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CentOS 7.0 - man page for fcntl (centos section 2)

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 argument is required  is
       determined  by cmd.  The required argument type is indicated in parentheses after each cmd
       name (in most cases, the required type is int, and we identify the argument using the name
       arg), or void is specified if the argument is not required.

   Duplicating a file descriptor
       F_DUPFD (int)
	      Find the lowest numbered available file descriptor greater than or equal to arg and
	      make it be a copy of fd.	This is different from dup2(2), which  uses  exactly  the
	      descriptor specified.

	      On success, the new descriptor is returned.

	      See dup(2) for further details.

       F_DUPFD_CLOEXEC (int; since Linux 2.6.24)
	      As  for  F_DUPFD,  but  additionally  set  the close-on-exec flag for the duplicate
	      descriptor.  Specifying this flag permits a program to avoid an additional  fcntl()
	      F_SETFD  operation to set the FD_CLOEXEC flag.  For an explanation of why this flag
	      is useful, see the description of O_CLOEXEC in open(2).

   File descriptor flags
       The following commands manipulate the flags associated with a file descriptor.  Currently,
       only  one such flag is defined: FD_CLOEXEC, the close-on-exec flag.  If the FD_CLOEXEC bit
       is 0, the file descriptor will remain open across  an  execve(2),  otherwise  it  will  be

       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, initialized by open(2) and
       possibly  modified  by  fcntl().   Duplicated  file   descriptors   (made   with   dup(2),
       fcntl(F_DUPFD), fork(2), etc.) refer to the same open file description, and thus share the
       same file status flags.

       The file status flags and their semantics are described in open(2).

       F_GETFL (void)
	      Get the file access mode and the file status flags; arg is ignored.

       F_SETFL (int)
	      Set the file status flags  to  the  value  specified  by	arg.   File  access  mode
	      (O_RDONLY,  O_WRONLY,  O_RDWR)  and  file  creation  flags  (i.e., O_CREAT, O_EXCL,
	      O_NOCTTY, O_TRUNC) in arg are ignored.  On Linux this command can change	only  the

   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.  Bytes past the end of the file may be locked,  but  not  bytes  before  the
       start of the file.

       l_start	is  the  starting offset for the lock, and is interpreted relative to either: the
       start of the file (if l_whence is SEEK_SET); the  current  file	offset	(if  l_whence  is
       SEEK_CUR);  or  the  end  of  the file (if l_whence is SEEK_END).  In the final two cases,
       l_start can be a negative number provided the offset does not lie before the start of  the

       l_len specifies the number of bytes to be locked.  If l_len is positive, then the range to
       be locked covers bytes l_start up to and  including  l_start+l_len-1.   Specifying  0  for
       l_len  has  the	special  meaning:  lock  all  bytes starting at the location specified by
       l_whence and l_start through to the end of file, no matter how large the file grows.

       POSIX.1-2001 allows (but does not require) an implementation to support a  negative  l_len
       value;  if l_len is negative, the interval described by lock covers bytes l_start+l_len up
       to and including l_start-1.  This is supported by Linux since kernel versions  2.4.21  and

       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,  shrink-
       ing,  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  inter-
	      rupted and (after the signal handler has returned) returns immediately (with return
	      value -1 and errno set to EINTR; see signal(7)).

       F_GETLK (struct flock *)
	      On input to this call, lock describes a lock we would like to place  on  the  file.
	      If  the  lock  could  be	placed,  fcntl()  does not actually place it, but returns
	      F_UNLCK in the l_type field of lock and leaves the other fields  of  the	structure
	      unchanged.  If one or more incompatible locks would prevent this lock being placed,
	      then fcntl() returns details about one of 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 hold-
	      ing 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 automatically 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

       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 mandatory, 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  incom-
       patible	access	(e.g.,	read(2)  or  write(2))	on a file region that has an incompatible
       mandatory lock, then the result depends upon whether the O_NONBLOCK flag  is  enabled  for
       its  open  file	description.   If the O_NONBLOCK flag is not enabled, then system call is
       blocked until the lock is removed or converted to a  mode  that	is  compatible	with  the
       access.	 If  the  O_NONBLOCK  flag  is enabled, then the system call fails with the error

       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).  Mandatory locking is enabled on a file by disabling group execute permission on
       the file and enabling the set-group-ID permission bit (see chmod(1) and chmod(2)).

       The Linux implementation of mandatory locking is unreliable.  See BUGS below.

   Managing signals
       F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG and F_SETSIG are used to manage I/O
       availability signals:

       F_GETOWN (void)
	      Return (as the function result) the process ID or process group currently receiving
	      SIGIO and SIGURG signals for  events  on	file  descriptor  fd.	Process  IDs  are
	      returned as positive values; process group IDs are returned as negative values (but
	      see BUGS below).	arg is ignored.

       F_SETOWN (int)
	      Set the process ID or process group ID that will receive SIGIO and  SIGURG  signals
	      for events on file descriptor fd to the ID given in arg.	A process ID is specified
	      as a positive value; a process group ID is specified as  a  negative  value.   Most
	      commonly, the calling process specifies itself as the owner (that is, arg is speci-
	      fied as getpid(2)).

	      If you set the O_ASYNC status flag on a file descriptor by using the  F_SETFL  com-
	      mand  of	fcntl(), a SIGIO signal is sent whenever input or output becomes possible
	      on that file descriptor.	F_SETSIG can be used to obtain delivery of a signal other
	      than SIGIO.  If this permission check fails, then the signal is silently discarded.

	      Sending  a  signal to the owner process (group) specified by F_SETOWN is subject to
	      the same permissions checks as are described for kill(2), where the sending process
	      is the one that employs F_SETOWN (but see BUGS below).

	      If  the  file descriptor fd refers to a socket, F_SETOWN also selects the recipient
	      of SIGURG signals that are delivered when out-of-band data arrives on that  socket.
	      (SIGURG  is sent in any situation where select(2) would report the socket as having
	      an "exceptional condition".)

	      The following was true in 2.6.x kernels up to and including kernel 2.6.11:

		     If a nonzero value is given to F_SETSIG in a multithreaded  process  running
		     with  a  threading  library that supports thread groups (e.g., NPTL), then a
		     positive value given to F_SETOWN has a different meaning: instead of being a
		     process ID identifying a whole process, it is a thread ID identifying a spe-
		     cific thread within a process.  Consequently, it may be  necessary  to  pass
		     F_SETOWN  the  result  of	gettid(2)  instead  of	getpid(2) to get sensible
		     results when F_SETSIG is used.  (In current Linux threading implementations,
		     a	main thread's thread ID is the same as its process ID.	This means that a
		     single-threaded program can equally use gettid(2) or getpid(2) in this  sce-
		     nario.)   Note,  however, that the statements in this paragraph do not apply
		     to the SIGURG signal generated for out-of-band data on a socket: this signal
		     is  always  sent  to  either  a process or a process group, depending on the
		     value given to F_SETOWN.

	      The above behavior was accidentally dropped in Linux 2.6.12, and won't be restored.
	      From  Linux  2.6.32 onward, use F_SETOWN_EX to target SIGIO and SIGURG signals at a
	      particular thread.

       F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
	      Return the current  file	descriptor  owner  settings  as  defined  by  a  previous
	      F_SETOWN_EX  operation.  The information is returned in the structure pointed to by
	      arg, which has the following form:

		  struct f_owner_ex {
		      int   type;
		      pid_t pid;

	      The  type  field	will  have  one  of  the  values  F_OWNER_TID,	F_OWNER_PID,   or
	      F_OWNER_PGRP.   The  pid	field  is  a  positive	integer representing a thread ID,
	      process ID, or process group ID.	See F_SETOWN_EX for more details.

       F_SETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
	      This operation performs a similar task to F_SETOWN.  It allows the caller to direct
	      I/O  availability  signals  to  a  specific thread, process, or process group.  The
	      caller specifies the target of signals via arg, which is a pointer to a  f_owner_ex
	      structure.  The type field has one of the following values, which define how pid is

		     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 pid.

		     Send  the	signal	to the process group whose ID is specified in pid.  (Note
		     that, unlike with F_SETOWN, a process group ID is specified  as  a  positive
		     value here.)

       F_GETSIG (void)
	      Return (as the function result) the signal sent when input or output becomes possi-
	      ble.  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 sig-
	      nal handler if installed with SA_SIGINFO.  arg is ignored.

       F_SETSIG (int)
	      Set the signal sent when input or output becomes possible to  the  value	given  in
	      arg.   A	value  of  zero  means to send the default SIGIO signal.  Any other value
	      (including SIGIO) is the signal to send instead, and in this case  additional  info
	      is available to the signal handler if installed with SA_SIGINFO.

	      By  using F_SETSIG with a nonzero value, and setting SA_SIGINFO for the signal han-
	      dler (see sigaction(2)), extra information about I/O events is passed to	the  han-
	      dler  in	a  siginfo_t  structure.   If  the  si_code field indicates the source is
	      SI_SIGIO, the si_fd field gives the file	descriptor  associated	with  the  event.
	      Otherwise,  there  is  no  indication  which  file descriptors are pending, and you
	      should use the usual mechanisms (select(2), poll(2), read(2)  with  O_NONBLOCK  set
	      etc.) to determine which file descriptors are available for I/O.

	      By  selecting  a	real  time signal (value >= SIGRTMIN), multiple I/O events may be
	      queued using the same signal numbers.  (Queuing is dependent on available  memory).
	      Extra  information  is  available  if  SA_SIGINFO is set for the signal handler, as

	      Note that Linux imposes a limit on the number of	real-time  signals  that  may  be
	      queued  to a process (see getrlimit(2) and signal(7)) and if this limit is reached,
	      then the kernel reverts to delivering SIGIO, and this signal is  delivered  to  the
	      entire process rather than to a specific thread.

       Using  these  mechanisms,  a  program  can  implement fully asynchronous I/O without using
       select(2) or poll(2) most of the time.

       The use of O_ASYNC, F_GETOWN,  F_SETOWN	is  specific  to  BSD  and  Linux.   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 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

       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 writing 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 dupli-
       cate  file  descriptors	(created  by,  for  example, fork(2) or dup(2)) refer to the same
       lease, and this lease may be modified or released using any of  these  descriptors.   Fur-
       thermore,  the  lease  is released by either an explicit F_UNLCK operation on any of these
       duplicate descriptors, or when all such descriptors have been closed.

       Leases may be taken out only on regular files.  An unprivileged process	may  take  out	a
       lease  only  on	a  file  whose UID (owner) matches the file system 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 suf-
       ficient for the lease holder to downgrade the lease to a read lease.  This is done by per-
       forming an F_SETLEASE command specifying arg as F_RDLCK.

       If  the	lease  holder fails to downgrade or remove the lease within the number of seconds
       specified in /proc/sys/fs/lease-break-time then the kernel forcibly removes or  downgrades
       the lease holder's lease.

       Once  a	lease  break has been initiated, F_GETLEASE returns the target lease type (either
       F_RDLCK or F_UNLCK, depending on what would be compatible with the  lease  breaker)  until
       the  lease  holder voluntarily downgrades or removes the lease or the kernel forcibly does
       so after the lease break timer expires.

       Once the lease has been voluntarily or forcibly removed or downgraded,  and  assuming  the
       lease  breaker  has  not unblocked its system call, the kernel permits the lease breaker's
       system call to proceed.

       If the lease breaker's blocked open(2) or truncate(2) is interrupted by a signal  handler,
       then  the  system  call	fails  with  the  error EINTR, but the other steps still occur as
       described above.  If the lease breaker is killed by a signal while blocked in  open(2)  or
       truncate(2),  then  the	other steps still occur as described above.  If the lease breaker
       specifies the O_NONBLOCK flag when calling open(2), then the call immediately  fails  with
       the error EWOULDBLOCK, but the other steps still occur as described above.

       The default signal used to notify the lease holder is SIGIO, but this can be changed using
       the F_SETSIG command to fcntl().  If a F_SETSIG command is performed (even one  specifying
       SIGIO),	and  the  signal  handler  is established using SA_SIGINFO, then the handler will
       receive a siginfo_t structure as its second argument, and the si_fd field of this argument
       will  hold  the	descriptor  of the leased file that has been accessed by another process.
       (This is useful if the caller holds leases against multiple files).

   File and directory change notification (dnotify)
       F_NOTIFY (int)
	      (Linux 2.4 onward) Provide notification when the directory referred to by fd or any
	      of  the files that it contains is changed.  The events to be notified are specified
	      in arg, which is a bit mask specified by ORing together zero or more of the follow-
	      ing bits:

	      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 before including any header files.)

	      Directory notifications are normally "one-shot", and the application must  reregis-
	      ter  to  receive further notifications.  Alternatively, if DN_MULTISHOT is included
	      in arg, then notification will remain in effect until explicitly removed.

	      A series of F_NOTIFY requests is cumulative, with the events in arg being added  to
	      the set already monitored.  To disable notification of all events, make an F_NOTIFY
	      call specifying arg as 0.

	      Notification occurs via delivery of a signal.  The default  signal  is  SIGIO,  but
	      this can be changed using the F_SETSIG command to fcntl().  In the latter case, the
	      signal handler receives a siginfo_t structure as its second argument (if	the  han-
	      dler  was  established using SA_SIGINFO) and the si_fd field of this structure con-
	      tains the file descriptor which generated the notification (useful when  establish-
	      ing notification on multiple directories).

	      Especially when using DN_MULTISHOT, a real time signal should be used for notifica-
	      tion, so that multiple notifications can be queued.

	      NOTE: New applications should use the inotify  interface	(available  since  kernel
	      2.6.13),	which  provides  a much superior interface for obtaining notifications of
	      file system events.  See inotify(7).

   Changing the capacity of a pipe
       F_SETPIPE_SZ (int; since Linux 2.6.35)
	      Change the capacity of the pipe referred to by fd to be at  least  arg  bytes.   An
	      unprivileged  process  can adjust the pipe capacity to any value between the system
	      page size and  the  limit  defined  in  /proc/sys/fs/pipe-max-size  (see	proc(5)).
	      Attempts	to  set  the pipe capacity below the page size are silently rounded up to
	      the page size.  Attempts by an unprivileged process to set the pipe capacity  above
	      the limit in /proc/sys/fs/pipe-max-size yield the error EPERM; a privileged process
	      (CAP_SYS_RESOURCE) can override the limit.  When	allocating  the  buffer  for  the
	      pipe,  the kernel may use a capacity larger than arg, if that is convenient for the
	      implementation.  The F_GETPIPE_SZ operation returns the actual size used.  Attempt-
	      ing to set the pipe capacity smaller than the amount of buffer space currently used
	      to store data produces the error EBUSY.

       F_GETPIPE_SZ (void; since Linux 2.6.35)
	      Return (as the function result) the capacity of the pipe referred to by fd.

       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

       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 signal(7).  For F_GETLK
	      and F_SETLK, the command was interrupted by a signal before the lock was checked or
	      acquired.  Most likely when locking a remote file (e.g., locking over NFS), but can
	      sometimes happen locally.

       EINVAL 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

       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

       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() system call was added in Linux 2.4.  The
       newer system call employs a different structure for file locking, flock64, and correspond-
       ing commands, F_GETLK64, F_SETLK64, and F_SETLKW64.  However, these details can be ignored
       by applications using glibc, whose fcntl() wrapper function transparently employs the more
       recent system call where it is available.

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

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

       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 sys-
       tem  call;  that  is,  the  return value of fcntl() will be -1, and errno will contain the
       (positive) process group ID.  The Linux-specific F_GETOWN_EX operation avoids  this  prob-
       lem.   Since  glibc  version  2.11,  glibc  makes the kernel F_GETOWN problem invisible by
       implementing F_GETOWN using F_GETOWN_EX.

       In Linux 2.4 and earlier, there is bug that can occur when an  unprivileged  process  uses
       F_SETOWN  to specify the owner of a socket file descriptor as a process (group) other than
       the caller.  In this case, fcntl() can return -1 with errno set to EPERM,  even	when  the
       owner  process  (group) is one that the caller has permission to send signals to.  Despite
       this error return, the file descriptor owner is set, and  signals  will	be  sent  to  the

       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  mod-
       ify  data  after  the mandatory lock is acquired; a read(2) call that overlaps with a lock
       may detect changes to data that were made only after a write lock was  acquired.   Similar
       races  exist  between mandatory locks and mmap(2).  It is therefore inadvisable to rely on
       mandatory locking.

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

       locks.txt, mandatory-locking.txt, and dnotify.txt in the  Linux	kernel	source	directory
       Documentation/filesystems/  (on older kernels, these files are directly under the Documen-
       tation/ directory, and mandatory-locking.txt is called mandatory.txt)

       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2012-04-15					 FCNTL(2)

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