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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.

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.

       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, or the command was F_SETLK
              or F_SETLKW and the file descriptor open mode doesn't match
              with the type of lock requested.

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

       EFAULT lock is outside your accessible address space.

       EINTR  For F_SETLKW, the command was interrupted by a signal; see
              signal(7).  For F_GETLK and F_SETLK, the command 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 For F_DUPFD, arg is negative or is greater than the maximum
              allowable value.  For F_SETSIG, 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 For F_DUPFD, 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.

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.

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 3.75 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                            2014-09-06                         FCNTL(2)