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
       required type is int, and we identify the argument using the name arg),
       or void is specified if the argument is not required.

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

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

              On success, the new file descriptor is returned.

              See dup(2) for further details.

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

   File descriptor flags
       The  following  commands  manipulate  the  flags associated with a file
       descriptor.  Currently, only one such flag is defined: FD_CLOEXEC,  the
       close-on-exec  flag.  If the FD_CLOEXEC bit is set, the file descriptor
       will automatically be closed during a successful  execve(2).   (If  the
       execve(2)  fails, the file descriptor is left open.)  If the FD_CLOEXEC
       bit is not  set,  the  file  descriptor  will  remain  open  across  an
       execve(2).

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

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

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

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

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

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

       F_SETFL (int)
              Set the file status flags to the value specified by  arg.   File
              access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags
              (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg  are  ignored.
              On  Linux,  this  command can change only the O_APPEND, O_ASYNC,
              O_DIRECT, O_NOATIME, and O_NONBLOCK flags.  It is  not  possible
              to change the O_DSYNC and O_SYNC flags; see BUGS, below.

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

       F_SETLK,  F_SETLKW,  and F_GETLK are used to acquire, release, and test
       for the existence of record locks (also known as byte-range,  file-seg-
       ment, or file-region locks).  The third argument, lock, is a pointer to
       a structure that has at least  the  following  fields  (in  unspecified
       order).

           struct flock {
               ...
               short l_type;    /* Type of lock: F_RDLCK,
                                   F_WRLCK, F_UNLCK */
               short l_whence;  /* How to interpret l_start:
                                   SEEK_SET, SEEK_CUR, SEEK_END */
               off_t l_start;   /* Starting offset for lock */
               off_t l_len;     /* Number of bytes to lock */
               pid_t l_pid;     /* PID of process blocking our lock
                                   (set by F_GETLK and F_OFD_GETLK) */
               ...
           };

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

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

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

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

       The l_type field can be used to place  a  read  (F_RDLCK)  or  a  write
       (F_WRLCK) lock on a file.  Any number of processes may hold a read lock
       (shared lock) on a file region, but only one process may hold  a  write
       lock  (exclusive  lock).   An  exclusive lock excludes all other locks,
       both shared and exclusive.  A single process can hold only one type  of
       lock  on  a  file region; if a new lock is applied to an already-locked
       region, then the existing lock is  converted  to  the  new  lock  type.
       (Such  conversions may involve splitting, shrinking, or coalescing with
       an existing lock if the byte range specified by the new lock  does  not
       precisely coincide with the range of the existing lock.)

       F_SETLK (struct flock *)
              Acquire  a lock (when l_type is F_RDLCK or F_WRLCK) or release a
              lock (when l_type is F_UNLCK) on  the  bytes  specified  by  the
              l_whence,  l_start,  and l_len fields of lock.  If a conflicting
              lock is held by another process, this call returns -1  and  sets
              errno  to  EACCES  or  EAGAIN.  (The error returned in this case
              differs across implementations, so  POSIX  requires  a  portable
              application to check for both errors.)

       F_SETLKW (struct flock *)
              As  for  F_SETLK, but if a conflicting lock is held on the file,
              then wait for that lock to be released.  If a signal  is  caught
              while  waiting, then the call is interrupted and (after the sig-
              nal handler has returned) returns immediately (with return value
              -1 and errno set to EINTR; see signal(7)).

       F_GETLK (struct flock *)
              On  input  to  this call, lock describes a lock we would like to
              place on the file.  If the lock could be  placed,  fcntl()  does
              not  actually  place it, but returns F_UNLCK in the l_type field
              of lock and leaves the other fields of the structure unchanged.

              If one or more incompatible locks would prevent this lock  being
              placed, then fcntl() returns details about one of those locks in
              the l_type, l_whence, l_start, and l_len fields of lock.  If the
              conflicting  lock  is  a traditional (process-associated) record
              lock, then the l_pid field is set to  the  PID  of  the  process
              holding  that  lock.   If  the  conflicting lock is an open file
              description lock, then l_pid  is  set  to  -1.   Note  that  the
              returned  information may already be out of date by the time the
              caller inspects it.

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

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

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

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

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

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

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

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

       Open file description locks solve both of these problems.

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

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

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

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

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

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

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

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

       F_OFD_SETLKW (struct flock *)
              As  for  F_OFD_SETLK,  but  if a conflicting lock is held on the
              file, then wait for that lock to be released.  If  a  signal  is
              caught  while  waiting,  then the call is interrupted and (after
              the signal  handler  has  returned)  returns  immediately  (with
              return value -1 and errno set to EINTR; see signal(7)).

       F_OFD_GETLK (struct flock *)
              On  input  to this call, lock describes an open file description
              lock we would like to place on the file.  If the lock  could  be
              placed,  fcntl() does not actually place it, but returns F_UNLCK
              in the l_type field of lock and leaves the other fields  of  the
              structure  unchanged.   If  one or more incompatible locks would
              prevent this lock being placed, then details about one of  these
              locks are returned via lock, as described above for F_GETLK.

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

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

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

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

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

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

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

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

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

              Sending a signal to  the  owner  process  (group)  specified  by
              F_SETOWN  is  subject  to  the  same  permissions  checks as are
              described for kill(2), where the sending process is the one that
              employs F_SETOWN (but see BUGS below).  If this permission check
              fails,  then  the  signal  is  silently  discarded.   Note:  The
              F_SETOWN  operation records the caller's credentials at the time
              of the fcntl() call, and it is these saved credentials that  are
              used for the permission checks.

              If  the  file  descriptor  fd  refers to a socket, F_SETOWN also
              selects the recipient of SIGURG signals that are delivered  when
              out-of-band data arrives on that socket.  (SIGURG is sent in any
              situation where select(2) would report the socket as  having  an
              "exceptional condition".)

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

                     If a nonzero value is  given  to  F_SETSIG  in  a  multi-
                     threaded  process  running  with a threading library that
                     supports thread groups  (e.g.,  NPTL),  then  a  positive
                     value  given to F_SETOWN has a different meaning: instead
                     of being a process ID identifying a whole process, it  is
                     a  thread  ID  identifying  a  specific  thread  within a
                     process.  Consequently,  it  may  be  necessary  to  pass
                     F_SETOWN  the result of gettid(2) instead of getpid(2) to
                     get sensible results when F_SETSIG is used.  (In  current
                     Linux  threading  implementations, a main thread's thread
                     ID is the same as its process ID.  This means that a sin-
                     gle-threaded  program  can  equally use gettid(2) or get-
                     pid(2) in this scenario.)  Note, however, that the state-
                     ments in this paragraph do not apply to the SIGURG signal
                     generated for out-of-band data on a socket:  this  signal
                     is  always  sent  to either a process or a process group,
                     depending on the value given to F_SETOWN.

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

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

                  struct f_owner_ex {
                      int   type;
                      pid_t pid;
                  };

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

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

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

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

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

       F_GETSIG (void)
              Return (as the function result) the signal sent  when  input  or
              output  becomes  possible.  A value of zero means SIGIO is sent.
              Any other value (including SIGIO) is the  signal  sent  instead,
              and in this case additional info is available to the signal han-
              dler if installed with SA_SIGINFO.  arg is ignored.

       F_SETSIG (int)
              Set the signal sent when input or output becomes possible to the
              value  given  in arg.  A value of zero means to send the default
              SIGIO signal.  Any other value (including SIGIO) is  the  signal
              to  send  instead, and in this case additional info is available
              to the signal handler if installed with SA_SIGINFO.

              By using F_SETSIG with a nonzero value, and  setting  SA_SIGINFO
              for  the  signal  handler  (see sigaction(2)), extra information
              about I/O events is passed to the handler in a siginfo_t  struc-
              ture.   If  the  si_code field indicates the source is SI_SIGIO,
              the si_fd field gives the file descriptor  associated  with  the
              event.  Otherwise, there is no indication which file descriptors
              are pending, and you should use the usual mechanisms (select(2),
              poll(2),  read(2)  with  O_NONBLOCK set etc.) to determine which
              file descriptors are available for I/O.

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

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

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

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

       The  use  of  O_ASYNC  is  specific  to BSD and Linux.  The only use of
       F_GETOWN and F_SETOWN specified in POSIX.1 is in conjunction  with  the
       use of the SIGURG signal on sockets.  (POSIX does not specify the SIGIO
       signal.)  F_GETOWN_EX, F_SETOWN_EX, F_GETSIG, and F_SETSIG  are  Linux-
       specific.  POSIX has asynchronous I/O and the aio_sigevent structure to
       achieve similar things; these are also available in Linux  as  part  of
       the GNU C Library (Glibc).

   Leases
       F_SETLEASE and F_GETLEASE (Linux 2.4 onward) are used (respectively) to
       establish a new lease, and retrieve the current lease, on the open file
       description  referred  to by the file descriptor fd.  A file lease pro-
       vides a mechanism whereby the process holding  the  lease  (the  "lease
       holder")  is  notified  (via  delivery of a signal) when a process (the
       "lease breaker") tries to open(2) or truncate(2) the file  referred  to
       by that file descriptor.

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

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

              F_WRLCK
                     Take out a write lease.  This will cause the caller to be
                     notified when the file is opened for reading  or  writing
                     or  is  truncated.  A write lease may be placed on a file
                     only if there are no other open file descriptors for  the
                     file.

              F_UNLCK
                     Remove our lease from the file.

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

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

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

       When a process (the "lease breaker") performs an open(2) or truncate(2)
       that conflicts with a lease established via F_SETLEASE, the system call
       is  blocked  by  the kernel and the kernel notifies the lease holder by
       sending it a signal  (SIGIO  by  default).   The  lease  holder  should
       respond to receipt of this signal by doing whatever cleanup is required
       in preparation for the file to be accessed by  another  process  (e.g.,
       flushing cached buffers) and then either remove or downgrade its lease.
       A lease is removed by performing an F_SETLEASE command  specifying  arg
       as  F_UNLCK.   If the lease holder currently holds a write lease on the
       file, and the lease breaker is opening the file for reading, then it is
       sufficient for the lease holder to downgrade the lease to a read lease.
       This is done by performing an  F_SETLEASE  command  specifying  arg  as
       F_RDLCK.

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

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

       Once  the lease has been voluntarily or forcibly removed or downgraded,
       and assuming the lease breaker has not unblocked its system  call,  the
       kernel permits the lease breaker's system call to proceed.

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

       The default signal used to notify the lease holder is SIGIO,  but  this
       can  be  changed  using the F_SETSIG command to fcntl().  If a F_SETSIG
       command is performed (even one specifying SIGIO), and the  signal  han-
       dler  is  established using SA_SIGINFO, then the handler will receive a
       siginfo_t structure as its second argument, and the si_fd field of this
       argument will hold the file descriptor of the leased file that has been
       accessed by another process.  (This  is  useful  if  the  caller  holds
       leases against multiple files.)

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

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

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

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

              A series of F_NOTIFY requests is cumulative, with the events  in
              arg  being added to the set already monitored.  To disable noti-
              fication of all events, make an F_NOTIFY call specifying arg  as
              0.

              Notification  occurs via delivery of a signal.  The default sig-
              nal is SIGIO, but this can be changed using the F_SETSIG command
              to  fcntl().  (Note that SIGIO is one of the nonqueuing standard
              signals; switching to the use of a real-time signal  means  that
              multiple  notifications  can  be queued to the process.)  In the
              latter case, the signal handler receives a  siginfo_t  structure
              as  its  second  argument  (if the handler was established using
              SA_SIGINFO) and the si_fd field of this structure  contains  the
              file  descriptor  which  generated the notification (useful when
              establishing notification on multiple directories).

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

              NOTE: New applications should use the inotify interface  (avail-
              able since kernel 2.6.13), which provides a much superior inter-
              face for obtaining notifications of filesystem events.  See ino-
              tify(7).

   Changing the capacity of a pipe
       F_SETPIPE_SZ (int; since Linux 2.6.35)
              Change the capacity of the pipe referred to by fd to be at least
              arg bytes.  An unprivileged process can adjust the pipe capacity
              to  any value between the system page size and the limit defined
              in /proc/sys/fs/pipe-max-size (see proc(5)).   Attempts  to  set
              the pipe capacity below the page size are silently rounded up to
              the page size.  Attempts by an unprivileged process to  set  the
              pipe  capacity  above  the  limit  in /proc/sys/fs/pipe-max-size
              yield the error EPERM; a privileged  process  (CAP_SYS_RESOURCE)
              can override the limit.

              When  allocating  the  buffer for the pipe, the kernel may use a
              capacity larger than arg, if that is convenient for  the  imple-
              mentation.   (In  the  current implementation, the allocation is
              the next higher power-of-two page-size multiple of the requested
              size.)   The  actual capacity (in bytes) that is set is returned
              as the function result.

              Attempting to set the pipe capacity smaller than the  amount  of
              buffer  space  currently  used  to store data produces the error
              EBUSY.

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

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

       Currently, 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 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.

   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
              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.  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
              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  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
                inode referred to by fd.

       All other commands
                Zero.

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

ERRORS
       EACCES or EAGAIN
              Operation is prohibited by locks held by other processes.

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

       EBADF  fd is not an open file descriptor

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

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

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

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

       EFAULT lock is outside your accessible address space.

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

       EINTR  cmd is F_GETLK, F_SETLK, F_OFD_GETLK, or  F_OFD_SETLK,  and  the
              operation  was  interrupted  by  a  signal  before  the lock was
              checked or acquired.  Most likely when  locking  a  remote  file
              (e.g., locking over NFS), but can sometimes happen locally.

       EINVAL The value specified in cmd is not recognized by this kernel.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       F_ADD_SEALS and F_GET_SEALS are Linux-specific.

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

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

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

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

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

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

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

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

   F_GETOWN
       A limitation of the Linux system call conventions on some architectures
       (notably i386) means that if  a  (negative)  process  group  ID  to  be
       returned  by  F_GETOWN  falls in the range -1 to -4095, then the return
       value is wrongly interpreted by glibc as an error in the  system  call;
       that is, the return value of fcntl() will be -1, and errno will contain
       the (positive) process group ID.  The Linux-specific F_GETOWN_EX opera-
       tion  avoids  this  problem.  Since glibc version 2.11, glibc makes the
       kernel  F_GETOWN  problem  invisible  by  implementing  F_GETOWN  using
       F_GETOWN_EX.

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

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

   Mandatory locking
       The Linux implementation of mandatory locking is subject to race condi-
       tions which render it unreliable: a write(2) call that overlaps with  a
       lock  may  modify  data after the mandatory lock is acquired; a read(2)
       call that overlaps with a lock may detect changes  to  data  that  were
       made only after a write lock was acquired.  Similar races exist between
       mandatory locks and mmap(2).  It is therefore inadvisable  to  rely  on
       mandatory locking.

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

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

COLOPHON
       This  page  is  part of release 4.15 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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