NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |

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

## NAME top

           fcntl - manipulate file descriptor

## SYNOPSIS top

           #include <unistd.h>
           #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)
                  Find the lowest numbered available file descriptor greater
                  than or equal to arg and make it be a copy of fd.  This is
                  different from dup2(2), which uses exactly the descriptor
                  specified.

                  On success, the new descriptor is returned.

                  See dup(2) for further details.

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

       File descriptor flags
           The following commands manipulate the flags associated with a file
           descriptor.  Currently, only one such flag is defined: FD_CLOEXEC,
           the close-on-exec flag.  If the FD_CLOEXEC bit is 0, the file
           descriptor will remain open across an execve(2), otherwise it will be
           closed.

           F_GETFD (void)
                  Read the file descriptor flags; arg is ignored.

           F_SETFD (int)
                  Set the file descriptor flags to the value specified by arg.

           In multithreaded programs, using fcntl() F_SETFD to set the close-on-
           exec flag at the same time as another thread performs a fork(2) plus
           execve(2) is vulnerable to a race condition that may unintentionally
           leak the file descriptor to the program executed in the child
           process.  See the discussion of the O_CLOEXEC flag in open(2) for
           details and a remedy to the problem.

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

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

           F_GETFL (void)
                  Get the file access mode and the file status flags; arg is
                  ignored.

           F_SETFL (int)
                  Set the file status flags to the value specified by arg.  File
                  access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation
                  flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are
                  ignored.  On Linux this command can change only the O_APPEND,
                  O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.  It is not
                  possible to change the O_DSYNC and O_SYNC flags; see BUGS,
                  below.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

           Open file description locks solve both of these problems.

       Open file description locks (non-POSIX)
           Open file description locks are advisory byte-range locks whose
           operation is in most respects identical to the traditional record
           locks described above.  This lock type is Linux-specific, and
           available since Linux 3.15.  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.

           Open file description locks always conflict with traditional record
           locks, even when they are acquired by the same process on the same
           file descriptor.

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

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

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

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

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

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

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

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

       Mandatory locking
           Warning: the Linux implementation of mandatory locking is unreliable.
           See BUGS below.

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

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

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

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

       Managing signals
           F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG and F_SETSIG
           are used to manage I/O availability signals:

           F_GETOWN (void)
                  Return (as the function result) the process ID or process
                  group currently receiving SIGIO and SIGURG signals for events
                  on file descriptor fd.  Process IDs are returned as positive
                  values; process group IDs are returned as negative values (but
                  see BUGS below).  arg is ignored.

           F_SETOWN (int)
                  Set the process ID or process group ID that will receive SIGIO
                  and SIGURG signals for events on file descriptor fd to the ID
                  given in arg.  A process ID is specified as a positive value;
                  a process group ID is specified as a negative value.  Most
                  commonly, the calling process specifies itself as the owner
                  (that is, arg is specified as getpid(2)).

                  If you set the O_ASYNC status flag on a file descriptor by
                  using the F_SETFL command of fcntl(), a SIGIO signal is sent
                  whenever input or output becomes possible on that file
                  descriptor.  F_SETSIG can be used to obtain delivery of a
                  signal other than SIGIO.  If this permission check fails, then
                  the signal is silently discarded.

                  Sending a signal to the owner process (group) specified by
                  F_SETOWN is subject to the same permissions checks as are
                  described for kill(2), where the sending process is the one
                  that employs F_SETOWN (but see BUGS below).

                  If the file descriptor fd refers to a socket, F_SETOWN also
                  selects the recipient of SIGURG signals that are delivered
                  when out-of-band data arrives on that socket.  (SIGURG is sent
                  in any situation where select(2) would report the socket as
                  having an "exceptional condition".)

                  The following was true in 2.6.x kernels up to and including
                  kernel 2.6.11:

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

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

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

                      struct f_owner_ex {
                          int   type;
                          pid_t pid;
                      };

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                  F_UNLCK
                         Remove our lease from the file.

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

           Leases may be taken out only on regular files.  An unprivileged
           process may take out a lease only on a file whose UID (owner) matches
           the filesystem UID of the process.  A process with the CAP_LEASE
           capability may take out leases on arbitrary files.

           F_GETLEASE (void)
                  Indicates what type of lease is associated with the file
                  descriptor fd by returning either F_RDLCK, F_WRLCK, or
                  F_UNLCK, indicating, respectively, a read lease , a write
                  lease, or no lease.  arg is ignored.

           When a process (the "lease breaker") performs an open(2) or
           truncate(2) that conflicts with a lease established via F_SETLEASE,
           the system call is blocked by the kernel and the kernel notifies the
           lease holder by sending it a signal (SIGIO by default).  The lease
           holder should respond to receipt of this signal by doing whatever
           cleanup is required in preparation for the file to be accessed by
           another process (e.g., flushing cached buffers) and then either
           remove or downgrade its lease.  A lease is removed by performing an
           F_SETLEASE command specifying arg as F_UNLCK.  If the lease holder
           currently holds a write lease on the file, and the lease breaker is
           opening the file for reading, then it is sufficient for the lease
           holder to downgrade the lease to a read lease.  This is done by
           performing an F_SETLEASE command specifying arg as F_RDLCK.

           If the lease holder fails to downgrade or remove the lease within the
           number of seconds specified in /proc/sys/fs/lease-break-time, then
           the kernel forcibly removes or downgrades the lease holder's lease.

           Once a lease break has been initiated, F_GETLEASE returns the target
           lease type (either F_RDLCK or F_UNLCK, depending on what would be
           compatible with the lease breaker) until the lease holder voluntarily
           downgrades or removes the lease or the kernel forcibly does so after
           the lease break timer expires.

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

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

           The default signal used to notify the lease holder is SIGIO, but this
           can be changed using the F_SETSIG command to fcntl().  If a F_SETSIG
           command is performed (even one specifying SIGIO), and the signal
           handler is established using SA_SIGINFO, then the handler will
           receive a siginfo_t structure as its second argument, and the si_fd
           field of this argument will hold the descriptor of the leased file
           that has been accessed by another process.  (This is useful if the
           caller holds leases against multiple files).

       File and directory change notification (dnotify)
           F_NOTIFY (int)
                  (Linux 2.4 onward) Provide notification when the directory
                  referred to by fd or any of the files that it contains is
                  changed.  The events to be notified are specified in arg,
                  which is a bit mask specified by ORing together zero or more
                  of the following bits:

                  DN_ACCESS   A file was accessed (read(2), pread(2), readv(2),
                              and similar)
                  DN_MODIFY   A file was modified (write(2), pwrite(2),
                              writev(2), truncate(2), ftruncate(2), and
                              similar).
                  DN_CREATE   A file was created (open(2), creat(2), mknod(2),
                              mkdir(2), link(2), symlink(2), rename(2) into this
                              directory).
                  DN_DELETE   A file was unlinked (unlink(2), rename(2) to
                              another directory, rmdir(2)).
                  DN_RENAME   A file was renamed within this directory
                              (rename(2)).
                  DN_ATTRIB   The attributes of a file were changed (chown(2),
                              chmod(2), utime(2), utimensat(2), and similar).

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

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

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

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

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

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

       Changing the capacity of a pipe
           F_SETPIPE_SZ (int; since Linux 2.6.35)
                  Change the capacity of the pipe referred to by fd to be at
                  least arg bytes.  An unprivileged process can adjust the pipe
                  capacity to any value between the system page size and the
                  limit defined in /proc/sys/fs/pipe-max-size (see proc(5)).
                  Attempts to set the pipe capacity below the page size are
                  silently rounded up to the page size.  Attempts by an
                  unprivileged process to set the pipe capacity above the limit
                  in /proc/sys/fs/pipe-max-size yield the error EPERM; a
                  privileged process (CAP_SYS_RESOURCE) can override the limit.
                  When allocating the buffer for the pipe, the kernel may use a
                  capacity larger than arg, if that is convenient for the
                  implementation.  The actual capacity that is set is returned
                  as the function result.  Attempting to set the pipe capacity
                  smaller than the amount of buffer space currently used to
                  store data produces the error EBUSY.

           F_GETPIPE_SZ (void; since Linux 2.6.35)
                  Return (as the function result) the capacity of the pipe
                  referred to by fd.

       File Sealing
           File seals limit the set of allowed operations on a given file.  For
           each seal that is set on a file, a specific set of operations will
           fail with EPERM on this file from now on.  The file is said to be
           sealed.  The default set of seals depends on the type of the
           underlying file and filesystem.  For an overview of file sealing, a
           discussion of its purpose, and some code examples, see
           memfd_create(2).

           Currently, only the tmpfs filesystem supports sealing.  On other
           filesystems, all fcntl(2) operations that operate on seals will
           return EINVAL.

           Seals are a property of an inode.  Thus, all open file descriptors
           referring to the same inode share the same set of seals.
           Furthermore, seals can never be removed, only added.

           F_ADD_SEALS (int; since Linux 3.17)
                  Add the seals given in the bit-mask argument arg to the set of
                  seals of the inode referred to by the file descriptor fd.
                  Seals cannot be removed again.  Once this call succeeds, the
                  seals are enforced by the kernel immediately.  If the current
                  set of seals includes F_SEAL_SEAL (see below), then this call
                  will be rejected with EPERM.  Adding a seal that is already
                  set is a no-op, in case F_SEAL_SEAL is not set already.  In
                  order to place a seal, the file descriptor fd must be
                  writable.

           F_GET_SEALS (void; since Linux 3.17)
                  Return (as the function result) the current set of seals of
                  the inode referred to by fd.  If no seals are set, 0 is
                  returned.  If the file does not support sealing, -1 is
                  returned and errno is set to EINVAL.

           The following seals are available:

           F_SEAL_SEAL
                  If this seal is set, any further call to fcntl(2) with
                  F_ADD_SEALS will fail with EPERM.  Therefore, this seal
                  prevents any modifications to the set of seals itself.  If the
                  initial set of seals of a file includes F_SEAL_SEAL, then this
                  effectively causes the set of seals to be constant and locked.

           F_SEAL_SHRINK
                  If this seal is set, the file in question cannot be reduced in
                  size.  This affects open(2) with the O_TRUNC flag as well as
                  truncate(2) and ftruncate(2).  Those calls will fail with
                  EPERM if you try to shrink the file in question.  Increasing
                  the file size is still possible.

           F_SEAL_GROW
                  If this seal is set, the size of the file in question cannot
                  be increased.  This affects write(2) beyond the end of the
                  file, truncate(2), ftruncate(2), and fallocate(2).  These
                  calls will fail with EPERM if you use them to increase the
                  file size.  If you keep the size or shrink it, those calls
                  still work as expected.

           F_SEAL_WRITE
                  If this seal is set, you cannot modify the contents of the
                  file.  Note that shrinking or growing the size of the file is
                  still possible and allowed.  Thus, this seal is normally used
                  in combination with one of the other seals.  This seal affects
                  write(2) and fallocate(2) (only in combination with the
                  FALLOC_FL_PUNCH_HOLE flag).  Those calls will fail with EPERM
                  if this seal is set.  Furthermore, trying to create new
                  shared, writable memory-mappings via mmap(2) will also fail
                  with EPERM.

                  Setting F_SEAL_WRITE via fcntl(2) with F_ADD_SEALS will fail
                  with EBUSY if any writable, shared mapping exists.  Such
                  mappings must be unmapped before you can add this seal.
                  Furthermore, if there are any asynchronous I/O operations
                  (io_submit(2)) pending on the file, all outstanding writes
                  will be discarded.

## RETURN VALUE top

           For a successful call, the return value depends on the operation:

           F_DUPFD  The new descriptor.

           F_GETFD  Value of file descriptor flags.

           F_GETFL  Value of file status flags.

           F_GETLEASE
                    Type of lease held on file descriptor.

           F_GETOWN Value of descriptor owner.

           F_GETSIG Value of signal sent when read or write becomes possible, or
                    zero for traditional SIGIO behavior.

           F_GETPIPE_SZ, F_SETPIPE_SZ
                    The pipe capacity.

           F_GET_SEALS
                    A bit mask identifying the seals that have been set for the
                    inode referred to by fd.

           All other commands
                    Zero.

           On error, -1 is returned, and errno is set appropriately.

## ERRORS 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 process already has the maximum number
                  of file descriptors open.

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

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

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

           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 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 _BSD_SOURCE, or _XOPEN_SOURCE with the
           value 500 or greater, or _POSIX_C_SOURCE with the value 200809L or
           greater.)

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

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

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

           F_ADD_SEALS and F_GET_SEALS are Lnux-specific.

## 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.

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

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

           Several systems have more fields in struct flock such as, for
           example, l_sysid.  Clearly, l_pid alone is not going to be very
           useful if the process holding the lock may live on a different
           machine.

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

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

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

## BUGS 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
           version 2.11, glibc makes the kernel F_GETOWN problem invisible by
           implementing F_GETOWN using F_GETOWN_EX.

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

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

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

## SEE ALSO top

           dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),
           feature_test_macros(7)

           locks.txt, mandatory-locking.txt, and dnotify.txt in the Linux kernel
           source directory Documentation/filesystems/ (on older kernels, these
           files are directly under the Documentation/ directory, and mandatory-
           locking.txt is called mandatory.txt)

## COLOPHON top

           This page is part of release 3.79 of the Linux man-pages project.  A
           description of the project, information about reporting bugs, and the
           latest version of this page, can be found at
           http://www.kernel.org/doc/man-pages/.

    Linux                            2015-01-22                         FCNTL(2)

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