most  com-
	      monly,  the  calling process specifies itself as the owner (that
	      is, arg is specified as getpid(2)).

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

	      Sending  a  signal  to  the  owner  process (group) specified by
	      F_SETOWN is subject  to  the  same  permissions  checks  as  are
	      described	for kill(2), where the sending process is the one that
	      employs F_SETOWN (but see	BUGS below).

	      If the file descriptor fd	refers	to  a  socket,	F_SETOWN  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	select(2) would	report the socket as having an
	      "exceptional condition".)

	      The following was	true in	2.6.x kernels up to and	including ker-
	      nel 2.6.11:

		     If	 a  nonzero  value  is	given  to 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 F_SETOWN has a different meaning:  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
		     F_SETOWN the result of gettid(2) instead of getpid(2)  to
		     get  sensible results when	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 sin-
		     gle-threaded program can equally use  gettid(2)  or  get-
		     pid(2) in this scenario.)	Note, however, that the	state-
		     ments in this paragraph do	not apply to the 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 F_SETOWN.

	      The above	behavior was accidentally dropped in Linux 2.6.12, and
	      won't be restored.  From Linux 2.6.32 onward, use	F_SETOWN_EX to
	      target SIGIO and SIGURG signals at a particular thread.

       F_GETOWN_EX (struct f_owner_ex *) (since	Linux 2.6.32)
	      Return  the current file descriptor owner	settings as defined by
	      a	previous F_SETOWN_EX operation.	 The information  is  returned
	      in  the  structure  pointed  to  by arg, which has the following

		  struct f_owner_ex {
		      int   type;
		      pid_t pid;

	      The  type	 field	will  have  one	 of  the  values  F_OWNER_TID,
	      F_OWNER_PID, or F_OWNER_PGRP.  The pid field is a	positive inte-
	      ger representing a thread	ID, process ID,	or process  group  ID.
	      See F_SETOWN_EX for more details.

       F_SETOWN_EX (struct f_owner_ex *) (since	Linux 2.6.32)
	      This  operation  performs	a similar task to F_SETOWN.  It	allows
	      the caller to direct I/O	availability  signals  to  a  specific
	      thread,  process,	 or  process  group.  The caller specifies the
	      target of	signals	via arg, which is a pointer  to	 a  f_owner_ex
	      structure.   The	type  field  has  one of the following values,
	      which define how pid is interpreted:

		     Send the signal to	the thread whose thread	ID (the	 value
		     returned by a call	to clone(2) or gettid(2)) is specified
		     in	pid.

		     Send the signal to	the process whose ID is	 specified  in

		     Send  the	signal to the process group whose ID is	speci-
		     fied in pid.  (Note that, unlike with F_SETOWN, a process
		     group ID is specified as a	positive value here.)

       F_GETSIG	(void)
	      Return  (as  the	function result) 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 han-
	      dler if installed	with SA_SIGINFO.  arg is ignored.

       F_SETSIG	(int)
	      Set the signal sent when input or	output becomes possible	to the
	      value given in arg.  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.

	      By  using	 F_SETSIG with a nonzero value,	and setting SA_SIGINFO
	      for the signal handler  (see  sigaction(2)),  extra  information
	      about  I/O events	is passed to the handler in a siginfo_t	struc-
	      ture.  If	the si_code field indicates the	 source	 is  SI_SIGIO,
	      the  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 (select(2),
	      poll(2), read(2) with O_NONBLOCK set etc.)  to  determine	 which
	      file descriptors are available for I/O.

	      By  selecting  a	real time signal (value	>= SIGRTMIN), multiple
	      I/O events may be	queued using the same signal numbers.	(Queu-
	      ing  is  dependent  on  available	memory).  Extra	information is
	      available	if SA_SIGINFO is set for the signal handler, as	above.

	      Note that	Linux imposes a	limit on the number of real-time  sig-
	      nals  that may be	queued to a process (see getrlimit(2) and sig-
	      nal(7)) and if this limit	is reached, then the kernel reverts to
	      delivering  SIGIO,  and  this  signal is delivered to the	entire
	      process rather than to a specific	thread.

       Using these mechanisms, a program can implement fully asynchronous  I/O
       without using select(2) or poll(2) most of the time.

       The  use	 of  O_ASYNC, F_GETOWN,	F_SETOWN is specific to	BSD and	Linux.
       F_GETOWN_EX, F_SETOWN_EX, F_GETSIG, and	F_SETSIG  are  Linux-specific.
       POSIX  has  asynchronous	 I/O and the aio_sigevent structure to achieve
       similar things; these are also available	in Linux as part of the	GNU  C
       Library (Glibc).

       F_SETLEASE and F_GETLEASE (Linux	2.4 onward) are	used (respectively) to
       establish a new lease, and retrieve the current lease, on the open file
       description  referred  to by the	file descriptor	fd.  A file lease pro-
       vides 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 open(2) or truncate(2)	the file  referred  to
       by that file descriptor.

       F_SETLEASE (int)
	      Set  or  remove a	file lease according to	which of the following
	      values is	specified in the integer arg:

		     Take out a	read  lease.   This  will  cause  the  calling
		     process  to be notified when the file is opened for writ-
		     ing or is truncated.  A read lease	can be placed only  on
		     a file descriptor that is opened read-only.

		     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

		     Remove our	lease from the file.

       Leases  are  associated	with  an  open file description	(see open(2)).
       This means that duplicate file descriptors (created  by,	 for  example,
       fork(2) or dup(2)) refer	to the same lease, and this lease may be modi-
       fied or released	using any  of  these  descriptors.   Furthermore,  the
       lease  is  released  by	either an explicit F_UNLCK operation on	any of
       these duplicate descriptors, or when all	 such  descriptors  have  been

       Leases may be taken out only on regular files.  An unprivileged process
       may take	out a lease only on a file whose UID (owner) matches the  file
       system UID of the process.  A process with the CAP_LEASE	capability may
       take out	leases on arbitrary files.

       F_GETLEASE (void)
	      Indicates	what  type  of	lease  is  associated  with  the  file
	      descriptor  fd by	returning either F_RDLCK, F_WRLCK, or F_UNLCK,
	      indicating, respectively,	a read lease , a write	lease,	or  no
	      lease.  arg is ignored.

       When a process (the "lease breaker") performs an	open(2)	or truncate(2)
       that conflicts with a lease established via F_SETLEASE, the system call
       is  blocked  by	the kernel and 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 F_SETLEASE command  specifying  arg
       as  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  F_SETLEASE  command  specifying  arg  as

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

       Once  a	lease  break has been initiated, F_GETLEASE returns the	target
       lease type (either F_RDLCK or F_UNLCK, depending	on what	would be  com-
       patible	with  the  lease  breaker)  until the lease holder voluntarily
       downgrades or removes the lease or the kernel forcibly  does  so	 after
       the lease break timer expires.

       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 open(2) or truncate(2) is	interrupted by
       a signal	handler, then the system call fails with the error EINTR,  but
       the  other  steps still occur as	described above.  If the lease breaker
       is killed by a signal while blocked in open(2) or truncate(2), then the
       other steps still occur as described above.  If the lease breaker spec-
       ifies the O_NONBLOCK flag when calling open(2), then the	 call  immedi-
       ately fails with	the error EWOULDBLOCK, but the other steps still occur
       as described above.

       The default signal used to notify the lease holder is SIGIO,  but  this
       can  be	changed	 using the F_SETSIG command to fcntl().	 If a F_SETSIG
       command is performed (even one specifying SIGIO), and the  signal  han-
       dler  is	 established using SA_SIGINFO, then the	handler	will receive a
       siginfo_t structure as its second argument, and the 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).

   File	and directory change notification (dnotify)
       F_NOTIFY	(int)
	      (Linux  2.4  onward)  Provide  notification  when	 the directory
	      referred to by fd	or any	of  the	 files	that  it  contains  is
	      changed.	 The events to be notified are specified in arg, which
	      is a bit mask specified by ORing together	zero or	 more  of  the
	      following	bits:

	      DN_ACCESS	  A file was accessed (read, pread, readv)
	      DN_MODIFY	  A  file  was	modified (write, pwrite, writev, trun-
			  cate,	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,

	      (In  order  to obtain these definitions, the _GNU_SOURCE feature
	      test macro must be defined before	including any header files.)

	      Directory	notifications are normally "one-shot", and the	appli-
	      cation must reregister to	receive	further	notifications.	Alter-
	      natively,	if DN_MULTISHOT	is included in arg, then  notification
	      will remain in effect until explicitly removed.

	      A	 series	of F_NOTIFY requests is	cumulative, with the events in
	      arg being	added to the set already monitored.  To	disable	 noti-
	      fication	of all events, make an F_NOTIFY	call specifying	arg as

	      Notification occurs via delivery of a signal.  The default  sig-
	      nal is SIGIO, but	this can be changed using the F_SETSIG command
	      to fcntl().  In the latter case, the signal handler  receives  a
	      siginfo_t	 structure  as its second argument (if the handler was
	      established using	SA_SIGINFO) and	the si_fd field	of this	struc-
	      ture  contains the file descriptor which generated the notifica-
	      tion (useful when	establishing notification on multiple directo-

	      Especially when using DN_MULTISHOT, a real time signal should be
	      used for notification, so	that  multiple	notifications  can  be

	      NOTE:  New applications should use the inotify interface (avail-
	      able since kernel	2.6.13), which provides	a much superior	inter-
	      face  for	 obtaining  notifications  of file system events.  See

   Changing the	capacity of a pipe
       F_SETPIPE_SZ (int; since	Linux 2.6.35)
	      Change the capacity of the pipe referred to by fd	to be at least
	      arg bytes.  An unprivileged process can adjust the pipe capacity
	      to any value between the system page size	and the	limit  defined
	      in  /proc/sys/fs/pipe-max-size  (see  proc(5)).  Attempts	to set
	      the pipe capacity	below the page size are	silently rounded up to
	      the  page	 size.	Attempts by an unprivileged process to set the
	      pipe capacity  above  the	 limit	in  /proc/sys/fs/pipe-max-size
	      yield  the  error	EPERM; a privileged process (CAP_SYS_RESOURCE)
	      can override the limit.  When  allocating	 the  buffer  for  the
	      pipe,  the kernel	may use	a capacity larger than arg, if that is
	      convenient for the implementation.  The  F_GETPIPE_SZ  operation
	      returns the actual size used.  Attempting	to set the pipe	capac-
	      ity smaller than the amount of buffer space  currently  used  to
	      store data produces the error EBUSY.

       F_GETPIPE_SZ (void; since Linux 2.6.35)
	      Return  (as  the	function  result)  the	capacity  of  the pipe
	      referred to by fd.

       For a successful	call, the return value depends on the operation:

       F_DUPFD	The new	descriptor.

       F_GETFD	Value of file descriptor flags.

       F_GETFL	Value of file status flags.

		Type of	lease held on file descriptor.

       F_GETOWN	Value of descriptor owner.

       F_GETSIG	Value of signal	sent when read or write	becomes	 possible,  or
		zero for traditional SIGIO behavior.

		The pipe capacity.

       All other commands

       On error, -1 is returned, and errno is set appropriately.

	      Operation	is prohibited by locks held by other processes.

       EAGAIN The  operation  is  prohibited because the file has been memory-
	      mapped by	another	process.

       EBADF  fd is not	an open	file descriptor, or the	command	was F_SETLK or
	      F_SETLKW	and  the  file descriptor open mode doesn't match with
	      the type of lock requested.

	      It was detected that the specified F_SETLKW command would	 cause
	      a	deadlock.

       EFAULT lock is outside your accessible address space.

       EINTR  For  F_SETLKW, the command was interrupted by a signal; see sig-
	      nal(7).  For 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.

       EINVAL For  F_DUPFD,  arg  is  negative	or is greater than the maximum
	      allowable	value.	For F_SETSIG, arg is not an  allowable	signal

       EMFILE For  F_DUPFD, the	process	already	has the	maximum	number of file
	      descriptors open.

       ENOLCK Too many segment locks open, lock	table is  full,	 or  a	remote
	      locking protocol failed (e.g., locking over NFS).

       EPERM  Attempted	 to  clear  the	 O_APPEND  flag	on a file that has the
	      append-only attribute set.

       SVr4, 4.3BSD, POSIX.1-2001.   Only  the	operations  F_DUPFD,  F_GETFD,
       F_SETFD,	F_GETFL, F_SETFL, F_GETLK, F_SETLK and F_SETLKW, are specified
       in POSIX.1-2001.

       F_GETOWN	and F_SETOWN are specified in  POSIX.1-2001.   (To  get	 their
       definitions,  define BSD_SOURCE,	or _XOPEN_SOURCE with the value	500 or
       greater,	or define _POSIX_C_SOURCE with the value 200809L or greater.)

       F_DUPFD_CLOEXEC is specified in POSIX.1-2008.  (To get this definition,
       define	_POSIX_C_SOURCE	  with	 the  value  200809L  or  greater,  or
       _XOPEN_SOURCE with the value 700	or greater.)

       SIG,  F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific.  (Define
       the _GNU_SOURCE macro to	obtain these definitions.)

       The original Linux fcntl() system call was not designed to handle large
       file offsets (in	the flock structure).  Consequently, an	fcntl64() sys-
       tem call	was added in Linux 2.4.	 The newer system call employs a  dif-
       ferent structure	for file locking, flock64, and corresponding commands,
       F_GETLK64, F_SETLK64, and F_SETLKW64.  However, these  details  can  be
       ignored	by  applications  using	 glibc,	whose fcntl() wrapper function
       transparently employs the more recent system call where	it  is	avail-

       The  errors  returned  by  dup2(2) are different	from those returned by

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

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

       A limitation of the Linux system	call conventions on some architectures
       (notably	i386) means that if  a	(negative)  process  group  ID	to  be
       returned	 by  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;
       that is,	the return value of fcntl() will be -1,	and errno will contain
       the (positive) process group ID.	 The Linux-specific F_GETOWN_EX	opera-
       tion  avoids  this  problem.  Since glibc version 2.11, glibc makes the
       kernel  F_GETOWN	 problem  invisible  by	 implementing  F_GETOWN	 using

       In  Linux 2.4 and earlier, there	is bug that can	occur when an unprivi-
       leged process uses F_SETOWN to specify  the  owner  of  a  socket  file
       descriptor  as  a process (group) other than the	caller.	 In this case,
       fcntl() can return -1 with errno	set to	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 race conditions which render it  unreliable:  a  write(2)
       call that overlaps with a lock may modify data after the	mandatory lock
       is acquired; a 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 mmap(2).	 It is	there-
       fore inadvisable	to rely	on mandatory locking.

       dup2(2),	 flock(2), open(2), socket(2), lockf(3), capabilities(7), fea-

       locks.txt, mandatory-locking.txt, and dnotify.txt in the	 Linux	kernel
       source  directory  Documentation/filesystems/  (on older	kernels, these
       files are directly under	the Documentation/ directory,  and  mandatory-
       locking.txt is called mandatory.txt)

       This  page  is  part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found	at

Linux				  2012-04-15			      FCNTL(2)


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