fcntl(2) — Linux manual page

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fcntl(2)                   System Calls Manual                  fcntl(2)

NAME         top

       fcntl - manipulate file descriptor

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <fcntl.h>

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

DESCRIPTION         top

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

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

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

   Duplicating a file descriptor
       F_DUPFD (int)
              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,
       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, O_TRUNC)
              in arg are ignored.  On Linux, this command can change
              only the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and
              O_NONBLOCK flags.  It is not possible to change the
              O_DSYNC and O_SYNC flags; see BUGS, below.

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

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

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

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

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

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

       POSIX.1-2001 allows (but does not require) an implementation to
       support a negative l_len value; if l_len is negative, the
       interval described by lock covers bytes l_start+l_len up to and
       including l_start-1.  This is supported since Linux 2.4.21 and
       Linux 2.5.49.

       The l_type field can be used to place a read (F_RDLCK) or a write
       (F_WRLCK) lock on a file.  Any number of processes may hold a
       read lock (shared lock) on a file region, but only one process
       may hold a write lock (exclusive lock).  An exclusive lock
       excludes all other locks, both shared and exclusive.  A single
       process can hold only one type of lock on a file region; if a new
       lock is applied to an already-locked region, then the existing
       lock is converted to the new lock type.  (Such conversions may
       involve splitting, shrinking, or coalescing with an existing lock
       if the byte range specified by the new lock does not precisely
       coincide with the range of the existing lock.)

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

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

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

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

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

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

       As well as being removed by an explicit F_UNLCK, record locks are
       automatically released when the process terminates.

       Record locks are not inherited by a child created via fork(2),
       but are preserved across an execve(2).

       Because of the buffering performed by the stdio(3) library, the
       use of record locking with routines in that package should be
       avoided; use read(2) and write(2) instead.

       The record locks described above are associated with the process
       (unlike the open file description locks described below).  This
       has some unfortunate consequences:

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

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

       Open file description locks solve both of these problems.

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

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

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

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

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

       As with traditional advisory locks, the third argument to
       fcntl(), lock, is a pointer to an flock structure.  By contrast
       with traditional record locks, the l_pid field of that structure
       must be set to zero when using the commands described below.

       The commands for working with open file description locks are
       analogous to those used with traditional locks:

       F_OFD_SETLK (struct flock *)
              Acquire an open file description lock (when l_type is
              F_RDLCK or F_WRLCK) or release an open file description
              lock (when l_type is F_UNLCK) on the bytes specified by
              the l_whence, l_start, and l_len fields of lock.  If a
              conflicting lock is held by another process, this call
              returns -1 and sets errno to EAGAIN.

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

       F_OFD_GETLK (struct flock *)
              On input to this call, lock describes an open file
              description lock we would like to place on the file.  If
              the lock could be placed, fcntl() does not actually place
              it, but returns F_UNLCK in the l_type field of lock and
              leaves the other fields of the structure unchanged.  If
              one or more incompatible locks would prevent this lock
              being placed, then details about one of these locks are
              returned via lock, as described above for F_GETLK.

       In the current implementation, no deadlock detection is performed
       for open file description locks.  (This contrasts with process-
       associated record locks, for which the kernel does perform
       deadlock detection.)

   Mandatory locking
       Warning: the Linux implementation of mandatory locking is
       unreliable.  See BUGS below.  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 (CONFIG_MANDATORY_FILE_LOCKING).  This
       feature is no longer supported at all in Linux 5.15 and above.

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

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

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

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

   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 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 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 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 process
              group ID 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 Linux 2.6.x up to and including
              Linux 2.6.11:

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

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

       F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
              Return the current file descriptor owner settings as
              defined by a previous F_SETOWN_EX operation.  The
              information is returned in the structure pointed to by
              arg, which has the following form:

                  struct f_owner_ex {
                      int   type;
                      pid_t pid;
                  };

              The type field will have one of the values F_OWNER_TID,
              F_OWNER_PID, or F_OWNER_PGRP.  The pid field is a positive
              integer representing a thread ID, process ID, or process
              group ID.  See F_SETOWN_EX for more details.

       F_SETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
              This operation performs a similar task to F_SETOWN.  It
              allows the caller to direct I/O availability signals to a
              specific thread, process, or process group.  The caller
              specifies the target of signals via arg, which is a
              pointer to a f_owner_ex structure.  The type field has one
              of the following values, which define how pid is
              interpreted:

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

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

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

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

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

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

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

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

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

       Using these mechanisms, a program can implement fully
       asynchronous I/O without using select(2) or poll(2) most of the
       time.

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

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

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

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

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

              F_UNLCK
                     Remove our lease from the file.

       Leases are associated with an open file description (see
       open(2)).  This means that duplicate file descriptors (created
       by, for example, fork(2) or dup(2)) refer to the same lease, and
       this lease may be modified or released using any of these
       descriptors.  Furthermore, the lease is released by either an
       explicit F_UNLCK operation on any of these duplicate 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 compatible with the lease breaker) until the lease
       holder voluntarily downgrades or removes the lease or the kernel
       forcibly does so after the lease break timer expires.

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

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

       The default signal used to notify the lease holder is SIGIO, but
       this can be changed using the F_SETSIG command to fcntl().  If a
       F_SETSIG command is performed (even one specifying SIGIO), and
       the signal handler is established using SA_SIGINFO, then the
       handler will receive a siginfo_t structure as its second
       argument, and the si_fd field of this argument will hold the 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
              application must reregister to receive further
              notifications.  Alternatively, if DN_MULTISHOT is included
              in arg, then notification will remain in effect until
              explicitly removed.

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

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

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

              NOTE: New applications should use the inotify interface
              (available since Linux 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 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.

              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 depends on the type of
       the underlying file and filesystem.  For an overview of file
       sealing, a discussion of its purpose, and some code examples, see
       memfd_create(2).

       Currently, 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 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 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.  Furthermore, if there are any asynchronous 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 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
       individual absolute meaning should be attributed to them.

RETURN VALUE         top

       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 to indicate the error.

ERRORS         top

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

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

       EBADF  fd is not an open file descriptor

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

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

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

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

       EFAULT lock is outside your accessible address space.

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

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

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

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

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

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

       EINVAL cmd is F_SETSIG and arg is not an allowable signal number.

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

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

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

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

       EPERM  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.

STANDARDS         top

       POSIX.1-2008.

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

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

       F_ADD_SEALS and F_GET_SEALS are Linux-specific.

HISTORY         top

       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.)

NOTES         top

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

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

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

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

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

BUGS         top

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

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

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

   Deadlock detection
       The deadlock-detection algorithm employed by the kernel when
       dealing with F_SETLKW requests can yield both false negatives
       (failures to detect deadlocks, leaving a set of deadlocked
       processes blocked indefinitely) and false positives (EDEADLK
       errors when there is no deadlock).  For example, the kernel
       limits the lock depth of its dependency search to 10 steps,
       meaning that circular deadlock chains that exceed that size will
       not be detected.  In addition, the kernel may falsely indicate a
       deadlock when two or more processes created using the clone(2)
       CLONE_FILES flag place locks that appear (to the kernel) to
       conflict.

   Mandatory locking
       The Linux implementation of mandatory locking is subject to race
       conditions which render it unreliable: a write(2) call that
       overlaps with a lock may modify data after the mandatory lock is
       acquired; a read(2) call that overlaps with a lock may detect
       changes to data that were made only after a write lock was
       acquired.  Similar races exist between mandatory locks and
       mmap(2).  It is therefore inadvisable to rely on mandatory
       locking.

SEE ALSO         top

       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)

Linux man-pages (unreleased)     (date)                         fcntl(2)

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