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FCNTL(2)                            Linux Programmer's Manual                            FCNTL(2)



NAME
       fcntl - manipulate file descriptor

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

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

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

       fcntl() can take an optional third argument.  Whether or not this 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 ver‐
       sion.  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, shrink‐
       ing, 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 inter‐
              rupted 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-asso‐
              ciated)  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 pro‐
       cesses 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 pro‐
       cesses 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.  For an explanation of open file descrip‐
       tions, 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.

       Open file description locks always conflict with traditional record locks, 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  conver‐
       sions 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 pro‐
       gram 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  speci‐
              fied  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  descrip‐
       tion  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 speci‐
              fied as getpid(2)).

              If  you  set the O_ASYNC status flag on a file descriptor by using the F_SETFL com‐
              mand 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 spe‐
                     cific  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 sce‐
                     nario.)  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 possi‐
              ble.   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  sig‐
              nal 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  han‐
              dler  (see  sigaction(2)), extra information about I/O events is passed to the han‐
              dler 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.

              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_SET‐
       SIG 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 dupli‐
       cate 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.  Fur‐
       thermore, 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  suf‐
       ficient for the lease holder to downgrade the lease to a read lease.  This is done by per‐
       forming 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 follow‐
              ing 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 reregis‐
              ter 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 estab‐
              lishing notification on multiple directories).

              Especially when using DN_MULTISHOT, a real time signal should be used for notifica‐
              tion, 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.

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.

       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, or the command was F_SETLK or F_SETLKW and the
              file descriptor open mode doesn't match with the type of lock requested.

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

       EFAULT lock is outside your accessible address space.

       EINTR  For F_SETLKW, the command was interrupted by a signal; see signal(7).  For  F_GETLK
              and F_SETLK, the command 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 For  F_DUPFD,  arg is negative or is greater than the maximum allowable value.  For
              F_SETSIG, 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 For F_DUPFD, the process already has the maximum number of file descriptors open.

       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.

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.

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 correspond‐
       ing 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 correspond‐
       ing 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 sys‐
       tem 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 prob‐
       lem.  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 pro‐
       cesses 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  manda‐
       tory 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  manda‐
       tory 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  Documen‐
       tation/ directory, and mandatory-locking.txt is called mandatory.txt)

COLOPHON
       This  page  is  part of release 3.74 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/.



Linux                                       2014-09-06                                   FCNTL(2)


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