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       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 closed.

       F_GETFD (void)
              Read the file descriptor flags; arg is ignored.

       F_SETFD (int)
              Set the file descriptor flags to the value specified by arg.

       In  multithreaded  programs, using fcntl() F_SETFD to set the close-on-
       exec flag at the same time as another thread performs  a  fork(2)  plus
       execve(2)  is  vulnerable  to a race condition that may unintentionally
       leak the file descriptor to the program executed in the child  process.
       See  the  discussion of the O_CLOEXEC flag in open(2) for details and a
       remedy to the problem.

   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

       F_SETFL (int)
              Set  the  file status flags to the value specified by arg.  File
              access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags
              (i.e.,  O_CREAT,  O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored.
              On Linux this command can change  only  the  O_APPEND,  O_ASYNC,
              O_DIRECT,  O_NOATIME,  and O_NONBLOCK flags.  It is not possible
              to change the O_DSYNC and O_SYNC flags; see BUGS, below.

   Advisory locking
       F_SETLK, F_SETLKW, and F_GETLK are used to acquire, release,  and  test
       for  the existence of record locks (also known as file-segment or file-
       region locks).  The third argument, lock, is a pointer to  a  structure
       that has at least the following fields (in unspecified order).

           struct flock {
               short l_type;    /* Type of lock: F_RDLCK,
                                   F_WRLCK, F_UNLCK */
               short l_whence;  /* How to interpret l_start:
                                   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.  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 file.

       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

       POSIX.1-2001 allows (but does not require) an implementation to support
       a negative l_len value; if l_len is negative, the interval described by
       lock covers bytes l_start+l_len up to and including l_start-1.  This is
       supported by Linux since kernel versions 2.4.21 and 2.5.49.

       The l_type field can be used to place  a  read  (F_RDLCK)  or  a  write
       (F_WRLCK) lock on a file.  Any number of processes may hold a read lock
       (shared lock) on a file region, but only one process may hold  a  write
       lock  (exclusive  lock).   An  exclusive lock excludes all other locks,
       both shared and exclusive.  A single process can hold only one type  of
       lock  on  a  file region; if a new lock is applied to an already-locked
       region, then the existing lock is  converted  to  the  new  lock  type.
       (Such  conversions may involve splitting, shrinking, or coalescing with
       an existing lock if the byte range specified by the new lock  does  not
       precisely coincide with the range of the existing lock.)

       F_SETLK (struct flock *)
              Acquire  a lock (when l_type is F_RDLCK or F_WRLCK) or release a
              lock (when l_type is F_UNLCK) on  the  bytes  specified  by  the
              l_whence,  l_start,  and l_len fields of lock.  If a conflicting
              lock is held by another process, this call returns -1  and  sets
              errno to EACCES or EAGAIN.

       F_SETLKW (struct flock *)
              As  for  F_SETLK, but if a conflicting lock is held on the file,
              then wait for that lock to be released.  If a signal  is  caught
              while  waiting,  then  the  call  is  interrupted and (after the
              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 holding that lock.  Note that
              the  information  returned by F_GETLK may already be out of date
              by the time the caller inspects it.

       In order to place a read lock, fd must be open for reading.   In  order
       to  place  a  write  lock,  fd must be open for writing.  To place both
       types of lock, open a file read-write.

       As well as being removed by  an  explicit  F_UNLCK,  record  locks  are
       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 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
       mandatory, 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
       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 EAGAIN.

       To  make use of mandatory locks, mandatory locking must be enabled both
       on the filesystem that contains the file to be locked, and on the  file
       itself.   Mandatory  locking  is  enabled on a filesystem using the "-o
       mand" option  to  mount(8),  or  the  MS_MANDLOCK  flag  for  mount(2).
       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

   Managing signals
       used to manage I/O availability signals:

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

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

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

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

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

              The following was true in 2.6.x  kernels  up  to  and  including
              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  specific  thread  within  a
                     process.   Consequently,  it  may  be  necessary  to pass
                     F_SETOWN the result of gettid(2) instead of getpid(2)  to
                     get  sensible results when F_SETSIG is used.  (In current
                     Linux threading implementations, a main  thread's  thread
                     ID  is  the  same  as  its process ID.  This means that a
                     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.

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

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

                  struct f_owner_ex {
                      int   type;
                      pid_t pid;

              The  type  field  will  have  one  of  the  values  F_OWNER_TID,
              F_OWNER_PID,  or  F_OWNER_PGRP.   The  pid  field  is a positive
              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 interpreted:

                     Send the signal to the thread whose thread ID (the  value
                     returned by a call to clone(2) or gettid(2)) is specified
                     in pid.

                     Send the signal to the process whose ID is  specified  in

                     Send  the  signal  to  the  process  group  whose  ID  is
                     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 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.  arg is ignored.

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

              By  using  F_SETSIG with a nonzero value, and setting SA_SIGINFO
              for the signal handler  (see  sigaction(2)),  extra  information
              about  I/O  events  is  passed  to  the  handler  in a siginfo_t
              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 is specific to BSD  and  Linux.   The  only  use  of
       F_GETOWN  and  F_SETOWN specified in POSIX.1 is in conjunction with the
       use of the SIGURG signal on sockets.  (POSIX does not specify the SIGIO
       signal.)   F_GETOWN_EX,  F_SETOWN_EX, F_GETSIG, and F_SETSIG are Linux-
       specific.  POSIX has asynchronous I/O and the aio_sigevent structure to
       achieve  similar  things;  these are also available in Linux as part of
       the GNU C Library (Glibc).

       F_SETLEASE and F_GETLEASE (Linux 2.4 onward) are used (respectively) to
       establish a new lease, and retrieve the current lease, on the open file
       description referred to by  the  file  descriptor  fd.   A  file  lease
       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.

       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  duplicate 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.  Furthermore, the
       lease is released by either an explicit F_UNLCK  operation  on  any  of
       these  duplicate  descriptors,  or  when all such descriptors have been

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

       F_GETLEASE (void)
              Indicates  what  type  of  lease  is  associated  with  the file
              descriptor fd by returning either F_RDLCK, F_WRLCK, or  F_UNLCK,
              indicating,  respectively,  a  read lease , a write lease, or no
              lease.  arg is ignored.

       When a process (the "lease breaker") performs an open(2) or truncate(2)
       that conflicts with a lease established via F_SETLEASE, the system call
       is blocked by the kernel and the kernel notifies the  lease  holder  by
       sending  it  a  signal  (SIGIO  by  default).   The lease holder should
       respond to receipt of this signal by doing whatever cleanup is required
       in  preparation  for  the file to be accessed by another process (e.g.,
       flushing cached buffers) and then either remove or downgrade its lease.
       A  lease  is removed by performing an F_SETLEASE command specifying arg
       as F_UNLCK.  If the lease holder currently holds a write lease  on  the
       file, and the lease breaker is opening the file for reading, then it is
       sufficient for the lease holder to downgrade the lease to a read lease.
       This  is  done  by  performing  an F_SETLEASE command specifying arg as

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

       Once a lease break has been initiated, F_GETLEASE  returns  the  target
       lease  type  (either  F_RDLCK  or  F_UNLCK,  depending on what would be
       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
              following 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,

              (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 reregister 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
              handler was established using SA_SIGINFO) and the si_fd field of
              this structure contains the file descriptor which generated  the
              notification  (useful when establishing notification on multiple

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

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

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

       EBADF  fd is not an open file descriptor, or the command was F_SETLK or
              F_SETLKW and the file descriptor open mode  doesn't  match  with
              the type of lock requested.

              It  was detected that the specified F_SETLKW command would cause
              a deadlock.

       EFAULT lock is outside your accessible address space.

       EINTR  For F_SETLKW, the command  was  interrupted  by  a  signal;  see
              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

       EMFILE For F_DUPFD, the process already has the maximum number of  file
              descriptors open.

       ENOLCK Too  many  segment  locks  open, lock table is full, or a remote
              locking protocol failed (e.g., locking over NFS).

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


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

       F_GETOWN  and  F_SETOWN  are  specified in POSIX.1-2001.  (To get their
       definitions, define _BSD_SOURCE, or _XOPEN_SOURCE with the value 500 or
       greater, or define _POSIX_C_SOURCE with the value 200809L or greater.)

       F_DUPFD_CLOEXEC is specified in POSIX.1-2008.  (To get this definition,
       define  _POSIX_C_SOURCE  with  the  value  200809L   or   greater,   or
       _XOPEN_SOURCE with the value 700 or greater.)

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


       The original Linux fcntl() system call was not designed to handle large
       file offsets (in the  flock  structure).   Consequently,  an  fcntl64()
       system  call  was  added in Linux 2.4.  The newer system call employs a
       different  structure  for  file  locking,  flock64,  and  corresponding
       commands, F_GETLK64, F_SETLK64, and F_SETLKW64.  However, these details
       can be ignored by  applications  using  glibc,  whose  fcntl()  wrapper
       function  transparently employs the more recent system call where it is

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

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

       Several systems have more fields in struct flock such as, for  example,
       l_sysid.   Clearly,  l_pid  alone is not going to be very useful if the
       process holding the lock may live on a different machine.


       It is not possible to use F_SETFL to change the state  of  the  O_DSYNC
       and  O_SYNC  flags.   Attempts  to  change the state of these flags are
       silently ignored.

       A limitation of the Linux system call conventions on some architectures
       (notably  i386)  means  that  if  a  (negative)  process group ID to be
       returned by F_GETOWN falls in the range -1 to -4095,  then  the  return
       value  is  wrongly interpreted by glibc as an error in the system call;
       that is, the return value of fcntl() will be -1, and errno will contain
       the  (positive)  process  group  ID.   The  Linux-specific  F_GETOWN_EX
       operation avoids this problem.  Since glibc version 2.11,  glibc  makes
       the  kernel  F_GETOWN  problem invisible by implementing F_GETOWN using

       In Linux 2.4  and  earlier,  there  is  bug  that  can  occur  when  an
       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 owner.

   Mandatory locking
       The implementation of mandatory locking in all known versions of  Linux
       is  subject  to  race conditions which render it unreliable: a write(2)
       call that overlaps with a lock may modify data after the mandatory lock
       is  acquired;  a  read(2)  call  that  overlaps  with a lock may detect
       changes to data that were made only after a write  lock  was  acquired.
       Similar  races  exist  between  mandatory  locks  and  mmap(2).   It is
       therefore inadvisable to rely on mandatory locking.


       dup2(2),  flock(2),  open(2),  socket(2),  lockf(3),   capabilities(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 Documentation/ directory, and mandatory-
       locking.txt is called mandatory.txt)


       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

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