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



NAME
       fcntl - manipulate file descriptor

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

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

DESCRIPTION
       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.

       Certain of the operations below are supported only since	 a  particular
       Linux  kernel  version.	 The  preferred method of checking whether the
       host kernel supports a particular operation is to invoke	 fcntl()  with
       the  desired  cmd value and then test whether the call failed with EIN-
       VAL, indicating that the kernel does not recognize this value.

   Duplicating a file descriptor
       F_DUPFD (int)
	      Duplicate the  file  descriptor  fd  using  the  lowest-numbered
	      available file descriptor greater than or equal to arg.  This is
	      different from dup2(2), which uses exactly the  file  descriptor
	      specified.

	      On success, the new file 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 file descriptor.  Specifying this flag  permits  a
	      program  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 set, the file descriptor
       will automatically be closed during a successful	 execve(2).   (If  the
       execve(2)  fails, the file descriptor is left open.)  If the FD_CLOEXEC
       bit is not  set,	 the  file  descriptor	will  remain  open  across  an
       execve(2).

       F_GETFD (void)
	      Return  (as  the function result) the file descriptor flags; arg
	      is ignored.

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

       In multithreaded programs, using fcntl() F_SETFD to set	the  close-on-
       exec  flag  at  the same time as another thread performs a fork(2) plus
       execve(2) is vulnerable to a race condition  that  may  unintentionally
       leak  the file descriptor to the program executed in the child process.
       See the discussion of the O_CLOEXEC flag in open(2) for details	and  a
       remedy to the problem.

   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
       flags.

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

       F_GETFL (void)
	      Return  (as  the	function  result) the file access mode and the
	      file status flags; arg is ignored.

       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.  It is	 not  possible
	      to change the O_DSYNC and O_SYNC flags; see BUGS, below.

   Advisory record locking
       Linux  implements traditional ("process-associated") UNIX record locks,
       as standardized by POSIX.  For a Linux-specific alternative with better
       semantics, see the discussion of open file description locks below.

       F_SETLK,	 F_SETLKW,  and F_GETLK are used to acquire, release, and test
       for the existence of record locks (also known as byte-range,  file-seg-
       ment, 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:
				   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
				   (set by F_GETLK and F_OFD_GETLK) */
	       ...
	   };

       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
       grows.

       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.  (The error returned in this case
	      differs across implementations, so  POSIX	 requires  a  portable
	      application to check for both errors.)

       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 those locks in
	      the l_type, l_whence, l_start, and l_len fields of lock.	If the
	      conflicting  lock	 is  a traditional (process-associated) record
	      lock, then the l_pid field is set to  the	 PID  of  the  process
	      holding  that  lock.   If	 the  conflicting lock is an open file
	      description lock, then l_pid  is	set  to	 -1.   Note  that  the
	      returned	information may already be out of date by the time the
	      caller inspects it.

       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.

       When placing locks with F_SETLKW, the kernel detects deadlocks, whereby
       two  or	more  processes	 have  their lock requests mutually blocked by
       locks held by the other processes.   For	 example,  suppose  process  A
       holds  a	 write lock on byte 100 of a file, and process B holds a write
       lock on byte 200.  If each process  then	 attempts  to  lock  the  byte
       already locked by the other process using F_SETLKW, then, without dead-
       lock detection, both processes would remain blocked indefinitely.  When
       the  kernel  detects such deadlocks, it causes one of the blocking lock
       requests to immediately fail with the  error  EDEADLK;  an  application
       that encounters such an error should release some of its locks to allow
       other applications to proceed before attempting regain the  locks  that
       it  requires.  Circular deadlocks involving more than two processes are
       also detected.  Note, however, that there are limitations to  the  ker-
       nel's deadlock-detection algorithm; see BUGS.

       As well as being removed by an explicit F_UNLCK, record locks are auto-
       matically released when the process terminates.

       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.

       The  record  locks  described  above  are  associated  with the process
       (unlike the open file description locks	described  below).   This  has
       some unfortunate consequences:

       *  If  a	 process  closes any file descriptor referring to a file, then
	  all of the process's locks on that file are released, regardless  of
	  the  file  descriptor(s)  on which the locks were obtained.  This is
	  bad: it means that a process can lose its locks on a	file  such  as
	  /etc/passwd  or  /etc/mtab  when  for some reason a library function
	  decides to open, read, and close the same file.

       *  The threads in a process share locks.	  In  other  words,  a	multi-
	  threaded  program  can't  use	 record locking to ensure that threads
	  don't simultaneously access the same region of a file.

       Open file description locks solve both of these problems.

   Open file description locks (non-POSIX)
       Open file description locks are advisory byte-range locks whose	opera-
       tion  is	 in  most  respects  identical to the traditional record locks
       described above.	 This lock type is Linux-specific, and available since
       Linux 3.15.  (There is a proposal with the Austin Group to include this
       lock type in the next revision of POSIX.1.)  For an explanation of open
       file descriptions, see open(2).

       The  principal  difference  between  the two lock types is that whereas
       traditional record locks are  associated	 with  a  process,  open  file
       description  locks  are	associated  with  the open file description on
       which they are acquired, much like locks acquired with flock(2).	  Con-
       sequently  (and	unlike	traditional  advisory record locks), open file
       description locks are  inherited	 across	 fork(2)  (and	clone(2)  with
       CLONE_FILES),  and are only automatically released on the last close of
       the open file description, instead of being released on	any  close  of
       the file.

       Conflicting  lock  combinations	(i.e., a read lock and a write lock or
       two write locks) where one lock is an open file	description  lock  and
       the  other  is  a  traditional  record lock conflict even when they are
       acquired by the same process on the same file descriptor.

       Open file description locks placed via the same open  file  description
       (i.e.,  via  the	 same  file descriptor, or via a duplicate of the file
       descriptor created by fork(2), dup(2), fcntl() F_DUPFD, and so on)  are
       always compatible: if a new lock is placed on an already locked region,
       then the existing lock is converted to the new lock type.   (Such  con-
       versions	 may  result  in  splitting,  shrinking, or coalescing with an
       existing lock as discussed above.)

       On the other hand, open file description locks may conflict  with  each
       other  when  they  are  acquired	 via different open file descriptions.
       Thus, the threads in a multithreaded program can use open file descrip-
       tion locks to synchronize access to a file region by having each thread
       perform its own open(2) on the file and applying locks via the  result-
       ing file descriptor.

       As  with	 traditional  advisory	locks,	the third argument to fcntl(),
       lock, is a pointer to an flock structure.  By contrast with traditional
       record  locks,  the  l_pid  field of that structure must be set to zero
       when using the commands described below.

       The commands for working with open file description locks are analogous
       to those used with traditional locks:

       F_OFD_SETLK (struct flock *)
	      Acquire an open file description lock (when l_type is F_RDLCK or
	      F_WRLCK) or release an open file description 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 EAGAIN.

       F_OFD_SETLKW (struct flock *)
	      As  for  F_OFD_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 errno set to EINTR; see signal(7)).

       F_OFD_GETLK (struct flock *)
	      On  input	 to this call, lock describes an open file description
	      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 details about one of	 these
	      locks are returned via lock, as described above for F_GETLK.

       In  the	current implementation, no deadlock detection is performed for
       open file description locks.  (This contrasts  with  process-associated
       record locks, for which the kernel does perform deadlock detection.)

   Mandatory locking
       Warning:	 the  Linux implementation of mandatory locking is unreliable.
       See BUGS below.	Because of these bugs, and the fact that  the  feature
       is  believed  to be little used, since Linux 4.5, mandatory locking has
       been made an optional feature, governed by a configuration option (CON-
       FIG_MANDATORY_FILE_LOCKING).   This  is an initial step toward removing
       this feature completely.

       By  default,  both  traditional	(process-associated)  and  open	  file
       description record locks are advisory.  Advisory locks are not enforced
       and are useful only between cooperating processes.

       Both lock types can also be mandatory.  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	incom-
       patible mandatory lock, then the result depends upon whether the O_NON-
       BLOCK flag is enabled for its open file description.  If the O_NONBLOCK
       flag  is not enabled, then the system call is blocked until the lock is
       removed or converted to a mode that is compatible 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 filesystem that contains the file to be locked, and on the file
       itself.	Mandatory locking is enabled on a  filesystem  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)).

       Mandatory locking is not specified by POSIX.  Some other	 systems  also
       support	mandatory  locking,  although  the details of how to enable it
       vary across systems.

   Managing signals
       F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG and F_SETSIG are
       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 the file	 descriptor  fd.   The
	      target  process  or  process  group  ID  is specified in arg.  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, arg is specified
	      as getpid(2)).

	      As  well	as  setting  the  file descriptor owner, one must also
	      enable generation of signals on the file	descriptor.   This  is
	      done  by	using  the  fcntl() F_SETFL command to set the O_ASYNC
	      file status flag on the file descriptor.	Subsequently, a	 SIGIO
	      signal  is sent whenever input or output becomes possible on the
	      file descriptor.	The fcntl() F_SETSIG command can  be  used  to
	      obtain delivery of a signal other than SIGIO.

	      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 this permission check
	      fails,  then  the	 signal	 is  silently  discarded.   Note:  The
	      F_SETOWN operation records the caller's credentials at the  time
	      of  the fcntl() call, and it is these saved credentials that are
	      used for the permission checks.

	      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
	      form:

		  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:

	      F_OWNER_TID
		     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.

	      F_OWNER_PID
		     Send the signal to the process whose ID is	 specified  in
		     pid.

	      F_OWNER_PGRP
		     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.

	      Note  that the file descriptor provided in si_fd is the one that
	      was specified during the F_SETSIG operation.  This can  lead  to
	      an  unusual  corner  case.  If the file descriptor is duplicated
	      (dup(2) or similar), and the original file descriptor is closed,
	      then  I/O	 events	 will  continue to be generated, but the si_fd
	      field will contain the number of the now closed file descriptor.

	      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 is specific to BSD  and  Linux.   The	 only  use  of
       F_GETOWN	 and  F_SETOWN specified in POSIX.1 is in conjunction with the
       use of the SIGURG signal on sockets.  (POSIX does not specify the SIGIO
       signal.)	  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).

   Leases
       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:

	      F_RDLCK
		     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.

	      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.

	      F_UNLCK
		     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	 file  descriptors,  or when all such file descriptors
       have been closed.

       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
       filesystem 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
       F_RDLCK.

       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 file 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(2),  pread(2),  readv(2),
			  and similar)
	      DN_MODIFY	  A file was modified (write(2), pwrite(2), writev(2),
			  truncate(2), ftruncate(2), and similar).
	      DN_CREATE	  A file was  created  (open(2),  creat(2),  mknod(2),
			  mkdir(2),  link(2),  symlink(2), rename(2) into this
			  directory).
	      DN_DELETE	  A file was unlinked (unlink(2), rename(2) to another
			  directory, rmdir(2)).
	      DN_RENAME	  A   file   was   renamed   within   this   directory
			  (rename(2)).
	      DN_ATTRIB	  The attributes of a  file  were  changed  (chown(2),
			  chmod(2), utime(2), utimensat(2), and similar).

	      (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
	      0.

	      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().  (Note that SIGIO is one of the nonqueuing  standard
	      signals;	switching  to the use of a real-time signal means that
	      multiple notifications can be queued to the  process.)   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 structure contains the
	      file descriptor which generated the  notification	 (useful  when
	      establishing notification on multiple directories).

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

	      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 filesystem events.  See ino-
	      tify(7).

   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 imple-
	      mentation.  (In the current implementation,  the	allocation  is
	      the next higher power-of-two page-size multiple of the requested
	      size.)  The actual capacity (in bytes) that is set  is  returned
	      as the function result.

	      Attempting  to  set the pipe capacity 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.

   File Sealing
       File seals limit the set of allowed operations on a  given  file.   For
       each seal that is set on a file, a specific set of operations will fail
       with EPERM on this file from now on.  The file is said  to  be  sealed.
       The default set of seals depends on the type of the underlying file and
       filesystem.  For an overview of file sealing, a discussion of its  pur-
       pose, and some code examples, see memfd_create(2).

       Currently,  only	 the  tmpfs(5)	filesystem supports sealing.  On other
       filesystems, all fcntl() operations that operate on seals  will	return
       EINVAL.

       Seals  are  a  property	of  an inode.  Thus, all open file descriptors
       referring to the same inode share the same set of seals.	  Furthermore,
       seals can never be removed, only added.

       F_ADD_SEALS (int; since Linux 3.17)
	      Add  the	seals given in the bit-mask argument arg to the set of
	      seals of the inode referred to by the file descriptor fd.	 Seals
	      cannot be removed again.	Once this call succeeds, the seals are
	      enforced by the kernel immediately.  If the current set of seals
	      includes	F_SEAL_SEAL  (see  below),  then  this	call  will  be
	      rejected with EPERM.  Adding a seal that is already set is a no-
	      op, in case F_SEAL_SEAL is not set already.  In order to place a
	      seal, the file descriptor fd must be writable.

       F_GET_SEALS (void; since Linux 3.17)
	      Return (as the function result) the current set of seals of  the
	      inode  referred  to  by fd.  If no seals are set, 0 is returned.
	      If the file does not support sealing, -1 is returned  and	 errno
	      is set to EINVAL.

       The following seals are available:

       F_SEAL_SEAL
	      If   this	 seal  is  set,	 any  further  call  to	 fcntl()  with
	      F_ADD_SEALS will fail with EPERM.	 Therefore, this seal prevents
	      any  modifications  to  the set of seals itself.	If the initial
	      set of seals of a file includes F_SEAL_SEAL,  then  this	effec-
	      tively causes the set of seals to be constant and locked.

       F_SEAL_SHRINK
	      If  this	seal is set, the file in question cannot be reduced in
	      size.  This affects open(2) with the O_TRUNC  flag  as  well  as
	      truncate(2)  and ftruncate(2).  Those calls will fail with EPERM
	      if you try to shrink the file in question.  Increasing the  file
	      size is still possible.

       F_SEAL_GROW
	      If  this seal is set, the size of the file in question cannot be
	      increased.  This affects write(2) beyond the end	of  the	 file,
	      truncate(2),  ftruncate(2),  and fallocate(2).  These calls will
	      fail with EPERM if you use them to increase the file  size.   If
	      you  keep	 the  size  or	shrink	it,  those calls still work as
	      expected.

       F_SEAL_WRITE
	      If this seal is set, you cannot modify the contents of the file.
	      Note  that  shrinking  or	 growing the size of the file is still
	      possible and allowed.  Thus, this seal is normally used in  com-
	      bination	with  one  of  the  other  seals.   This  seal affects
	      write(2) and fallocate(2) (only in  combination  with  the  FAL-
	      LOC_FL_PUNCH_HOLE	 flag).	  Those	 calls will fail with EPERM if
	      this seal is set.	 Furthermore, trying  to  create  new  shared,
	      writable memory-mappings via mmap(2) will also fail with EPERM.

	      Using  the  F_ADD_SEALS  operation  to set the F_SEAL_WRITE seal
	      will fail with EBUSY if any  writable,  shared  mapping  exists.
	      Such  mappings  must  be	unmapped before you can add this seal.
	      Furthermore,  if	there  are  any	 asynchronous  I/O  operations
	      (io_submit(2))  pending on the file, all outstanding writes will
	      be discarded.

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

       F_DUPFD	The new file descriptor.

       F_GETFD	Value of file descriptor flags.

       F_GETFL	Value of file status flags.

       F_GETLEASE
		Type of lease held on file descriptor.

       F_GETOWN Value of file descriptor owner.

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

       F_GETPIPE_SZ, F_SETPIPE_SZ
		The pipe capacity.

       F_GET_SEALS
		A  bit	mask  identifying the seals that have been set for the
		inode referred to by fd.

       All other commands
		Zero.

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

ERRORS
       EACCES or EAGAIN
	      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

       EBADF  cmd  is  F_SETLK	or  F_SETLKW and the file descriptor open mode
	      doesn't match with the type of lock requested.

       EBUSY  cmd is F_SETPIPE_SZ and the new pipe capacity specified  in  arg
	      is  smaller  than	 the  amount of buffer space currently used to
	      store data in the pipe.

       EBUSY  cmd is F_ADD_SEALS, arg includes F_SEAL_WRITE, and there	exists
	      a writable, shared mapping on the file referred to by fd.

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

       EFAULT lock is outside your accessible address space.

       EINTR  cmd is F_SETLKW or F_OFD_SETLKW and  the	operation  was	inter-
	      rupted by a signal; see signal(7).

       EINTR  cmd  is  F_GETLK,	 F_SETLK, F_OFD_GETLK, or F_OFD_SETLK, and the
	      operation 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 The value specified in cmd is not recognized by this kernel.

       EINVAL cmd is F_ADD_SEALS and arg includes an unrecognized sealing bit.

       EINVAL cmd is F_ADD_SEALS or F_GET_SEALS and the filesystem  containing
	      the inode referred to by fd does not support sealing.

       EINVAL cmd  is F_DUPFD and arg is negative or is greater than the maxi-
	      mum allowable value (see	the  discussion	 of  RLIMIT_NOFILE  in
	      getrlimit(2)).

       EINVAL cmd is F_SETSIG and arg is not an allowable signal number.

       EINVAL cmd  is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and l_pid was
	      not specified as zero.

       EMFILE cmd is F_DUPFD and the per-process limit on the number  of  open
	      file descriptors has been reached.

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

       ENOTDIR
	      F_NOTIFY was specified in cmd, but fd does not refer to a direc-
	      tory.

       EPERM  cmd  is  F_SETPIPE_SZ  and  the soft or hard user pipe limit has
	      been reached; see pipe(7).

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

       EPERM  cmd was F_ADD_SEALS, but fd was not open for writing or the cur-
	      rent set of seals on the file already includes F_SEAL_SEAL.

CONFORMING TO
       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 either _XOPEN_SOURCE with the value 500 or greater,
       or _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.)

       F_GETOWN_EX, F_SETOWN_EX, F_SETPIPE_SZ, F_GETPIPE_SZ, F_GETSIG,	F_SET-
       SIG,  F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific.  (Define
       the _GNU_SOURCE macro to obtain these definitions.)

       F_OFD_SETLK, F_OFD_SETLKW, and F_OFD_GETLK are Linux-specific (and  one
       must define _GNU_SOURCE to obtain their definitions), but work is being
       done to have them included in the next version of POSIX.1.

       F_ADD_SEALS and F_GET_SEALS are Linux-specific.

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

   File locking
       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-
       able.


   Record locks
       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.

       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-
       able.

   Record locking and NFS
       Before Linux 3.12, if an NFSv4 client loses contact with the server for
       a  period  of  time (defined as more than 90 seconds with no communica-
       tion), it might lose and regain a lock without ever being aware of  the
       fact.  (The period of time after which contact is assumed lost is known
       as the NFSv4 leasetime.	On a Linux NFS server, this can be  determined
       by  looking at /proc/fs/nfsd/nfsv4leasetime, which expresses the period
       in seconds.  The default value for this file  is	 90.)	This  scenario
       potentially  risks data corruption, since another process might acquire
       a lock in the intervening period and perform file I/O.

       Since Linux 3.12, if an NFSv4 client loses contact with the server, any
       I/O  to	the file by a process which "thinks" it holds a lock will fail
       until that process closes and reopens the file.	 A  kernel  parameter,
       nfs.recover_lost_locks,	can  be set to 1 to obtain the pre-3.12 behav-
       ior, whereby the client will attempt to recover lost locks when contact
       is  reestablished  with	the  server.  Because of the attendant risk of
       data corruption, this parameter defaults to 0 (disabled).

BUGS
   F_SETFL
       It is not possible to use F_SETFL to change the state  of  the  O_DSYNC
       and  O_SYNC  flags.   Attempts  to  change the state of these flags are
       silently ignored.

   F_GETOWN
       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
       F_GETOWN_EX.

   F_SETOWN
       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.

   Deadlock detection
       The deadlock-detection algorithm employed by the	 kernel	 when  dealing
       with  F_SETLKW  requests	 can  yield  both false negatives (failures to
       detect deadlocks, leaving a set of deadlocked processes blocked indefi-
       nitely) and false positives (EDEADLK errors when there is no deadlock).
       For example, the kernel limits the lock depth of its dependency	search
       to  10  steps,  meaning	that circular deadlock chains that exceed that
       size will not be detected.  In addition, the kernel may	falsely	 indi-
       cate  a	deadlock when two or more processes created using the clone(2)
       CLONE_FILES flag place locks that appear (to the kernel) to conflict.

   Mandatory locking
       The Linux implementation of mandatory locking is subject to race condi-
       tions  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 therefore inadvisable to rely on
       mandatory locking.

SEE ALSO
       dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),  fea-
       ture_test_macros(7), lslocks(8)

       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)

COLOPHON
       This page is part of release 4.10 of the Linux  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/.



Linux				  2016-10-08			      FCNTL(2)