oracular (2) fcntl.2.gz

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NAME

       fcntl - manipulate file descriptor

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <fcntl.h>

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

DESCRIPTION

       fcntl()  performs one of the operations described below on the open file descriptor fd.  The operation is
       determined by op.

       fcntl() can take an optional third argument.  Whether or not this argument is required is  determined  by
       op.   The  required  argument  type  is  indicated  in parentheses after each op 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 op value and then test whether the call failed with EINVAL, indicating that the kernel
       does not recognize this value.

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

              On success, the new file descriptor is returned.

              See dup(2) for further details.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Open file description locks solve both of these problems.

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

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

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

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

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

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

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

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

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

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

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

   Mandatory locking
       Warning: the Linux implementation of mandatory locking is unreliable.  See BUGS below.  Because of  these
       bugs, and the fact that the feature is believed to be little used, since Linux 4.5, mandatory locking has
       been made an optional feature, governed by a configuration option (CONFIG_MANDATORY_FILE_LOCKING).   This
       feature is no longer supported at all in Linux 5.15 and above.

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

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

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

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

   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 functioning.

       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 asynchronous notification of lost locks.

   Managing signals
       F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG, and F_SETSIG are used to manage I/O  availability
       signals:

       F_GETOWN (void)
              Return  (as  the function result) the process ID or process group ID currently receiving SIGIO and
              SIGURG signals for events on file descriptor fd.  Process IDs are  returned  as  positive  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 operation  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 operation 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 Linux 2.6.x up to and including Linux 2.6.11:

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

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

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

                  struct f_owner_ex {
                      int   type;
                      pid_t pid;
                  };

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              F_UNLCK
                     Remove our lease from the file.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              NOTE: New applications should use the inotify interface  (available  since  Linux  2.6.13),  which
              provides  a  much  superior  interface  for  obtaining  notifications  of  filesystem events.  See
              inotify(7).

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

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

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

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

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

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

       Currently, file seals can be applied only to a  file  descriptor  returned  by  memfd_create(2)  (if  the
       MFD_ALLOW_SEALING was employed).  On other filesystems, all fcntl() operations that operate on seals will
       return EINVAL.

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

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

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

       The following seals are available:

       F_SEAL_SEAL
              If this seal is set, any further call to fcntl() with F_ADD_SEALS  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
              combination with one of the other seals.  This seal affects write(2)  and  fallocate(2)  (only  in
              combination with the FALLOC_FL_PUNCH_HOLE flag).  Those calls 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.  Furthermore,
              if  there  are any asynchronous I/O operations (io_submit(2)) pending on the file, all outstanding
              writes will be discarded.

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

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

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

RETURN VALUE

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

       F_DUPFD
              The new file descriptor.

       F_GETFD
              Value of file descriptor flags.

       F_GETFL
              Value of file status flags.

       F_GETLEASE
              Type of lease held on file descriptor.

       F_GETOWN
              Value of file descriptor owner.

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

       F_GETPIPE_SZ
       F_SETPIPE_SZ
              The pipe capacity.

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

       All other operations
              Zero.

       On error, -1 is returned, and errno is set to indicate the error.

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

       EBUSY  op 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  op 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 operation would cause a deadlock.

       EFAULT lock is outside your accessible address space.

       EINTR  op is F_SETLKW or F_OFD_SETLKW and the operation was interrupted by a signal; see signal(7).

       EINTR  op 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 op is not recognized by this kernel.

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

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

       EINVAL op is F_DUPFD and arg is negative or  is  greater  than  the  maximum  allowable  value  (see  the
              discussion of RLIMIT_NOFILE in getrlimit(2)).

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

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

       EMFILE op 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 op, but fd does not refer to a directory.

       EPERM  op 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  op was F_ADD_SEALS, but fd was not open for writing or the  current  set  of  seals  on  the  file
              already includes F_SEAL_SEAL.

STANDARDS

       POSIX.1-2008.

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

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

       F_ADD_SEALS and F_GET_SEALS are Linux-specific.

HISTORY

       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.)

NOTES

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

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

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

       Several systems have more fields in struct flock such as, for example, l_sysid (to identify  the  machine
       where  the lock is held).  Clearly, l_pid alone is not going to be very useful if the process holding the
       lock may live on a different machine; on Linux, while present on some  architectures  (such  as  MIPS32),
       this field is not used.

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

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

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

BUGS

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

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

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

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

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

SEE ALSO

       dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), feature_test_macros(7), lslocks(8)

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