fcntl

       #include <unistd.h>
       #include <fcntl.h>

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

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

       fcntl() can take an optional third argument.  Whether or not this argu-
       ment  is  required is determined by cmd.  The required argument type is
       indicated in parentheses after  each  cmd  name  (in  most  cases,  the
       required type is int, and we identify the argument using the name arg),
       or void is specified if the argument is not required.

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

   Duplicating a file descriptor
       F_DUPFD (int)
              Find the lowest numbered available file descriptor greater  than
              or  equal to arg and make it be a copy of fd.  This is different
              from dup2(2), which uses exactly the descriptor specified.

              On success, the new 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  descriptor.  Specifying this flag permits a pro-
              gram 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 0,  the  file  descriptor
       will remain open across an execve(2), otherwise it will be closed.

       F_GETFD (void)
              Read 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.
       F_GETFL (void)
              Get the file access mode and  the  file  status  flags;  arg  is
              ignored.

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

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

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

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

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

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

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

       (Such conversions may involve splitting, shrinking, or coalescing  with
       an  existing  lock if the byte range specified by the new lock does not
       precisely coincide with the range of the existing lock.)

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

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

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

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

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

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

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

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

       Open file description locks solve both of these problems.

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

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

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

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

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

              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.

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

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

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

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

   Managing signals
              and  SIGURG  signals  for events on file descriptor fd to the ID
              given in arg.  A process ID is specified as a positive value;  a
              process  group  ID  is specified as a negative value.  Most com-
              monly, the calling process specifies itself as the  owner  (that
              is, arg is specified as getpid(2)).

              If you set the O_ASYNC status flag on a file descriptor by using
              the F_SETFL command of fcntl(), a SIGIO signal is sent  whenever
              input  or  output  becomes  possible  on  that  file descriptor.
              F_SETSIG can be used to obtain delivery of a signal  other  than
              SIGIO.   If  this  permission  check  fails,  then the signal is
              silently discarded.

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

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

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

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

       F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
              Return the current file descriptor owner settings as defined  by
              a  previous  F_SETOWN_EX operation.  The information is returned
              in the structure pointed to by  arg,  which  has  the  following
              form:
              This operation performs a similar task to F_SETOWN.   It  allows
              the  caller  to  direct  I/O  availability signals to a specific
              thread, process, or process group.   The  caller  specifies  the
              target  of  signals  via arg, which is a pointer to a f_owner_ex
              structure.  The type field has  one  of  the  following  values,
              which define how pid is interpreted:

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

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

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

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

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

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

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

              By  selecting  a  real time signal (value >= SIGRTMIN), multiple
              I/O events may be queued using the same signal numbers.   (Queu-

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

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

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

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

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

              F_UNLCK
                     Remove our lease from the file.

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

       sufficient for the lease holder to downgrade the lease to a read lease.
       This is done by performing an  F_SETLEASE  command  specifying  arg  as
       F_RDLCK.

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

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

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

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

       The default signal used to notify the lease holder is SIGIO,  but  this
       can  be  changed  using the F_SETSIG command to fcntl().  If a F_SETSIG
       command is performed (even one specifying SIGIO), and the  signal  han-
       dler  is  established using SA_SIGINFO, then the handler will receive a
       siginfo_t structure as its second argument, and the si_fd field of this
       argument  will  hold  the  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
              A series of F_NOTIFY requests is cumulative, with the events  in
              arg  being added to the set already monitored.  To disable noti-
              fication of all events, make an F_NOTIFY call specifying arg  as
              0.

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

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

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

   Changing the capacity of a pipe
       F_SETPIPE_SZ (int; since Linux 2.6.35)
              Change the capacity of the pipe referred to by fd to be at least
              arg bytes.  An unprivileged process can adjust the pipe capacity
              to  any value between the system page size and the limit defined
              in /proc/sys/fs/pipe-max-size (see proc(5)).   Attempts  to  set
              the pipe capacity below the page size are silently rounded up to
              the page size.  Attempts by an unprivileged process to  set  the
              pipe  capacity  above  the  limit  in /proc/sys/fs/pipe-max-size
              yield the error EPERM; a privileged  process  (CAP_SYS_RESOURCE)
              can  override  the  limit.   When  allocating the buffer for the
              pipe, the kernel may use a capacity larger than arg, if that  is
              convenient  for the implementation.  The actual capacity that is
              set is returned as the function result.  Attempting to  set  the
              pipe  capacity smaller than the amount of buffer space currently
              used to store data produces the error EBUSY.

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

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

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

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

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

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

              Setting F_SEAL_WRITE via fcntl(2)  with  F_ADD_SEALS  will  fail
              with  EBUSY  if  any writable, shared mapping exists.  Such map-
              pings must be unmapped before you can add this  seal.   Further-
              more,  if  there  are  any  asynchronous I/O operations (io_sub-
              mit(2)) pending on the file, all outstanding writes will be dis-
              carded.

RETURN VALUE

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

       F_GETPIPE_SZ, F_SETPIPE_SZ
                The pipe capacity.

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

       All other commands
                Zero.

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

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

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

       EBADF  fd is not an open file descriptor

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

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

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

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

       EFAULT lock is outside your accessible address space.

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

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

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

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

       EINVAL cmd  is F_ADD_SEALS or F_GET_SEALS and the filesystem containing
              file descriptors has been reached.

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

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

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

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

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

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

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

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

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

       F_ADD_SEALS and F_GET_SEALS are Linux-specific.

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

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

       The errors returned by dup2(2) are different  from  those  returned  by
       F_DUPFD.
       ferent structure for file locking, flock64, and corresponding commands,
       F_GETLK64, F_SETLK64, and F_SETLKW64.  However, these  details  can  be
       ignored  by  applications  using  glibc, whose fcntl() wrapper function
       transparently employs the more recent system call where  it  is  avail-
       able.

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

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

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

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

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

   Deadlock detection
       The  deadlock-detection  algorithm  employed by the kernel when dealing
       with F_SETLKW requests can yield  both  false  negatives  (failures  to
       call that overlaps with a lock may detect changes  to  data  that  were
       made only after a write lock was acquired.  Similar races exist between
       mandatory locks and mmap(2).  It is therefore inadvisable  to  rely  on
       mandatory locking.

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

       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 4.04 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
       http://www.kernel.org/doc/man-pages/.

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