fcntl(2)



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  re-
       quired  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 de-
       scriptor.  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  ex-
       ecve(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  or-
       der).

           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 in-
       cluding l_start+l_len-1.  Specifying 0 for l_len has the special  mean-
       ing:  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 ap-
              plication 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  de-
              scription 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 al-
       ready 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  (un-
       like  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 de-
          cides 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  de-
       scribed  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 de-
       scription locks are associated with the open file description on  which
       they  are  acquired,  much  like  locks acquired with flock(2).  Conse-
       quently (and unlike traditional advisory record locks), open  file  de-
       scription  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  ac-
       quired 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 de-
       scriptor  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  ex-
       isting 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 re-
              turn 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 de-
       scription 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  ac-
       cess (e.g., read(2) or write(2)) on a file region that has an incompat-
       ible 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 er-
       ror 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.

   Lost locks
       When an advisory lock is obtained on a networked filesystem such as NFS
       it is possible that the lock might get lost.  This may  happen  due  to
       administrative  action  on  the  server,  or due to a network partition
       (i.e., loss of network connectivity with the server) which  lasts  long
       enough  for the server to assume that the client is no longer function-
       ing.

       When the filesystem determines  that  a  lock  has  been  lost,  future
       read(2)  or  write(2) requests may fail with the error EIO.  This error
       will persist until the lock  is  removed  or  the  file  descriptor  is
       closed.   Since  Linux 3.12, this happens at least for NFSv4 (including
       all minor versions).

       Some versions of UNIX send a signal  (SIGLOST)  in  this  circumstance.
       Linux  does  not define this signal, and does not provide any asynchro-
       nous notification of lost locks.

   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
              ID currently receiving SIGIO and SIGURG signals  for  events  on
              file  descriptor  fd.  Process IDs are returned as positive val-
              ues; 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 en-
              able 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  sig-
              nal  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_SE-
              TOWN 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 oper-
              ation records the caller's credentials at the time  of  the  fc-
              ntl()  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  se-
              lects  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_SE-
                     TOWN 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  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 provides a  mecha-
       nism  whereby the process holding the lease (the "lease holder") is no-
       tified (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 de-
       scriptor.

       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 de-
              scriptor 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 re-
       spond 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  re-
              ferred 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 direc-
                     tory).
              DN_DELETE
                     A file was unlinked (unlink(2), rename(2) to another  di-
                     rectory, 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.

              Note that because of the way the pages of the  pipe  buffer  are
              employed  when  data is written to the pipe, the number of bytes
              that can be written may be less than the nominal size, depending
              on the size of the writes.

       F_GETPIPE_SZ (void; since Linux 2.6.35)
              Return  (as  the  function  result) the capacity of the pipe re-
              ferred 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, file seals can be applied only to a file descriptor returned
       by memfd_create(2) (if the MFD_ALLOW_SEALING was employed).   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  re-
       ferring  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  re-
              jected  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  fails  with  the error 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
              effectively 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 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  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 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
              fails with EBUSY if any writable, shared mapping  exists.   Such
              mappings  must  be  unmapped before you can add this seal.  Fur-
              thermore, if there are any asynchronous I/O operations  (io_sub-
              mit(2)) pending on the file, all outstanding writes will be dis-
              carded.

       F_SEAL_FUTURE_WRITE (since Linux 5.1)
              The effect of this seal is similar to F_SEAL_WRITE, but the con-
              tents of the file can still be modified via shared writable map-
              pings that were created prior to the seal being  set.   Any  at-
              tempt  to  create a new writable mapping on the file via mmap(2)
              will fail with EPERM.  Likewise, an attempt to write to the file
              via write(2) will fail with EPERM.

              Using  this seal, one process can create a memory buffer that it
              can continue to modify while sharing that  buffer  on  a  "read-
              only" basis with other processes.

   File read/write hints
       Write  lifetime  hints can be used to inform the kernel about the rela-
       tive expected lifetime of writes on a given inode or via  a  particular
       open  file  description.   (See open(2) for an explanation of open file
       descriptions.)  In this context, the term "write  lifetime"  means  the
       expected  time the data will live on media, before being overwritten or
       erased.

       An application may use the different hint  values  specified  below  to
       separate writes into different write classes, so that multiple users or
       applications running on a single storage back-end can  aggregate  their
       I/O  patterns in a consistent manner.  However, there are no functional
       semantics implied by these flags, and different I/O classes can use the
       write  lifetime  hints in arbitrary ways, so long as the hints are used
       consistently.

       The following operations can be applied to the file descriptor, fd:

       F_GET_RW_HINT (uint64_t *; since Linux 4.13)
              Returns the value of the read/write hint associated with the un-
              derlying inode referred to by fd.

       F_SET_RW_HINT (uint64_t *; since Linux 4.13)
              Sets  the  read/write  hint value associated with the underlying
              inode referred to by fd.  This hint persists until either it  is
              explicitly modified or the underlying filesystem is unmounted.

       F_GET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
              Returns  the  value  of  the read/write hint associated with the
              open file description referred to by fd.

       F_SET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
              Sets the read/write hint value associated with the open file de-
              scription referred to by fd.

       If  an  open  file description has not been assigned a read/write hint,
       then it shall use the value assigned to the inode, if any.

       The following read/write hints are valid since Linux 4.13:

       RWH_WRITE_LIFE_NOT_SET
              No specific hint has been set.  This is the default value.

       RWH_WRITE_LIFE_NONE
              No specific write lifetime is associated with this file  or  in-
              ode.

       RWH_WRITE_LIFE_SHORT
              Data  written to this inode or via this open file description is
              expected to have a short lifetime.

       RWH_WRITE_LIFE_MEDIUM
              Data written to this inode or via this open file description  is
              expected  to  have  a  lifetime  longer  than  data written with
              RWH_WRITE_LIFE_SHORT.

       RWH_WRITE_LIFE_LONG
              Data written to this inode or via this open file description  is
              expected  to  have  a  lifetime  longer  than  data written with
              RWH_WRITE_LIFE_MEDIUM.

       RWH_WRITE_LIFE_EXTREME
              Data written to this inode or via this open file description  is
              expected  to  have  a  lifetime  longer  than  data written with
              RWH_WRITE_LIFE_LONG.

       All the write-specific hints are relative to each other, and  no  indi-
       vidual absolute meaning should be attributed to them.

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  in-
              ode 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 ap-
              pend-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  (to  identify  the  machine where the lock is held).  Clearly,
       l_pid alone is not going to be very useful if the process  holding  the
       lock  may  live on a different machine; on Linux, while present on some
       architectures (such as MIPS32), this field is not used.

       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 po-
       tentially 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  re-
       turned  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  de-
       scriptor as a process (group) other than the caller.  In this case, fc-
       ntl() 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  de-
       tect  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 5.07 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/.

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