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

NAME
       fcntl - manipulate file descriptor

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

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

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

   Duplicating a file descriptor
       F_DUPFD
	      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.

   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
	      Read the file descriptor flags.

       F_SETFD
	      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(2).  Duplicated file
       descriptors  (made  with	 dup(), fcntl(F_DUPFD), fork(), 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
	      Read the file status flags.

       F_SETFL
	      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:
				 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
				 (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
       grows.

       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  con‐
       versions may involve splitting, shrinking, or coalescing with an exist‐
       ing lock if the byte range specified by the new lock does not precisely
       coincide with the range of the existing lock.)

       F_SETLK
	      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
	      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).

       F_GETLK
	      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
       processes.

       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)).

   Managing signals
       F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O avail‐
       ability signals:

       F_GETOWN
	      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).

       F_SETOWN
	      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‐
	      pid()).

	      If you set the O_ASYNC status flag on a file descriptor  (either
	      by  providing  this  flag with the open(2) call, or 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() instead of getpid() to get sen‐
	      sible results when F_SETSIG is used.  (In current Linux  thread‐
	      ing  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() or getpid() in this scenario.)  Note, how‐
	      ever, 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.

       F_GETSIG
	      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
	      SA_SIGINFO.

       F_SETSIG
	      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
	      details.

	      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).

   Leases
       F_SETLEASE and F_GETLEASE (Linux 2.4 onwards) are  used	(respectively)
       to  establish and retrieve the current setting of the calling process's
       lease on the file referred to by 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) that file.

       F_SETLEASE
	      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 writ‐
		     ing or is truncated.  A read lease can only be placed  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 no other process currently has the file open.

	      F_UNLCK
		     Remove our lease from the file.

       A process may hold only one type of lease on a file.

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

       F_GETLEASE
	      Indicates	 what type of lease we hold on the file referred to by
	      fd by returning either F_RDLCK, F_WRLCK, or F_UNLCK, indicating,
	      respectively, that the calling process holds a read, a write, or
	      no lease on the file.  (The third argument to fcntl()  is	 omit‐
	      ted.)

       When  a	process (the "lease breaker") performs an open() or truncate()
       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  we currently hold a write lease on the file, and the
       lease breaker is opening the file for reading, then it is sufficient 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 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() or truncate() 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() or truncate(), then the
       other steps still occur as described above.  If the lease breaker spec‐
       ifies  the  O_NONBLOCK  flag when calling open(), 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
	      (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:

	      Bit	  Description (event in directory)
	      ─────────────────────────────────────────────────────────────
	      DN_ACCESS	  A file was accessed (read, pread, readv)
	      DN_MODIFY	  A file was modified (write, pwrite,
			  writev, truncate, 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, utime[s])

	      (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 noti‐
	      fication 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‐
	      ries).

	      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 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).

RETURN VALUE
       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.

       F_GETOWN Value of descriptor owner.

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

       All other commands
		Zero.

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

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

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

       EBADF  fd is not an open file descriptor, 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.   For
	      F_GETLK  and  F_SETLK,  the  command was interrupted by a signal
	      before the lock was checked or acquired.	Most likely when lock‐
	      ing  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
	      number.

       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.

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

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

       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 e.g.  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.

BUGS
       A limitation of the Linux system call conventions on some architectures
       (notably x86) 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
       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.

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

       F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux spe‐
       cific.  (Define the _GNU_SOURCE macro to obtain these definitions.)

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

       See    also    locks.txt,    mandatory.txt,    and    dnotify.txt    in
       /usr/src/linux/Documentation.

Linux 2.6.14			  2005-05-20			      FCNTL(2)
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