xenial (2) fcntl.2.gz

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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)
              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 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 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.   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)
              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-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(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 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.

       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
              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 commonly, 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 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 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 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 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 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.
              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 purpose, and some code examples, see memfd_create(2).

       Currently,  only  the  tmpfs  filesystem supports sealing.  On other filesystems, all fcntl(2) 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(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 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.

              Setting  F_SEAL_WRITE  via  fcntl(2) with F_ADD_SEALS 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 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 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  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
       _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_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.

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

   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)

       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/.