fcntl64(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
   argument  is required is determined by cmd.  The required argument type
   is indicated in parentheses after each cmd name  (in  most  cases,  the
   required type is int, and we identify the argument using the name arg),
   or void is specified if the argument is not required.

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

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

          On success, the new file descriptor is returned.

          See dup(2) for further details.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Open file description locks solve both of these problems.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

          The following was true in 2.6.x  kernels  up  to  and  including
          kernel 2.6.11:

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

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

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

              struct f_owner_ex {
                  int   type;
                  pid_t pid;
              };

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

          F_UNLCK
                 Remove our lease from the file.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Currently, only the tmpfs(5) filesystem  supports  sealing.   On  other
   filesystems,  all  fcntl() operations that operate on seals will return
   EINVAL.

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

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

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

   The following seals are available:

   F_SEAL_SEAL
          If  this  seal  is  set,  any  further  call  to  fcntl()   with
          F_ADD_SEALS will fail with EPERM.  Therefore, this seal prevents
          any modifications to the set of seals itself.   If  the  initial
          set   of  seals  of  a  file  includes  F_SEAL_SEAL,  then  this
          effectively causes the set of seals to be constant and locked.

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

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

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

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

RETURN VALUE

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

   F_DUPFD  The new file descriptor.

   F_GETFD  Value of file descriptor flags.

   F_GETFL  Value of file status flags.

   F_GETLEASE
            Type of lease held on file descriptor.

   F_GETOWN Value of file descriptor owner.

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

   F_GETPIPE_SZ, F_SETPIPE_SZ
            The pipe capacity.

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

   All other commands
            Zero.

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

ERRORS

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

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

   EBADF  fd is not an open file descriptor

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

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

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

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

   EFAULT lock is outside your accessible address space.

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

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

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

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

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

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

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

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

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

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

   ENOTDIR
          F_NOTIFY  was  specified  in  cmd,  but  fd  does not refer to a
          directory.

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

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

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

CONFORMING TO

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

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

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

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

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

   F_ADD_SEALS and F_GET_SEALS are Linux-specific.

NOTES

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

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

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

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

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

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

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

BUGS

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

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

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

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

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

SEE ALSO

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

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

COLOPHON

   This  page  is  part of release 4.09 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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