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

       fcntl - manipulate file descriptor

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

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

       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 argu‐
       ment  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.

   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 pro‐
	      gram 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.

   File status flags
       Each  open  file	 description has certain associated status flags, ini‐
       tialized 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

       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

       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 locking
       F_SETLK,	 F_SETLKW,  and F_GETLK are used to acquire, release, and test
       for the existence of record locks (also known as 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:
	       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
				   (F_GETLK only) */

       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	 rela‐
       tive  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 posi‐
       tive, 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

       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.

       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  sig‐
	      nal 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 these locks in
	      the l_type, l_whence, l_start, and l_len fields of lock and sets
	      l_pid to be the PID of the process holding that lock.  Note that
	      the information returned by F_GETLK 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.

       As well as being removed by an explicit F_UNLCK, record locks are auto‐
       matically released when the process terminates or if it closes any file
       descriptor  referring  to a file on which locks are held.  This is bad:
       it means that a process can lose the locks on a file  like  /etc/passwd
       or  /etc/mtab  when for some reason a library function decides to open,
       read and close it.

       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.

   Mandatory locking
       (Non-POSIX.)   The  above record locks may be either advisory or manda‐
       tory, and are advisory by default.

       Advisory locks are not enforced and are useful only between cooperating

       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  descrip‐
       tion.   If  the	O_NONBLOCK  flag  is  not enabled, then system call is
       blocked until the lock is removed or converted to a mode that  is  com‐
       patible	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).	Manda‐
       tory 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)).

       The Linux implementation of mandatory locking is unreliable.  See  BUGS

   Managing signals
       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 com‐
	      monly, 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 ker‐
	      nel 2.6.11:

		     If a nonzero value is  given  to  F_SETSIG	 in  a	multi‐
		     threaded  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 sin‐
		     gle-threaded  program  can	 equally use gettid(2) or get‐
		     pid(2) in this scenario.)	Note, however, that the state‐
		     ments 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

		  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 inte‐
	      ger  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:

		     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.

		     Send  the	signal to the process whose ID is specified in

		     Send the signal to the process group whose ID  is	speci‐
		     fied 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 han‐
	      dler 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	struc‐
	      ture.   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.	(Queu‐
	      ing 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 sig‐
	      nals that may be queued to a process (see getrlimit(2) and  sig‐
	      nal(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).

       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  pro‐
       vides  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:

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

		     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

		     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 modi‐
       fied  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

       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

       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 com‐
       patible 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 spec‐
       ifies  the  O_NONBLOCK flag when calling open(2), then the call immedi‐
       ately 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 han‐
       dler 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, pread, readv)
	      DN_MODIFY	  A file was modified (write,  pwrite,	writev,	 trun‐
			  cate, ftruncate).
	      DN_CREATE	  A file was created (open, creat, mknod, mkdir, link,
			  symlink, rename).
	      DN_DELETE	  A file  was  unlinked	 (unlink,  rename  to  another
			  directory, rmdir).
	      DN_RENAME	  A file was renamed within this directory (rename).
	      DN_ATTRIB	  The attributes of a file were changed (chown, chmod,

	      (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 appli‐
	      cation must reregister to receive further notifications.	Alter‐
	      natively,	 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 noti‐
	      fication of all events, make an F_NOTIFY call specifying arg  as

	      Notification  occurs via delivery of a signal.  The default sig‐
	      nal is SIGIO, but this can be changed using the F_SETSIG command
	      to  fcntl().   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 struc‐
	      ture contains the file descriptor which generated the  notifica‐
	      tion (useful when establishing notification on multiple directo‐

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

	      NOTE: New applications should use the inotify interface  (avail‐
	      able since kernel 2.6.13), which provides a much superior inter‐
	      face for obtaining notifications of filesystem events.  See ino‐

   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 F_GETPIPE_SZ operation
	      returns the actual size used.  Attempting to set the pipe capac‐
	      ity  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.

       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.

		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.

		The pipe capacity.

       All other commands

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

	      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.

	      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  sig‐
	      nal(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 For F_DUPFD, arg is negative or  is  greater  than  the  maximum
	      allowable	 value.	  For F_SETSIG, arg is not an allowable signal

       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).

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

       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 BSD_SOURCE, or _XOPEN_SOURCE with the value 500  or
       greater, or define _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.)

       SIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific.   (Define
       the _GNU_SOURCE macro to obtain these definitions.)

       The original Linux fcntl() system call was not designed to handle large
       file offsets (in the flock structure).  Consequently, an fcntl64() sys‐
       tem  call was added in Linux 2.4.  The newer system call employs a dif‐
       ferent 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 avail‐

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

       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.

       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.

       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 opera‐
       tion avoids this problem.  Since glibc version 2.11,  glibc  makes  the
       kernel  F_GETOWN	 problem  invisible  by	 implementing  F_GETOWN	 using

       In Linux 2.4 and earlier, there is bug that can occur when an  unprivi‐
       leged  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.

   Mandatory locking
       The implementation of mandatory locking in all known versions of	 Linux
       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 there‐
       fore inadvisable to rely on mandatory locking.

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

       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)

       This page is part of release 3.63 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2014-02-20			      FCNTL(2)

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