fcntl

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



NAME
       fcntl - manipulate file descriptor

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

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

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

       fcntl() can take an optional third argument.  Whether or not this
       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 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.  (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 is an initial step toward
       removing this feature completely.

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

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

       To make use of mandatory locks, mandatory locking must be enabled both
       on the filesystem that contains the file to be locked, and on the file
       itself.  Mandatory locking is enabled on a filesystem using the "-o
       mand" option to mount(8), or the MS_MANDLOCK flag for mount(2).
       Mandatory 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
              currently receiving SIGIO and SIGURG signals for events on file
              descriptor fd.  Process IDs are returned as positive values;
              process group IDs are returned as negative values (but see BUGS
              below).  arg is ignored.

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

              As well as setting the file descriptor owner, one must also
              enable generation of signals on the file descriptor.  This is
              done by using the fcntl() F_SETFL command to set the O_ASYNC
              file status flag on the file descriptor.  Subsequently, a SIGIO
              signal is sent whenever input or output becomes possible on the
              file descriptor.  The fcntl() F_SETSIG command can be used to
              obtain delivery of a signal other than SIGIO.

              Sending a signal to the owner process (group) specified by
              F_SETOWN is subject to the same permissions checks as are
              described for kill(2), where the sending process is the one that
              employs F_SETOWN (but see BUGS below).  If this permission check
              fails, then the signal is silently discarded.  Note: The
              F_SETOWN operation records the caller's credentials at the time
              of the fcntl() call, and it is these saved credentials that are
              used for the permission checks.

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

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

              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 kernel 2.6.13), which provides a much superior
              interface for obtaining notifications of filesystem events.  See
              inotify(7).

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

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

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

       F_DUPFD  The new file descriptor.

       F_GETFD  Value of file descriptor flags.

       F_GETFL  Value of file status flags.

       F_GETLEASE
                Type of lease held on file descriptor.

       F_GETOWN Value of file descriptor owner.

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

       F_GETPIPE_SZ, F_SETPIPE_SZ
                The pipe capacity.

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

       All other commands
                Zero.

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

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

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

       EBADF  fd is not an open file descriptor

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

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

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

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

       EFAULT lock is outside your accessible address space.

       EINTR  cmd is F_SETLKW or F_OFD_SETLKW and the operation was
              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.

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

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

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

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

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

       F_ADD_SEALS and F_GET_SEALS are Linux-specific.

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

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

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

       Several systems have more fields in struct flock such as, for example,
       l_sysid (to identify the machine where the lock is held).  Clearly,
       l_pid alone is not going to be very 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
   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
       dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),
       feature_test_macros(7), lslocks(8)

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

COLOPHON
       This page is part of release 5.00 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-03-06                          FCNTL(2)