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man:fcntl

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 argu-
     ment  is  required is determined by cmd.  The required argument type is
     indicated in parentheses after  each  cmd  name  (in  most  cases,  the
     required type is int, and we identify the argument using the name arg),
     or void is specified if the argument is not required.
     Certain of the operations below are supported only since  a  particular
     Linux  kernel  version.   The  preferred method of checking whether the
     host kernel supports a particular operation is to invoke  fcntl()  with
     the  desired  cmd value and then test whether the call failed with EIN-
     VAL, indicating that the kernel does not recognize this value.
 Duplicating a file descriptor
     F_DUPFD (int)
            Duplicate the  file  descriptor  fd  using  the  lowest-numbered
            available file descriptor greater than or equal to arg.  This is
            different from dup2(2), which uses exactly the  file  descriptor
            specified.
            On success, the new file descriptor is returned.
            See dup(2) for further details.
     F_DUPFD_CLOEXEC (int; since Linux 2.6.24)
            As  for F_DUPFD, but additionally set the close-on-exec flag for
            the duplicate file descriptor.  Specifying this flag  permits  a
            program  to avoid an additional fcntl() F_SETFD operation to set
            the FD_CLOEXEC flag.  For an explanation of  why  this  flag  is
            useful, see the description of O_CLOEXEC in open(2).
 File descriptor flags
     The  following  commands  manipulate  the  flags associated with a file
     descriptor.  Currently, only one such flag is defined: FD_CLOEXEC,  the
     close-on-exec  flag.  If the FD_CLOEXEC bit is set, the file descriptor
     will automatically be closed during a successful  execve(2).   (If  the
     execve(2)  fails, the file descriptor is left open.)  If the FD_CLOEXEC
     bit is not  set,  the  file  descriptor  will  remain  open  across  an
     execve(2).
     F_GETFD (void)
            Return  (as  the function result) the file descriptor flags; arg
            is ignored.
     F_SETFD (int)
            Set the file descriptor flags to the value specified by arg.
     In multithreaded programs, using fcntl() F_SETFD to set  the  close-on-
     exec  flag  at  the same time as another thread performs a fork(2) plus
     execve(2) is vulnerable to a race condition  that  may  unintentionally
     leak  the file descriptor to the program executed in the child process.
     See the discussion of the O_CLOEXEC flag in open(2) for details  and  a
     remedy to the problem.
 File status flags
     Each  open  file  description has certain associated status flags, ini-
     tialized by open(2) and possibly modified by fcntl().  Duplicated  file
     descriptors  (made with dup(2), fcntl(F_DUPFD), fork(2), etc.) refer to
     the same open file description, and thus share  the  same  file  status
     flags.
     The file status flags and their semantics are described in open(2).
     F_GETFL (void)
            Return  (as  the  function  result) the file access mode and the
            file status flags; arg is ignored.
     F_SETFL (int)
            Set the file status flags to the value specified by  arg.   File
            access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags
            (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg  are  ignored.
            On  Linux,  this  command can change only the O_APPEND, O_ASYNC,
            O_DIRECT, O_NOATIME, and O_NONBLOCK flags.  It is  not  possible
            to change the O_DSYNC and O_SYNC flags; see BUGS, below.
 Advisory record locking
     Linux  implements traditional ("process-associated") UNIX record locks,
     as standardized by POSIX.  For a Linux-specific alternative with better
     semantics, see the discussion of open file description locks below.
     F_SETLK,  F_SETLKW,  and F_GETLK are used to acquire, release, and test
     for the existence of record locks (also known as byte-range,  file-seg-
     ment, or file-region locks).  The third argument, lock, is a pointer to
     a structure that has at least  the  following  fields  (in  unspecified
     order).
         struct flock {
             ...
             short l_type;    /* Type of lock: F_RDLCK,
                                 F_WRLCK, F_UNLCK */
             short l_whence;  /* How to interpret l_start:
                                 SEEK_SET, SEEK_CUR, SEEK_END */
             off_t l_start;   /* Starting offset for lock */
             off_t l_len;     /* Number of bytes to lock */
             pid_t l_pid;     /* PID of process blocking our lock
                                 (set by F_GETLK and F_OFD_GETLK) */
             ...  };
     The  l_whence,  l_start, and l_len fields of this structure specify the
     range of bytes we wish to lock.  Bytes past the end of the file may  be
     locked, but not bytes before the start of the file.
     l_start  is  the starting offset for the lock, and is interpreted rela-
     tive to either: the start of the file (if l_whence  is  SEEK_SET);  the
     current  file  offset (if l_whence is SEEK_CUR); or the end of the file
     (if l_whence is SEEK_END).  In the final two cases, l_start  can  be  a
     negative  number  provided  the offset does not lie before the start of
     the file.
     l_len specifies the number of bytes to be locked.  If  l_len  is  posi-
     tive,  then  the  range  to  be  locked  covers bytes l_start up to and
     including l_start+l_len-1.  Specifying 0  for  l_len  has  the  special
     meaning:  lock all bytes starting at the location specified by l_whence
     and l_start through to the end of file, no matter how  large  the  file
     grows.
     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 sig-
            nal handler has returned) returns immediately (with return value
            -1 and errno set to EINTR; see signal(7)).
     F_GETLK (struct flock *)
            On  input  to  this call, lock describes a lock we would like to
            place on the file.  If the lock could be  placed,  fcntl()  does
            not  actually  place it, but returns F_UNLCK in the l_type field
            of lock and leaves the other fields of the structure  unchanged.
            If  one or more incompatible locks would prevent this lock being
            placed, then fcntl() returns details about one of those locks in
            the l_type, l_whence, l_start, and l_len fields of lock.  If the
            conflicting lock is a  traditional  (process-associated)  record
            lock,  then  the  l_pid  field  is set to the PID of the process
            holding that lock.  If the conflicting  lock  is  an  open  file
            description  lock,  then  l_pid  is  set  to  -1.  Note that the
            returned information may already be out of date by the time  the
            caller inspects it.
     In  order  to place a read lock, fd must be open for reading.  In order
     to place a write lock, fd must be open  for  writing.   To  place  both
     types of lock, open a file read-write.
     When placing locks with F_SETLKW, the kernel detects deadlocks, whereby
     two or more processes have their  lock  requests  mutually  blocked  by
     locks  held  by  the  other  processes.  For example, suppose process A
     holds a write lock on byte 100 of a file, and process B holds  a  write
     lock  on  byte  200.   If  each  process then attempts to lock the byte
     already locked by the other process using F_SETLKW, then, without dead-
     lock detection, both processes would remain blocked indefinitely.  When
     the kernel detects such deadlocks, it causes one of the  blocking  lock
     requests  to  immediately  fail  with the error EDEADLK; an application
     that encounters such an error should release some of its locks to allow
     other  applications  to proceed before attempting regain the locks that
     it requires.  Circular deadlocks involving more than two processes  are
     also  detected.   Note, however, that there are limitations to the ker-
     nel's deadlock-detection algorithm; see BUGS.
     As well as being removed by an explicit F_UNLCK, record locks are auto-
     matically 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 multi-

threaded program can't use record locking to ensure that threads

        don't simultaneously access the same region of a file.
     Open file description locks solve both of these problems.
 Open file description locks (non-POSIX)
     Open  file description locks are advisory byte-range locks whose opera-
     tion is in most respects identical  to  the  traditional  record  locks
     described above.  This lock type is Linux-specific, and available since
     Linux 3.15.  (There is a proposal with the Austin Group to include this
     lock type in the next revision of POSIX.1.)  For an explanation of open
     file descriptions, see open(2).
     The principal difference between the two lock  types  is  that  whereas
     traditional  record  locks  are  associated  with  a process, open file
     description locks are associated with  the  open  file  description  on
     which  they are acquired, much like locks acquired with flock(2).  Con-
     sequently (and unlike traditional advisory  record  locks),  open  file
     description  locks  are  inherited  across  fork(2)  (and clone(2) with
     CLONE_FILES), and are only automatically released on the last close  of
     the  open  file  description, instead of being released on any close of
     the file.
     Conflicting lock combinations (i.e., a read lock and a  write  lock  or
     two  write  locks)  where one lock is an open file description lock and
     the other is a traditional record lock  conflict  even  when  they  are
     acquired by the same process on the same file descriptor.
     Open  file  description locks placed via the same open file description
     (i.e., via the same file descriptor, or via a  duplicate  of  the  file
     descriptor  created by fork(2), dup(2), fcntl() F_DUPFD, and so on) are
     always compatible: if a new lock is placed on an already locked region,
     then  the  existing lock is converted to the new lock type.  (Such con-
     versions may result in splitting,  shrinking,  or  coalescing  with  an
     existing lock as discussed above.)
     On  the  other hand, open file description locks may conflict with each
     other when they are acquired  via  different  open  file  descriptions.
     Thus, the threads in a multithreaded program can use open file descrip-
     tion locks to synchronize access to a file region by having each thread
     perform  its own open(2) on the file and applying locks via the result-
     ing file descriptor.
     As with traditional advisory locks,  the  third  argument  to  fcntl(),
     lock, is a pointer to an flock structure.  By contrast with traditional
     record locks, the l_pid field of that structure must  be  set  to  zero
     when using the commands described below.
     The commands for working with open file description locks are analogous
     to those used with traditional locks:
     F_OFD_SETLK (struct flock *)
            Acquire an open file description lock (when l_type is F_RDLCK or
            F_WRLCK)  or  release an open file description lock (when l_type
            is F_UNLCK) on the bytes specified by the l_whence, l_start, and
            l_len  fields of lock.  If a conflicting lock is held by another
            process, this call returns -1 and sets errno to EAGAIN.
     F_OFD_SETLKW (struct flock *)
            As for F_OFD_SETLK, but if a conflicting lock  is  held  on  the
            file,  then  wait  for that lock to be released.  If a signal is
            caught while waiting, then the call is  interrupted  and  (after
            the  signal  handler  has  returned)  returns  immediately (with
            return value -1 and errno set to EINTR; see signal(7)).
     F_OFD_GETLK (struct flock *)
            On input to this call, lock describes an open  file  description
            lock  we  would like to place on the file.  If the lock could be
            placed, fcntl() does not actually place it, but returns  F_UNLCK
            in  the  l_type field of lock and leaves the other fields of the
            structure unchanged.  If one or more  incompatible  locks  would
            prevent  this lock being placed, then details about one of these
            locks are returned via lock, as described above for F_GETLK.
     In the current implementation, no deadlock detection is  performed  for
     open  file  description locks.  (This contrasts with process-associated
     record locks, for which the kernel does perform deadlock detection.)
 Mandatory locking
     Warning: the Linux implementation of mandatory locking  is  unreliable.
     See  BUGS  below.  Because of these bugs, and the fact that the feature
     is believed to be little used, since Linux 4.5, mandatory  locking  has
     been made an optional feature, governed by a configuration option (CON-
     FIG_MANDATORY_FILE_LOCKING).  This is an initial step  toward  removing
     this feature completely.
     By   default,  both  traditional  (process-associated)  and  open  file
     description record locks are advisory.  Advisory locks are not enforced
     and are useful only between cooperating processes.
     Both  lock  types  can also be mandatory.  Mandatory locks are enforced
     for all processes.  If a  process  tries  to  perform  an  incompatible
     access  (e.g., read(2) or write(2)) on a file region that has an incom-
     patible mandatory lock, then the result depends upon whether the O_NON-
     BLOCK flag is enabled for its open file description.  If the O_NONBLOCK
     flag is not enabled, then the system call is blocked until the lock  is
     removed  or converted to a mode that is compatible with the access.  If
     the O_NONBLOCK flag is enabled, then the system  call  fails  with  the
     error EAGAIN.
     To  make use of mandatory locks, mandatory locking must be enabled both
     on the filesystem that contains the file to be locked, and on the  file
     itself.   Mandatory  locking  is  enabled on a filesystem using the "-o
     mand" option to mount(8), or the MS_MANDLOCK flag for mount(2).  Manda-
     tory locking is enabled on a file by disabling group execute permission
     on the file and enabling the set-group-ID permission bit (see  chmod(1)
     and chmod(2)).
     Mandatory  locking  is not specified by POSIX.  Some other systems also
     support mandatory locking, although the details of  how  to  enable  it
     vary across systems.
 Lost locks
     When an advisory lock is obtained on a networked filesystem such as NFS
     it is possible that the lock might get lost.  This may  happen  due  to
     administrative  action  on  the  server,  or due to a network partition
     (i.e., loss of network connectivity with the server) which  lasts  long
     enough  for the server to assume that the client is no longer function-
     ing.
     When the filesystem determines  that  a  lock  has  been  lost,  future
     read(2)  or  write(2) requests may fail with the error EIO.  This error
     will persist until the lock  is  removed  or  the  file  descriptor  is
     closed.   Since  Linux 3.12, this happens at least for NFSv4 (including
     all minor versions).
     Some versions of UNIX send a signal  (SIGLOST)  in  this  circumstance.
     Linux  does  not define this signal, and does not provide any asynchro-
     nous notification of lost locks.
 Managing signals
     F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG and F_SETSIG are
     used to manage I/O availability signals:
     F_GETOWN (void)
            Return  (as the function result) the process ID or process group
            currently receiving SIGIO and SIGURG signals for events on  file
            descriptor  fd.   Process  IDs  are returned as positive values;
            process group IDs are returned as negative values (but see  BUGS
            below).  arg is ignored.
     F_SETOWN (int)
            Set  the  process ID or process group ID that will receive SIGIO
            and SIGURG signals for events on the file  descriptor  fd.   The
            target  process  or  process  group  ID  is specified in arg.  A
            process ID is specified as a positive value; a process group  ID
            is  specified  as  a negative value.  Most commonly, the calling
            process specifies itself as the owner (that is, arg is specified
            as getpid(2)).
            As  well  as  setting  the  file descriptor owner, one must also
            enable generation of signals on the file  descriptor.   This  is
            done  by  using  the  fcntl() F_SETFL command to set the O_ASYNC
            file status flag on the file descriptor.  Subsequently, a  SIGIO
            signal  is sent whenever input or output becomes possible on the
            file descriptor.  The fcntl() F_SETSIG command can  be  used  to
            obtain delivery of a signal other than SIGIO.
            Sending  a  signal  to  the  owner  process (group) specified by
            F_SETOWN is subject  to  the  same  permissions  checks  as  are
            described for kill(2), where the sending process is the one that
            employs F_SETOWN (but see BUGS below).  If this permission check
            fails,  then  the  signal  is  silently  discarded.   Note:  The
            F_SETOWN operation records the caller's credentials at the  time
            of  the fcntl() call, and it is these saved credentials that are
            used for the permission checks.
            If the file descriptor fd refers  to  a  socket,  F_SETOWN  also
            selects  the recipient of SIGURG signals that are delivered when
            out-of-band data arrives on that socket.  (SIGURG is sent in any
            situation  where  select(2) would report the socket as having an
            "exceptional condition".)
            The following was true in 2.6.x kernels up to and including ker-
            nel 2.6.11:
                   If  a  nonzero  value  is  given  to F_SETSIG in a multi-
                   threaded process running with a  threading  library  that
                   supports  thread  groups  (e.g.,  NPTL),  then a positive
                   value given to F_SETOWN has a different meaning:  instead
                   of  being a process ID identifying a whole process, it is
                   a thread  ID  identifying  a  specific  thread  within  a
                   process.   Consequently,  it  may  be  necessary  to pass
                   F_SETOWN the result of gettid(2) instead of getpid(2)  to
                   get  sensible results when F_SETSIG is used.  (In current
                   Linux threading implementations, a main  thread's  thread
                   ID is the same as its process ID.  This means that a sin-
                   gle-threaded program can equally use  gettid(2)  or  get-
                   pid(2) in this scenario.)  Note, however, that the state-
                   ments in this paragraph do not apply to the SIGURG signal
                   generated  for  out-of-band data on a socket: this signal
                   is always sent to either a process or  a  process  group,
                   depending on the value given to F_SETOWN.
            The above behavior was accidentally dropped in Linux 2.6.12, and
            won't be restored.  From Linux 2.6.32 onward, use F_SETOWN_EX to
            target SIGIO and SIGURG signals at a particular thread.
     F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
            Return  the current file descriptor owner settings as defined by
            a previous F_SETOWN_EX operation.  The information  is  returned
            in  the  structure  pointed  to  by arg, which has the following
            form:
                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 inte-
            ger 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 speci-
                   fied in pid.  (Note that, unlike with F_SETOWN, a process
                   group ID is specified as a positive value here.)
     F_GETSIG (void)
            Return  (as  the  function result) the signal sent when input or
            output becomes possible.  A value of zero means SIGIO  is  sent.
            Any  other  value  (including SIGIO) is the signal sent instead,
            and in this case additional info is available to the signal han-
            dler if installed with SA_SIGINFO.  arg is ignored.
     F_SETSIG (int)
            Set the signal sent when input or output becomes possible to the
            value given in arg.  A value of zero means to send  the  default
            SIGIO  signal.   Any other value (including SIGIO) is the signal
            to send instead, and in this case additional info  is  available
            to the signal handler if installed with SA_SIGINFO.
            By  using  F_SETSIG with a nonzero value, and setting SA_SIGINFO
            for the signal handler  (see  sigaction(2)),  extra  information
            about  I/O events is passed to the handler in a siginfo_t struc-
            ture.  If the si_code field indicates the  source  is  SI_SIGIO,
            the  si_fd  field  gives the file descriptor associated with the
            event.  Otherwise, there is no indication which file descriptors
            are pending, and you should use the usual mechanisms (select(2),
            poll(2), read(2) with O_NONBLOCK set etc.)  to  determine  which
            file descriptors are available for I/O.
            Note  that the file descriptor provided in si_fd is the one that
            was specified during the F_SETSIG operation.  This can  lead  to
            an  unusual  corner  case.  If the file descriptor is duplicated
            (dup(2) or similar), and the original file descriptor is closed,
            then  I/O  events  will  continue to be generated, but the si_fd
            field will contain the number of the now closed file descriptor.
            By  selecting  a  real time signal (value >= SIGRTMIN), multiple
            I/O events may be queued using the same signal numbers.   (Queu-
            ing  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 sig-
            nals that may be queued to a process (see getrlimit(2) and  sig-
            nal(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  pro-
     vides  a  mechanism  whereby  the process holding the lease (the "lease
     holder") is notified (via delivery of a signal)  when  a  process  (the
     "lease  breaker")  tries to open(2) or truncate(2) the file referred to
     by that file descriptor.
     F_SETLEASE (int)
            Set or remove a file lease according to which of  the  following
            values is specified in the integer arg:
            F_RDLCK
                   Take  out  a  read  lease.   This  will cause the calling
                   process to be notified when the file is opened for  writ-
                   ing  or is truncated.  A read lease can be placed only on
                   a file descriptor that is opened read-only.
            F_WRLCK
                   Take out a write lease.  This will cause the caller to be
                   notified  when  the file is opened for reading or writing
                   or is truncated.  A write lease may be placed on  a  file
                   only  if there are no other open file descriptors for the
                   file.
            F_UNLCK
                   Remove our lease from the file.
     Leases are associated with an  open  file  description  (see  open(2)).
     This  means  that  duplicate file descriptors (created by, for example,
     fork(2) or dup(2)) refer to the same lease, and this lease may be modi-
     fied  or  released  using  any  of these descriptors.  Furthermore, the
     lease is released by either an explicit F_UNLCK  operation  on  any  of
     these  duplicate  file  descriptors,  or when all such file descriptors
     have been closed.
     Leases may be taken out only on regular files.  An unprivileged process
     may  take  out  a  lease  only  on a file whose UID (owner) matches the
     filesystem UID of the process.  A process with the CAP_LEASE capability
     may take out leases on arbitrary files.
     F_GETLEASE (void)
            Indicates  what  type  of  lease  is  associated  with  the file
            descriptor fd by returning either F_RDLCK, F_WRLCK, or  F_UNLCK,
            indicating,  respectively,  a  read lease , a write lease, or no
            lease.  arg is ignored.
     When a process (the "lease breaker") performs an open(2) or truncate(2)
     that conflicts with a lease established via F_SETLEASE, the system call
     is blocked by the kernel and the kernel notifies the  lease  holder  by
     sending  it  a  signal  (SIGIO  by  default).   The lease holder should
     respond to receipt of this signal by doing whatever cleanup is required
     in  preparation  for  the file to be accessed by another process (e.g.,
     flushing cached buffers) and then either remove or downgrade its lease.
     A  lease  is removed by performing an F_SETLEASE command specifying arg
     as F_UNLCK.  If the lease holder currently holds a write lease  on  the
     file, and the lease breaker is opening the file for reading, then it is
     sufficient for the lease holder to downgrade the lease to a read lease.
     This  is  done  by  performing  an F_SETLEASE command specifying arg as
     F_RDLCK.
     If the lease holder fails to downgrade or remove the lease  within  the
     number  of seconds specified in /proc/sys/fs/lease-break-time, then the
     kernel forcibly removes or downgrades the lease holder's lease.
     Once a lease break has been initiated, F_GETLEASE  returns  the  target
     lease  type (either F_RDLCK or F_UNLCK, depending on what would be com-
     patible with the lease breaker)  until  the  lease  holder  voluntarily
     downgrades  or  removes  the lease or the kernel forcibly does so after
     the lease break timer expires.
     Once the lease has been voluntarily or forcibly removed or  downgraded,
     and  assuming  the lease breaker has not unblocked its system call, the
     kernel permits the lease breaker's system call to proceed.
     If the lease breaker's blocked open(2) or truncate(2) is interrupted by
     a  signal handler, then the system call fails with the error EINTR, but
     the other steps still occur as described above.  If the  lease  breaker
     is killed by a signal while blocked in open(2) or truncate(2), then the
     other steps still occur as described above.  If the lease breaker spec-
     ifies  the  O_NONBLOCK flag when calling open(2), then the call immedi-
     ately fails with the error EWOULDBLOCK, but the other steps still occur
     as described above.
     The  default  signal used to notify the lease holder is SIGIO, but this
     can be changed using the F_SETSIG command to fcntl().   If  a  F_SETSIG
     command  is  performed (even one specifying SIGIO), and the signal han-
     dler is established using SA_SIGINFO, then the handler will  receive  a
     siginfo_t structure as its second argument, and the si_fd field of this
     argument will hold the file descriptor of the leased file that has been
     accessed  by  another  process.   (This  is  useful if the caller holds
     leases against multiple files.)
 File and directory change notification (dnotify)
     F_NOTIFY (int)
            (Linux 2.4  onward)  Provide  notification  when  the  directory
            referred  to  by  fd  or  any  of  the files that it contains is
            changed.  The events to be notified are specified in arg,  which
            is  a  bit  mask specified by ORing together zero or more of the
            following bits:
            DN_ACCESS   A file was accessed  (read(2),  pread(2),  readv(2),
                        and similar)
            DN_MODIFY   A file was modified (write(2), pwrite(2), writev(2),
                        truncate(2), ftruncate(2), and similar).
            DN_CREATE   A file was  created  (open(2),  creat(2),  mknod(2),
                        mkdir(2),  link(2),  symlink(2), rename(2) into this
                        directory).
            DN_DELETE   A file was unlinked (unlink(2), rename(2) to another
                        directory, rmdir(2)).
            DN_RENAME   A   file   was   renamed   within   this   directory
                        (rename(2)).
            DN_ATTRIB   The attributes of a  file  were  changed  (chown(2),
                        chmod(2), utime(2), utimensat(2), and similar).
            (In  order  to obtain these definitions, the _GNU_SOURCE feature
            test macro must be defined before including any header files.)
            Directory notifications are normally "one-shot", and the  appli-
            cation must reregister to receive further notifications.  Alter-
            natively, 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  noti-
            fication  of all events, make an F_NOTIFY call specifying arg as
            0.
            Notification occurs via delivery of a signal.  The default  sig-
            nal is SIGIO, but this can be changed using the F_SETSIG command
            to fcntl().  (Note that SIGIO is one of the nonqueuing  standard
            signals;  switching  to the use of a real-time signal means that
            multiple notifications can be queued to the  process.)   In  the
            latter  case,  the signal handler receives a siginfo_t structure
            as its second argument (if the  handler  was  established  using
            SA_SIGINFO)  and  the si_fd field of this structure contains the
            file descriptor which generated the  notification  (useful  when
            establishing notification on multiple directories).
            Especially when using DN_MULTISHOT, a real time signal should be
            used for notification, so that  multiple  notifications  can  be
            queued.
            NOTE:  New applications should use the inotify interface (avail-
            able since kernel 2.6.13), which provides a much superior inter-
            face for obtaining notifications of filesystem events.  See ino-
            tify(7).
 Changing the capacity of a pipe
     F_SETPIPE_SZ (int; since Linux 2.6.35)
            Change the capacity of the pipe referred to by fd to be at least
            arg bytes.  An unprivileged process can adjust the pipe capacity
            to any value between the system page size and the limit  defined
            in  /proc/sys/fs/pipe-max-size  (see  proc(5)).  Attempts to set
            the pipe capacity below the page size are silently rounded up to
            the  page  size.  Attempts by an unprivileged process to set the
            pipe capacity  above  the  limit  in  /proc/sys/fs/pipe-max-size
            yield  the  error EPERM; a privileged process (CAP_SYS_RESOURCE)
            can override the limit.
            When allocating the buffer for the pipe, the kernel  may  use  a
            capacity  larger  than arg, if that is convenient for the imple-
            mentation.  (In the current implementation,  the  allocation  is
            the next higher power-of-two page-size multiple of the requested
            size.)  The actual capacity (in bytes) that is set  is  returned
            as the function result.
            Attempting  to  set the pipe capacity smaller than the amount of
            buffer space currently used to store  data  produces  the  error
            EBUSY.
     F_GETPIPE_SZ (void; since Linux 2.6.35)
            Return  (as  the  function  result)  the  capacity  of  the pipe
            referred to by fd.
 File Sealing
     File seals limit the set of allowed operations on a  given  file.   For
     each seal that is set on a file, a specific set of operations will fail
     with EPERM on this file from now on.  The file is said  to  be  sealed.
     The default set of seals depends on the type of the underlying file and
     filesystem.  For an overview of file sealing, a discussion of its  pur-
     pose, and some code examples, see memfd_create(2).
     Currently, file seals can be applied only to a file descriptor returned
     by memfd_create(2) (if the MFD_ALLOW_SEALING was employed).   On  other
     filesystems,  all  fcntl() operations that operate on seals will return
     EINVAL.
     Seals are a property of an inode.   Thus,  all  open  file  descriptors
     referring  to the same inode share the same set of seals.  Furthermore,
     seals can never be removed, only added.
     F_ADD_SEALS (int; since Linux 3.17)
            Add the seals given in the bit-mask argument arg to the  set  of
            seals of the inode referred to by the file descriptor fd.  Seals
            cannot be removed again.  Once this call succeeds, the seals are
            enforced by the kernel immediately.  If the current set of seals
            includes  F_SEAL_SEAL  (see  below),  then  this  call  will  be
            rejected with EPERM.  Adding a seal that is already set is a no-
            op, in case F_SEAL_SEAL is not set already.  In order to place a
            seal, the file descriptor fd must be writable.
     F_GET_SEALS (void; since Linux 3.17)
            Return  (as the function result) the current set of seals of the
            inode referred to by fd.  If no seals are set,  0  is  returned.
            If  the  file does not support sealing, -1 is returned and errno
            is set to EINVAL.
     The following seals are available:
     F_SEAL_SEAL
            If  this  seal  is  set,  any  further  call  to  fcntl()   with
            F_ADD_SEALS  fails  with  the error EPERM.  Therefore, this seal
            prevents any modifications to the set of seals itself.   If  the
            initial  set  of seals of a file includes F_SEAL_SEAL, then this
            effectively causes the set of seals to be constant and locked.
     F_SEAL_SHRINK
            If this seal is set, the file in question cannot be  reduced  in
            size.   This  affects  open(2)  with the O_TRUNC flag as well as
            truncate(2) and ftruncate(2).  Those calls fail  with  EPERM  if
            you  try  to  shrink  the file in question.  Increasing the file
            size is still possible.
     F_SEAL_GROW
            If this seal is set, the size of the file in question cannot  be
            increased.   This  affects  write(2) beyond the end of the file,
            truncate(2), ftruncate(2), and fallocate(2).  These  calls  fail
            with  EPERM  if  you use them to increase the file size.  If you
            keep the size or shrink it, those calls still work as  expected.
     F_SEAL_WRITE
            If this seal is set, you cannot modify the contents of the file.
            Note that shrinking or growing the size of  the  file  is  still
            possible  and allowed.  Thus, this seal is normally used in com-
            bination with  one  of  the  other  seals.   This  seal  affects
            write(2)  and  fallocate(2)  (only  in combination with the FAL-
            LOC_FL_PUNCH_HOLE flag).  Those calls fail with  EPERM  if  this
            seal is set.  Furthermore, trying to create new shared, writable
            memory-mappings via mmap(2) will also fail with EPERM.
            Using the F_ADD_SEALS operation to  set  the  F_SEAL_WRITE  seal
            fails  with  EBUSY if any writable, shared mapping exists.  Such
            mappings must be unmapped before you can add  this  seal.   Fur-
            thermore,  if there are any asynchronous I/O operations (io_sub-
            mit(2)) pending on the file, all outstanding writes will be dis-
            carded.
 File read/write hints
     Write  lifetime  hints can be used to inform the kernel about the rela-
     tive expected lifetime of writes on a given inode or via  a  particular
     open  file  description.   (See open(2) for an explanation of open file
     descriptions.)  In this context, the term "write  lifetime"  means  the
     expected  time the data will live on media, before being overwritten or
     erased.
     An application may use the different hint  values  specified  below  to
     separate writes into different write classes, so that multiple users or
     applications running on a single storage back-end can  aggregate  their
     I/O  patterns in a consistent manner.  However, there are no functional
     semantics implied by these flags, and different I/O classes can use the
     write  lifetime  hints in arbitrary ways, so long as the hints are used
     consistently.
     The following operations can be applied to the file descriptor, fd:
     F_GET_RW_HINT (uint64_t *; since Linux 4.13)
            Returns the value of the read/write  hint  associated  with  the
            underlying inode referred to by fd.
     F_SET_RW_HINT (uint64_t *; since Linux 4.13)
            Sets  the  read/write  hint value associated with the underlying
            inode referred to by fd.  This hint persists until either it  is
            explicitly modified or the underlying filesystem is unmounted.
     F_GET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
            Returns  the  value  of  the read/write hint associated with the
            open file description referred to by fd.
     F_SET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
            Sets the read/write hint value associated  with  the  open  file
            description referred to by fd.
     If  an  open  file description has not been assigned a read/write hint,
     then it shall use the value assigned to the inode, if any.
     The following read/write hints are valid since Linux 4.13:
     RWH_WRITE_LIFE_NOT_SET
            No specific hint has been set.  This is the default value.
     RWH_WRITE_LIFE_NONE
            No specific write lifetime  is  associated  with  this  file  or
            inode.
     RWH_WRITE_LIFE_SHORT
            Data  written to this inode or via this open file description is
            expected to have a short lifetime.
     RWH_WRITE_LIFE_MEDIUM
            Data written to this inode or via this open file description  is
            expected  to  have  a  lifetime  longer  than  data written with
            RWH_WRITE_LIFE_SHORT.
     RWH_WRITE_LIFE_LONG
            Data written to this inode or via this open file description  is
            expected  to  have  a  lifetime  longer  than  data written with
            RWH_WRITE_LIFE_MEDIUM.
     RWH_WRITE_LIFE_EXTREME
            Data written to this inode or via this open file description  is
            expected  to  have  a  lifetime  longer  than  data written with
            RWH_WRITE_LIFE_LONG.
     All the write-specific hints are relative to each other, and  no  indi-
     vidual absolute meaning should be attributed to them.

RETURN VALUE

     For a successful call, the return value depends on the operation:
     F_DUPFD  The new file descriptor.
     F_GETFD  Value of file descriptor flags.
     F_GETFL  Value of file status flags.
     F_GETLEASE
              Type of lease held on file descriptor.
     F_GETOWN Value of file descriptor owner.
     F_GETSIG Value  of  signal sent when read or write becomes possible, or
              zero for traditional SIGIO behavior.
     F_GETPIPE_SZ, F_SETPIPE_SZ
              The pipe capacity.
     F_GET_SEALS
              A bit mask identifying the seals that have been  set  for  the
              inode referred to by fd.
     All other commands
              Zero.
     On error, -1 is returned, and errno is set appropriately.

ERRORS

     EACCES or EAGAIN
            Operation is prohibited by locks held by other processes.
     EAGAIN The  operation  is  prohibited because the file has been memory-
            mapped by another process.
     EBADF  fd is not an open file descriptor
     EBADF  cmd is F_SETLK or F_SETLKW and the  file  descriptor  open  mode
            doesn't match with the type of lock requested.
     EBUSY  cmd  is  F_SETPIPE_SZ and the new pipe capacity specified in arg
            is smaller than the amount of buffer  space  currently  used  to
            store data in the pipe.
     EBUSY  cmd  is F_ADD_SEALS, arg includes F_SEAL_WRITE, and there exists
            a writable, shared mapping on the file referred to by fd.
     EDEADLK
            It was detected that the specified F_SETLKW command would  cause
            a deadlock.
     EFAULT lock is outside your accessible address space.
     EINTR  cmd  is  F_SETLKW  or  F_OFD_SETLKW and the operation was inter-
            rupted by a signal; see signal(7).
     EINTR  cmd is F_GETLK, F_SETLK, F_OFD_GETLK, or  F_OFD_SETLK,  and  the
            operation  was  interrupted  by  a  signal  before  the lock was
            checked or acquired.  Most likely when  locking  a  remote  file
            (e.g., locking over NFS), but can sometimes happen locally.
     EINVAL The value specified in cmd is not recognized by this kernel.
     EINVAL cmd is F_ADD_SEALS and arg includes an unrecognized sealing bit.
     EINVAL cmd is F_ADD_SEALS or F_GET_SEALS and the filesystem  containing
            the inode referred to by fd does not support sealing.
     EINVAL cmd  is F_DUPFD and arg is negative or is greater than the maxi-
            mum 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 direc-
            tory.
     EPERM  cmd  is  F_SETPIPE_SZ  and  the soft or hard user pipe limit has
            been reached; see pipe(7).
     EPERM  Attempted to clear the O_APPEND flag on  a  file  that  has  the
            append-only attribute set.
     EPERM  cmd was F_ADD_SEALS, but fd was not open for writing or the cur-
            rent set of seals on the file already includes F_SEAL_SEAL.

CONFORMING TO

     SVr4, 4.3BSD, POSIX.1-2001.   Only  the  operations  F_DUPFD,  F_GETFD,
     F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK, and F_SETLKW are specified
     in POSIX.1-2001.
     F_GETOWN and F_SETOWN are specified in  POSIX.1-2001.   (To  get  their
     definitions, define either _XOPEN_SOURCE with the value 500 or greater,
     or _POSIX_C_SOURCE with the value 200809L or greater.)
     F_DUPFD_CLOEXEC is specified in POSIX.1-2008.  (To get this definition,
     define   _POSIX_C_SOURCE   with   the  value  200809L  or  greater,  or
     _XOPEN_SOURCE with the value 700 or greater.)
     F_GETOWN_EX, F_SETOWN_EX, F_SETPIPE_SZ, F_GETPIPE_SZ, F_GETSIG,  F_SET-
     SIG,  F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific.  (Define
     the _GNU_SOURCE macro to obtain these definitions.)
     F_OFD_SETLK, F_OFD_SETLKW, and F_OFD_GETLK are Linux-specific (and  one
     must define _GNU_SOURCE to obtain their definitions), but work is being
     done to have them included in the next version of POSIX.1.
     F_ADD_SEALS and F_GET_SEALS are Linux-specific.

NOTES

     The errors returned by dup2(2) are different  from  those  returned  by
     F_DUPFD.
 File locking
     The original Linux fcntl() system call was not designed to handle large
     file offsets (in the flock structure).  Consequently, an fcntl64() sys-
     tem  call was added in Linux 2.4.  The newer system call employs a dif-
     ferent structure for file locking, flock64, and corresponding commands,
     F_GETLK64,  F_SETLK64,  and  F_SETLKW64.  However, these details can be
     ignored by applications using glibc,  whose  fcntl()  wrapper  function
     transparently  employs  the  more recent system call where it is avail-
     able.
 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() sys-
     tem  call was added in Linux 2.4.  The newer system call employs a dif-
     ferent structure for file locking, flock64, and corresponding commands,
     F_GETLK64,  F_SETLK64,  and  F_SETLKW64.  However, these details can be
     ignored by applications using glibc,  whose  fcntl()  wrapper  function
     transparently  employs  the  more recent system call where it is avail-
     able.
 Record locking and NFS
     Before Linux 3.12, if an NFSv4 client loses contact with the server for
     a  period  of  time (defined as more than 90 seconds with no communica-
     tion), it might lose and regain a lock without ever being aware of  the
     fact.  (The period of time after which contact is assumed lost is known
     as the NFSv4 leasetime.  On a Linux NFS server, this can be  determined
     by  looking at /proc/fs/nfsd/nfsv4leasetime, which expresses the period
     in seconds.  The default value for this file  is  90.)   This  scenario
     potentially  risks data corruption, since another process might acquire
     a lock in the intervening period and perform file I/O.
     Since Linux 3.12, if an NFSv4 client loses contact with the server, any
     I/O  to  the file by a process which "thinks" it holds a lock will fail
     until that process closes and reopens the file.   A  kernel  parameter,
     nfs.recover_lost_locks,  can  be set to 1 to obtain the pre-3.12 behav-
     ior, whereby the client will attempt to recover lost locks when contact
     is  reestablished  with  the  server.  Because of the attendant risk of
     data corruption, this parameter defaults to 0 (disabled).

BUGS

 F_SETFL
     It is not possible to use F_SETFL to change the state  of  the  O_DSYNC
     and  O_SYNC  flags.   Attempts  to  change the state of these flags are
     silently ignored.
 F_GETOWN
     A limitation of the Linux system call conventions on some architectures
     (notably  i386)  means  that  if  a  (negative)  process group ID to be
     returned by F_GETOWN falls in the range -1 to -4095,  then  the  return
     value  is  wrongly interpreted by glibc as an error in the system call;
     that is, the return value of fcntl() will be -1, and errno will contain
     the (positive) process group ID.  The Linux-specific F_GETOWN_EX opera-
     tion avoids this problem.  Since glibc version 2.11,  glibc  makes  the
     kernel  F_GETOWN  problem  invisible  by  implementing  F_GETOWN  using
     F_GETOWN_EX.
 F_SETOWN
     In Linux 2.4 and earlier, there is bug that can occur when an  unprivi-
     leged  process  uses  F_SETOWN  to  specify  the owner of a socket file
     descriptor as a process (group) other than the caller.  In  this  case,
     fcntl()  can  return  -1  with  errno set to EPERM, even when the owner
     process (group) is one that the caller has permission to  send  signals
     to.   Despite  this error return, the file descriptor owner is set, and
     signals will be sent to the owner.
 Deadlock detection
     The deadlock-detection algorithm employed by the  kernel  when  dealing
     with  F_SETLKW  requests  can  yield  both false negatives (failures to
     detect deadlocks, leaving a set of deadlocked processes blocked indefi-
     nitely) 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  indi-
     cate  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 condi-
     tions  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),  fea-
     ture_test_macros(7), lslocks(8)
     locks.txt,  mandatory-locking.txt,  and dnotify.txt in the Linux kernel
     source directory Documentation/filesystems/ (on  older  kernels,  these
     files  are  directly under the Documentation/ directory, and mandatory-
     locking.txt is called mandatory.txt)

COLOPHON

     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

Linux 2018-02-02 FCNTL(2)

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