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FCNTL(2V)							     FCNTL(2V)

NAME
       fcntl - file control

SYNOPSIS
       #include	<sys/types.h>
       #include	<unistd.h>
       #include	<fcntl.h>

       int fcntl(fd, cmd, arg)
       int fd, cmd, arg;

DESCRIPTION
       fcntl() performs	a variety of functions on open descriptors.  The argu-
       ment fd is an open descriptor used by cmd as follows:

       F_DUPFD	      Returns a	new descriptor,	which has the  smallest	 value
		      greater than or equal to arg.  It	refers to the same ob-
		      ject as the original descriptor, and has the same	access
		      mode  (read,  write  or read/write).  The	new descriptor
		      shares descriptor	status flags with fd, and if  the  ob-
		      ject  was	a file,	the same file pointer.	It is also as-
		      sociated with a FD_CLOEXEC (close-on-exec) flag  set  to
		      remain open across execve(2V) system calls.

       F_GETFD	      Get  the FD_CLOEXEC (close-on-exec) flag associated with
		      fd.  If the low-order bit	is 0, the  file	 remains  open
		      after executing execve(),	otherwise it is	closed.

       F_SETFD	      Set  the FD_CLOEXEC (close-on-exec) flag associated with
		      fd to the	low order bit of arg (0	or 1 as	above).

		      Note: this is a  per-process  and	 per-descriptor	 flag.
		      Setting  or clearing it for a particular descriptor does
		      not affect the flag on descriptors  copied  from	it  by
		      dup(2V) or F_DUPFD, nor does it affect the flag on other
		      processes	of that	descriptor.

       F_GETFL	      Get descriptor status flags (see	fcntl(5)  for  defini-
		      tions).

       F_SETFL	      Set  descriptor  status  flags (see fcntl(5) for defini-
		      tions).  The following flags are	the  only  ones	 whose
		      values  may  change: O_APPEND, O_SYNC, and O_NDELAY, and
		      the FASYNC, FNDELAY, and FNBIO  flags  defined  in  <fc-
		      ntl.h>.

		      O_NDELAY and FNDELAY are identical.

		      Descriptor status	flag values set	by F_SETFL affects de-
		      scriptors	copied using dup(2V), F_DUPFD  or  other  pro-
		      cesses.

		      Setting  or  clearing  the  FNDELAY flag on a descriptor
		      causes an	FIONBIO	ioctl(2) request to  be	 performed  on
		      the  object  referred to by that descriptor.  Setting or
		      clearing non-blocking mode, and setting or clearing  the
		      FASYNC  flag on a	descriptor causes an FIOASYNC ioctl(2)
		      request to be performed on the  object  referred	to  by
		      that  descriptor,	setting	or clearing asynchronous mode.
		      Thus, all	descriptors referring to the  object  are  af-
		      fected.

       F_GETLK	      Get  a  description  of the first	lock which would block
		      the lock specified by the	flock structure	pointed	to  by
		      arg  (see	 the  definition of struct flock below).  If a
		      lock exists, The flock  structure	 is  overwritten  with
		      that  lock's  description.   Otherwise, the structure is
		      passed back with the lock	type set to  F_UNLOCK  and  is
		      otherwise	unchanged.

       F_SETLK	      Set  or clear a file segment lock	according to the flock
		      structure	pointed	to by arg.  F_SETLK  is	 used  to  set
		      shared (F_RDLCK) or exclusive (F_WRLCK) locks, or	to re-
		      move those locks (F_UNLCK).  If the specified lock  can-
		      not be applied, fcntl() fails and	returns	immediately.

       F_SETLKW	      This  cmd	is the same as F_SETLK except that if a	shared
		      or exclusive lock	is blocked by other locks, the process
		      waits  until  the	 requested  lock can be	applied.  If a
		      signal that is set to be caught (see signal(3V)) is  re-
		      ceived  while  fcntl() is	waiting	for a region, the call
		      to  fcntl()  is  interrupted.   Upon  return  from   the
		      process's	signal handler,	fcntl()	fails and returns, and
		      the requested lock is not	applied.

       F_GETOWN	      Get the process ID or process group currently  receiving
		      SIGIO and	SIGURG signals;	process	groups are returned as
		      negative values.

       F_SETOWN	      Set the process or process group to  receive  SIGIO  and
		      SIGURG signals.  Process groups are specified by supply-
		      ing arg as negative, otherwise arg is interpreted	 as  a
		      process ID.

       F_RSETLK
       F_RSETLKW
       F_RGETLK	      Are  used	by the network lock daemon, lockd(8C), to com-
		      municate with the	NFS server kernel to handle  locks  on
		      the NFS files.

       Record  locking	is  done  with	either	shared (F_RDLCK), or exclusive
       (F_WRLCK) locks.	 More than one process may hold	a  shared  lock	 on  a
       particular  file	segment, but if	one process holds an exclusive lock on
       the segment, no other process may hold any lock on  the	segment	 until
       the exclusive lock is removed.

       In  order to claim a shared lock, a descriptor must be opened with read
       access.	Descriptors for	exclusive locks	must be	opened with write  ac-
       cess.

       A shared	lock may be changed to an exclusive lock, and vice versa, sim-
       ply by specifying the appropriate lock type with	a F_SETLK or  F_SETLKW
       cmd.   Before  the  previous lock is released and the new lock applied,
       any other processes already in line must	gain and release their locks.

       If cmd is F_SETLKW and the requested lock cannot	be claimed immediately
       (for  instance, when another process holds an exclusive lock that over-
       laps the	current	request) the calling process is	blocked	until the lock
       may  be	acquired.   These  blocks may be interrupted by	signals.  Care
       should be taken to avoid	deadlocks caused  by  multiple	processes  all
       blocking	the same records.

       A shared	or exclusive lock is either advisory or	mandatory depending on
       the mode	bits of	the file containing the	locked segment.	 The  lock  is
       mandatory if the	set-GID	bit (S_ISGID) is set and the group execute bit
       (S_IXGRP) is clear (see stat(2V)	 for  information  about  mode	bits).
       Otherwise, the lock is advisory.

       If  a  process  holds  a	 mandatory shared lock on a segment of a file,
       other processes may read	from the segment, but write  operations	 block
       until  all locks	are removed.  If a process holds a mandatory exclusive
       lock on a segment of a file, both read and write	operations block until
       the lock	is removed (see	WARNINGS).

       An  advisory  lock  does	not affect read	and write access to the	locked
       segment.	 Advisory locks	may be used by cooperating processes  checking
       for  locks  using F_GETLCK and voluntarily observing the	indicated read
       and write restrictions.

       The record to be	locked or unlocked is described	by the flock structure
       defined in <fcntl.h> as follows:

	      struct flock {
		     short l_type;   /*	F_RDLCK, F_WRLCK, or F_UNLCK */
		     short l_whence; /*	flag to	choose starting	offset */
		     long  l_start;  /*	relative offset, in bytes */
		     long  l_len;    /*	length,	in bytes;  0 means lock	to EOF */
		     pid_t l_pid;    /*	returned with F_GETLK */
	      };

       The  flock  structure  describes	 the  type  (l_type),  starting	offset
       (l_whence), relative offset (l_start), and size	(l_len)	 of  the  file
       segment	to  be	affected.   l_whence  is set to	SEEK_SET, SEEK_CUR, or
       SEEK_END	(see lseek(2V))	to indicate that the relative offset is	to  be
       measured	 from the start	of the file, current position, or EOF, respec-
       tively.	The process id field (l_pid) is	only used with the F_GETLK cmd
       to return the description of a lock held	by another process.  Note:  do
       not confuse struct flock	with the function flock(2).   They  are	 unre-
       lated.

       Locks  may start	or extend beyond the current EOF, but may not be nega-
       tive relative to	the beginning of the file.  Setting l_len to zero  (0)
       extends	the  lock  to EOF.  If l_whence	is set to SEEK_SET and l_start
       and l_len are set to zero (0), the entire file is locked.  Changing  or
       unlocking the subset of a locked	segment	leaves the smaller segments at
       either end locked.  Locking a segment already  locked  by  the  calling
       process causes the old lock type	to be removed and the new lock type to
       take affect.  All locks associated with a file for a given process  are
       removed	when  the file is closed or the	process	terminates.  Locks are
       not inherited by	the child process in a fork(2V)	system call.

       fcntl() record locks are	implemented in the kernel for local locks, and
       throughout  the	network	by the network lock daemon (lockd(8C)) for re-
       mote locks on NFS files.	 If the	file server crashes and	has to be  re-
       booted, the lock	daemon attempts	to recover all locks that were associ-
       ated with that server.  If a lock cannot	be reclaimed, the process that
       held the	lock is	issued a SIGLOST signal.

       In  order to maintain consistency in the	network	case, data must	not be
       cached on client	machines.  For this reason, file buffering for an  NFS
       file  is	 turned	 off  when  the	 first	lock is	attempted on the file.
       Buffering remains off as	long as	the file is open.   Programs  that  do
       I/O buffering in	the user address space,	however, may have inconsistent
       results.	 The standard I/O package, for instance, is a common source of
       unexpected buffering.

SYSTEM V DESCRIPTION
       O_NDELAY	and FNBIO are identical.

RETURN VALUES
       On success, the value returned by fcntl() depends on cmd	as follows:

	      F_DUPFD	     A new descriptor.

	      F_GETFD	     Value  of	flag  (only  the  low-order bit	is de-
			     fined).

	      F_GETFL	     Value of flags.

	      F_GETOWN	     Value of descriptor owner.

	      other	     Value other than -1.

       On failure, fcntl() returns -1 and sets errno to	indicate the error.

ERRORS
       EACCES	      cmd is  F_SETLK,	the  lock  type	 (l_type)  is  F_RDLCK
		      (shared  lock), and the file segment to be locked	is al-
		      ready under an exclusive lock held by  another  process.
		      This  error is also returned if the lock type is F_WRLCK
		      (exclusive lock) and the file segment is already	locked
		      with a shared or exclusive lock.

		      Note: In future, fcntl() may generate EAGAIN under these
		      conditions, so applications testing  for	EACCES	should
		      also test	for EAGAIN.

       EBADF	      fd is not	a valid	open descriptor.

		      cmd is F_SETLK or	F_SETLKW and the process does not have
		      the appropriate read or write permissions	on the file.

       EDEADLK	      cmd is F_SETLKW, the lock	is blocked by one from another
		      process,	and putting the	calling-process	to sleep would
		      cause a deadlock.

       EFAULT	      cmd is F_GETLK, F_SETLK, or F_SETLKW and arg  points  to
		      an invalid address.

       EINTR	      cmd  is  F_SETLKW	 and  a	signal interrupted the process
		      while it was waiting for the lock	to be granted.

       EINVAL	      cmd is F_DUPFD and arg is	negative or greater  than  the
		      maximum allowable	number (see getdtablesize(2)).

		      cmd  is  F_GETLK,	F_SETLK, or F_SETLKW and arg points to
		      invalid data.

       EMFILE	      cmd is F_DUPFD and the maximum number of	open  descrip-
		      tors has been reached.

       ENOLCK	      cmd  is  F_SETLK	or F_SETLKW and	there are no more file
		      lock entries available.

SEE ALSO
       close(2V), execve(2V), flock(2),	fork(2V), getdtablesize(2),  ioctl(2),
       open(2V), sigvec(2), lockf(3), fcntl(5),	lockd(8C)

WARNINGS
       Mandatory  record  locks	are dangerous.	If a runaway or	otherwise out-
       of-control process should hold a	mandatory lock on a file  critical  to
       the  system and fail to release that lock, the entire system could hang
       or crash.  For this reason, mandatory record locks may be removed in  a
       future SunOS release.  Use advisory record locking whenever possible.

NOTES
       Advisory	locks allow cooperating	processes to perform consistent	opera-
       tions on	files, but do not guarantee exclusive access.	Files  can  be
       accessed	without	advisory files,	but inconsistencies may	result.

       read(2V)	 and write(2V) system calls on files are affected by mandatory
       file and	record locks (see chmod(2V)).

BUGS
       File locks obtained by fcntl() do  not  interact	 with  flock()	locks.
       They  do,  however,  work correctly with	the exclusive locks claimed by
       lockf(3).

       F_GETLK returns F_UNLCK if the requesting process holds	the  specified
       lock.   Thus, there is no way for a process to determine	if it is still
       holding a specific lock after catching a	SIGLOST	signal.

       In a network environment, the value of l_pid  returned  by  F_GETLK  is
       next to useless.

				21 January 1990			     FCNTL(2V)

NAME | SYNOPSIS | DESCRIPTION | SYSTEM V DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | WARNINGS | NOTES | BUGS

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