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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 argument 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 EINVAL, 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, initialized 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-segment, or file-region  locks).   The  third
       argument,  lock,  is  a  pointer to a structure that has at least the following fields (in
       unspecified order).

           struct flock {
               ...
               short l_type;    /* Type of lock: F_RDLCK,
                                   F_WRLCK, F_UNLCK */
               short l_whence;  /* How to interpret l_start:
                                   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 relative  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 positive, 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 signal 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 deadlock 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 kernel's deadlock-detection algorithm; see BUGS.

       As  well  as being removed by an explicit F_UNLCK, record locks are automatically 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 multithreaded 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 operation 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).
       Consequently (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
       conversions  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 description 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
       resulting 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  (CONFIG_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 incompatible mandatory lock, then the result depends upon  whether  the
       O_NONBLOCK  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).  Mandatory 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 kernel 2.6.11:

                     If a nonzero value is given to F_SETSIG in a multithreaded  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
                     single-threaded program can equally  use  gettid(2)  or  getpid(2)  in  this
                     scenario.)   Note,  however,  that  the  statements 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  integer 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 specified 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 handler 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  structure.  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.  (Queuing 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  signals  that  may  be
              queued  to a process (see getrlimit(2) and signal(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 provides a mechanism whereby the process holding the
       lease (the "lease holder") is notified (via delivery of a  signal)  when  a  process  (the
       "lease  breaker")  tries  to  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 writing 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  modified  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 compatible 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
       specifies the O_NONBLOCK flag when calling open(2), then the call immediately  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  handler  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   application   must
              reregister  to  receive  further  notifications.  Alternatively, 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 notification of all events, make an F_NOTIFY
              call specifying arg as 0.

              Notification occurs via delivery of a signal.  The default  signal  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  (available  since  kernel
              2.6.13),  which  provides  a much superior interface for obtaining notifications of
              filesystem events.  See inotify(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 implementation.  (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
       purpose, 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 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 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
              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 combination with one of the other seals.   This  seal
              affects    write(2)    and    fallocate(2)    (only   in   combination   with   the
              FALLOC_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.

   File read/write hints
       Write lifetime hints can be used to inform the kernel about the relative expected lifetime
       of writes on a given inode or via a particular open file description.  (See open(2) for an
       explanation  of  open file desriptions.)  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  underlying  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.

       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 description 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 inode.

       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 individual 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 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 interrupted 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 maximum  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 directory.

       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 current 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_SETSIG,   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() system call was added in Linux 2.4.  The
       newer  system  call  employs  a  different  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 available.

   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() system call was added in Linux 2.4.  The
       newer  system  call  employs  a  different  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 available.

   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 communication), 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 behavior, 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  operation  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 unprivileged 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 indefinitely) 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  indicate 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 conditions which render
       it unreliable: a write(2) call that overlaps  with  a  lock  may  modify  data  after  the
       mandatory lock is acquired; a read(2) call that overlaps with a lock may detect changes to
       data that were made only after a write lock was acquired.   Similar  races  exist  between
       mandatory locks and mmap(2).  It is therefore inadvisable to rely on mandatory locking.

SEE ALSO

       dup2(2),  flock(2), open(2), socket(2), lockf(3), capabilities(7), feature_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 4.13 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/.