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.

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

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

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

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

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

   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
       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 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 4.13 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/.

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