FCNTL(2)                  (2020-12-21)                   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 iden-
          tify 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 oper-
          ation is to invoke fcntl() with the desired cmd value and
          then test whether the call failed with EINVAL, indicating
          that the kernel does not recognize this value.

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

               On success, the new file descriptor is returned.

               See dup(2) for further details.

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

        File descriptor flags
          The following commands manipulate the flags associated with
          a file descriptor.  Currently, only one such flag is

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          defined: FD_CLOEXEC, the close-on-exec flag.  If the
          FD_CLOEXEC bit is set, the file descriptor will automati-
          cally 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 per-
          forms a fork(2) plus execve(2) is vulnerable to a race con-
          dition that may unintentionally leak the file descriptor to
          the program executed in the child process.  See the discus-
          sion of the O_CLOEXEC flag in open(2) for details and a rem-
          edy to the problem.

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

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

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

          F_SETFL (int)
               Set the file status flags to the value specified by
               arg. File access mode (O_RDONLY, O_WRONLY, O_RDWR and
               file creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY,
               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 discus-
          sion of open file description locks below.

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          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 inter-
          preted 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 nega-
          tive 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-

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          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 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 fields of lock. If the conflicting lock is a tradi-
               tional (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 descrip-
               tion 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 pro-
          cesses.  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

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          already locked by the other process using F_SETLKW, then,
          without deadlock detection, both processes would remain
          blocked indefinitely.  When the kernel detects such dead-
          locks, it causes one of the blocking lock requests to imme-
          diately 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 termi-
          nates.

          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 tradi-
          tional 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

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          whereas traditional record locks are associated with a pro-
          cess, 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 tra-
          ditional 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 exist-
          ing 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
          program can use open file description locks to synchronize
          access to a file region by having each thread perform its
          own open(2) on the file and applying locks via the resulting
          file descriptor.

          As with traditional advisory locks, the third argument to
          fcntl(), lock, is a pointer to an flock structure.  By con-
          trast 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

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               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 per-
          formed 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 fea-
          ture, governed by a configuration option
          (CONFIG_MANDATORY_FILE_LOCKING).  This is an initial step
          toward removing this feature completely.

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

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

          To make use of mandatory locks, mandatory locking must be
          enabled both on the filesystem that contains the file to be
          locked, and on the file itself.  Mandatory locking is
          enabled on a filesystem using the "-o mand" option to
          mount(8), or the MS_MANDLOCK flag for mount(2).  Mandatory

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          locking is enabled on a file by disabling group execute per-
          mission 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 connec-
          tivity with the server) which lasts long enough for the
          server to assume that the client is no longer functioning.

          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 hap-
          pens at least for NFSv4 (including all minor versions).

          Some versions of UNIX send a signal (SIGLOST) in this cir-
          cumstance.  Linux does not define this signal, and does not
          provide any asynchronous 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 pro-
               cess group ID currently receiving SIGIO and SIGURG sig-
               nals 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 speci-
               fied as getpid(2)).

               As well as setting the file descriptor owner, one must
               also enable generation of signals on the file descrip-
               tor.  This is done by using the fcntl() F_SETFL command
               to set the O_ASYNC file status flag on the file

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               descriptor.  Subsequently, a SIGIO signal is sent when-
               ever 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 condi-
               tion".)

               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 mul-
                    tithreaded process running with a threading
                    library that supports thread groups (e.g., NPTL),
                    then a positive value given to F_SETOWN has a dif-
                    ferent meaning: instead of being a process ID
                    identifying a whole process, it is a thread ID
                    identifying a specific thread within a process.
                    Consequently, it may be necessary to pass F_SETOWN
                    the result of gettid(2) instead of getpid(2) to
                    get sensible results when F_SETSIG is used.  (In
                    current Linux threading implementations, a main
                    thread's thread ID is the same as its process ID.
                    This means that a single-threaded program can
                    equally use gettid(2) or getpid(2) in this 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 sig-
               nals at a particular thread.

          F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)

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               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 posi-
               tive 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 speci-
                    fied in pid.

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

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

          F_SETSIG (int)
               Set the signal sent when input or output becomes possi-
               ble 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

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               this case additional info is available to the signal
               handler if installed with SA_SIGINFO.

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

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

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

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

          Using these mechanisms, a program can implement fully asyn-
          chronous 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 con-
          junction 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 to
          establish a new lease, and retrieve the current lease, on

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          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 call-
                    ing process to be notified when the file is opened
                    for writing or is truncated.  A read lease can be
                    placed only on a file descriptor that is opened
                    read-only.

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

               F_UNLCK
                    Remove our lease from the file.

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

          Leases may be taken out only on regular files.  An unprivi-
          leged process may take out a lease only on a file whose UID
          (owner) matches the filesystem UID of the process.  A pro-
          cess 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

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          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 pro-
          cess (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 com-
          mand 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 sys-
          tem 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 spec-
          ifying SIGIO), and the signal handler is established using
          SA_SIGINFO, then the handler will receive a siginfo_t struc-
          ture 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)

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               (Linux 2.4 onward) Provide notification when the direc-
               tory referred to by fd or any of the files that it con-
               tains is changed.  The events to be notified are speci-
               fied 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 simi-
                    lar).
               DN_CREATE
                    A file was created (open(2), creat(2), mknod(2),
                    mkdir(2), link(2), symlink(2), rename(2) into this
                    directory).
               DN_DELETE
                    A file was unlinked (unlink(2), rename(2) to
                    another directory, rmdir(2)).
               DN_RENAME
                    A file was renamed within this directory
                    (rename(2)).
               DN_ATTRIB
                    The attributes of a file were changed (chown(2),
                    chmod(2), utime(2), utimensat(2), and similar).

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

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

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

               Notification occurs via delivery of a signal.  The
               default signal is SIGIO, but this can be changed using
               the F_SETSIG command to fcntl().  (Note that SIGIO is
               one of the nonqueuing standard signals; switching to
               the use of a real-time signal means that multiple noti-
               fications 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

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               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 noti-
               fications can be queued.

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

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

               When allocating the buffer for the pipe, the kernel may
               use a capacity larger than arg, if that is convenient
               for the implementation.  (In the current implementa-
               tion, 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.

               Note that because of the way the pages of the pipe
               buffer are employed when data is written to the pipe,
               the number of bytes that can be written may be less
               than the nominal size, depending on the size of the
               writes.

          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

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          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 descrip-
          tor 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 ker-
               nel 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 seal-
               ing, -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

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               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 combination with one of the
               other seals.  This seal affects write(2) and
               fallocate(2) (only in combination with the
               FALLOC_FL_PUNCH_HOLE flag).  Those calls fail with
               EPERM if this seal is set.  Furthermore, trying to cre-
               ate 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.  Furthermore, if there are any asyn-
               chronous I/O operations (io_submit(2)) pending on the
               file, all outstanding writes will be discarded.

          F_SEAL_FUTURE_WRITE (since Linux 5.1)
               The effect of this seal is similar to F_SEAL_WRITE, but
               the contents of the file can still be modified via
               shared writable mappings that were created prior to the
               seal being set.  Any attempt to create a new writable
               mapping on the file via mmap(2) will fail with EPERM.
               Likewise, an attempt to write to the file via write(2)
               will fail with EPERM.

               Using this seal, one process can create a memory buffer
               that it can continue to modify while sharing that
               buffer on a "read-only" basis with other processes.

        File read/write hints
          Write lifetime hints can be used to inform the kernel about
          the relative 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 con-
          sistent 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

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          hints are used consistently.

          The following operations can be applied to the file descrip-
          tor, 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 underly-
               ing 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

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               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 individual absolute meaning should be attributed to them.

     RETURN VALUE
          For a successful call, the return value depends on the oper-
          ation:

          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 possi-
               ble, 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 pro-
               cesses.

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

          EBADF

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               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 cur-
               rently used to store data in the pipe.

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

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

          EFAULT
               lock is outside your accessible address space.

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

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

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

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

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

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

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          EINVAL
               cmd is F_SETSIG and arg is not an allowable signal num-
               ber.

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

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

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

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

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

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

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

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

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

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

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

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          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).  Conse-
          quently, an fcntl64() system call was added in Linux 2.4.
          The newer system call employs a different structure for file
          locking, flock64, and corresponding commands, F_GETLK64,
          F_SETLK64, and F_SETLKW64.  However, these details can be
          ignored by applications using glibc, whose fcntl() wrapper
          function transparently employs the more recent system call
          where it is available.

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

          Several systems have more fields in struct flock such as,
          for example, l_sysid (to identify the machine where the lock
          is held).  Clearly, l_pid alone is not going to be very use-
          ful if the process holding the lock may live on a different
          machine; on Linux, while present on some architectures (such
          as MIPS32), this field is not used.

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

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

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          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 (dis-
          abled).

     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) pro-
          cess group ID to be returned by F_GETOWN falls in the range
          -1 to -4095, then the return value is wrongly interpreted by
          glibc as an error in the system call; that is, the return
          value of fcntl() will be -1, and errno will contain the
          (positive) process group ID.  The Linux-specific F_GETOWN_EX
          operation avoids this problem.  Since glibc version 2.11,
          glibc makes the kernel F_GETOWN problem invisible by imple-
          menting 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 nega-
          tives (failures to detect deadlocks, leaving a set of dead-
          locked processes blocked indefinitely) and false positives
          (EDEADLK errors when there is no deadlock).  For example,
          the kernel limits the lock depth of its dependency search to
          10 steps, meaning that circular deadlock chains that exceed
          that size will not be detected.  In addition, the kernel may
          falsely indicate a deadlock when two or more processes

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

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