Quick ?s
Cheat Sheets
Man Pages
The Lynx
FCNTL(2)		   Linux Programmers Manual		     FCNTL(2)

       fcntl - manipulate file descriptor


       int fcntl(int fd, int cmd);
       int fcntl(int fd, int cmd, long arg);
       int fcntl(int fd, int cmd, struct flock *lock);

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

   Duplicating a file descriptor
	      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 (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.

	      Read the file descriptor flags.

	      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).

	      Read the file status flags.

	      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 only  change  the  O_APPEND,  O_ASYNC,
	      O_DIRECT, O_NOATIME, and O_NONBLOCK flags.

   Advisory locking
       F_GETLK,  F_SETLK  and  F_SETLKW 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.	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  is  a  non-negative
       integer	(but see the NOTES below) specifying the number of bytes to be
       locked.	Bytes past the end of the file may be locked,  but  not  bytes
       before  the  start of the file.	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

       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.)

	      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.

	      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)).

	      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.

       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 or EWOULDBLOCK.

       To  make use of mandatory locks, mandatory locking must be enabled both
       on the file system that contains the file to be locked, and on the file
       itself.	 Mandatory  locking  is enabled on a file system 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
       F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O avail
       ability signals:

	      Get  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).

	      Set the process ID or process group ID that will	receive  SIGIO
	      and  SIGURG signals for events on file descriptor fd.  A process
	      ID is specified as a positive value; a process group ID is spec
	      ified  as  a negative value.  Most commonly, the calling process
	      specifies itself as the owner (that is, arg is specified as get

	      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".)

	      If  a  non-zero  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 threads 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.	Note also 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.

	      Get 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

	      Sets the signal sent when input or output becomes  possible.   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.

	      Additionally, passing a non-zero value to F_SETSIG  changes  the
	      signal  recipient  from  a  whole  process  to a specific thread
	      within a process.  See the  description  of  F_SETOWN  for  more

	      By  using F_SETSIG with a non-zero 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.

       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, F_GETOWN, F_SETOWN is specific to  BSD  and	Linux.
       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 onwards) 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.

	      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  pro
		     cess  to  be notified when the file is opened for writing
		     or is truncated.  A read lease can only be  placed  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 only be taken out on regular files.  An unprivileged process
       may only take out a lease on a file whose UID (owner) matches the  file
       system UID of the process.  A process with the CAP_LEASE capability may
       take out leases on arbitrary files.

	      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.  (The third argument to fcntl() is omitted.)

       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 holders lease.

       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 breakers system call to proceed.

       If the lease breakers 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)
	      (Linux  2.4  onwards)  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.)

	      Directory notifications are normally "one-shot", and the	appli
	      cation   must  re-register  to  receive  further	notifications.
	      Alternatively, if DN_MULTISHOT is included in arg, then  notifi
	      cation 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  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 consider using the inotify inter
	      face (available since kernel 2.6.13), which provides a  superior
	      interface  for  obtaining  notifications	of file system events.
	      See inotify(7).

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

       F_DUPFD	The new descriptor.

       F_GETFD	Value of flags.

       F_GETFL	Value of 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.

       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 doesnt 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_SETOWN are specified in POSIX.1-2001.

       cific.  (Define the _GNU_SOURCE macro to obtain these definitions.)

       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().

       POSIX.1-2001  allows l_len to be negative.  (And if it is, the interval
       described by the lock covers bytes l_start+l_len up  to	and  including
       l_start-1.)   This is supported by Linux since Linux 2.4.21 and 2.5.49.

       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.

       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.

       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 pro
       cess  (group) is one that the caller has permission to send signals to.
       Despite this error return, the file descriptor owner is set,  and  sig
       nals will be sent to the owner.

       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

       See also Documentation/locks.txt, Documentation/mandatory.txt, and Doc
       umentation/dnotify.txt in the kernel source.

       This  page  is  part of release 3.05 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				  2007-12-12			      FCNTL(2)

Yals.net is © 1999-2009 Crescendo Communications
Sharing tech info on the web for more than a decade!
This page was generated Thu Apr 30 17:05:23 2009