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       fcntl - manipulate file descriptor


       #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);


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

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

              On success, the new descriptor is returned.

              See dup(2) for further details.

       F_DUPFD_CLOEXEC (since Linux 2.6.24)
              As for F_DUPFD, but additionally set the close-on-exec flag  for
              the  duplicate  descriptor.   Specifying  this  flag  permits  a
              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.

              Read the file descriptor flags.

              Set the file descriptor flags to the value specified by arg.

   File status flags
       Each  open  file  description  has  certain  associated  status  flags,
       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).

              Read the file status flags.

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

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

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

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

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

              As  for  F_SETLK, but if a conflicting lock is held on the file,
              then wait for that lock to be released.  If a signal  is  caught
              while  waiting,  then  the  call  is  interrupted and (after the
              signal handler has returned) returns  immediately  (with  return
              value -1 and errno set to EINTR).

              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
       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 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).
       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
       F_GETOWN, F_SETOWN, F_GETSIG  and  F_SETSIG  are  used  to  manage  I/O
       availability signals:

              Get  the  process  ID or process group currently receiving SIGIO
              and SIGURG signals for events on file  descriptor  fd.   Process
              IDs  are  returned  as  positive  values;  process group IDs are
              returned as negative values (but see BUGS below).

              Set the process ID or process group ID that will  receive  SIGIO
              and  SIGURG signals for events on file descriptor fd.  A process
              ID is specified as a positive  value;  a  process  group  ID  is
              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".)

              If  a  non-zero  value  is given to F_SETSIG in a multi-threaded
              process running with a threading library  that  supports  thread
              groups (e.g., NPTL), then a positive value given to F_SETOWN has
              a different meaning: instead of being a process ID identifying a
              whole  process,  it is a thread ID identifying a specific thread
              within a process.  Consequently, it may  be  necessary  to  pass
              F_SETOWN  the  result  of  gettid(2) instead of getpid(2) to get
              sensible results when  F_SETSIG  is  used.   (In  current  Linux
              threading implementations, a main 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.  Note also that
              Linux imposes a limit on the number of  real-time  signals  that
              may  be queued to a process (see getrlimit(2) and signal(7)) and
              if this limit is reached, then the kernel reverts to  delivering
              SIGIO, and this signal is delivered to the entire process rather
              than to a specific thread.

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

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

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

              By  using F_SETSIG with a non-zero value, and setting SA_SIGINFO
              for the signal handler  (see  sigaction(2)),  extra  information
              about  I/O  events  is  passed  to  the  handler  in a siginfo_t
              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

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

       The use of O_ASYNC, F_GETOWN, F_SETOWN is specific to  BSD  and  Linux.
       F_GETSIG  and  F_SETSIG are Linux-specific.  POSIX has asynchronous I/O
       and the aio_sigevent structure to achieve  similar  things;  these  are
       also available in Linux as part of the GNU C Library (Glibc).

       F_SETLEASE  and  F_GETLEASE (Linux 2.4 onwards) are used (respectively)
       to establish a new lease, and retrieve the current lease, on  the  open
       file  description  referred to by the file descriptor fd.  A file lease
       provides a mechanism whereby the process holding the lease (the  "lease
       holder")  is  notified  (via  delivery of a signal) when a process (the
       "lease breaker") tries to open(2) or truncate(2) the file  referred  to
       by that file descriptor.

              Set  or  remove a file lease according to which of the following
              values is specified in the integer arg:

                     Take out a read  lease.   This  will  cause  the  calling
                     process  to  be  notified  when  the  file  is opened for
                     writing or is truncated.  A read lease can only be placed
                     on a file descriptor that is opened read-only.

                     Take out a write lease.  This will cause the caller to be
                     notified when the file is opened for reading  or  writing
                     or  is  truncated.  A write lease may be placed on a file
                     only if there are no other open file descriptors for  the

                     Remove our lease from the file.

       Leases  are  associated  with  an  open file description (see open(2)).
       This means that duplicate file descriptors (created  by,  for  example,
       fork(2)  or  dup(2))  refer  to  the  same lease, and this lease may be
       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 only be taken out on regular files.  An unprivileged process
       may only take out a lease on a file whose UID (owner) matches the  file
       system UID of the process.  A process with the CAP_LEASE capability may
       take out leases on arbitrary files.

              Indicates what  type  of  lease  is  associated  with  the  file
              descriptor  fd by returning either F_RDLCK, F_WRLCK, or F_UNLCK,
              indicating, respectively, a read lease , a write  lease,  or  no
              lease.  (The third argument to fcntl() is omitted.)

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

       If  the  lease holder fails to downgrade or remove the lease within the
       number of seconds specified in /proc/sys/fs/lease-break-time  then  the
       kernel forcibly removes or downgrades the lease 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(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)
              (Linux  2.4  onwards)  Provide  notification  when the directory
              referred to by fd or any  of  the  files  that  it  contains  is
              changed.   The events to be notified are specified in arg, which
              is a bit mask specified by ORing together zero or  more  of  the
              following bits:

              DN_ACCESS   A file was accessed (read, pread, readv)
              DN_MODIFY   A   file   was   modified  (write,  pwrite,  writev,
                          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.)

              Directory  notifications  are  normally  "one-shot",   and   the
              application  must  re-register 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  consider  using  the  inotify
              interface (available since  kernel  2.6.13),  which  provides  a
              superior  interface  for  obtaining notifications of file system
              events.  See inotify(7).


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

       F_DUPFD  The new descriptor.

       F_GETFD  Value of flags.

       F_GETFL  Value of flags.

                Type of lease held on file descriptor.

       F_GETOWN Value of descriptor owner.

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

       All other commands

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


              Operation is prohibited by locks held by other processes.

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

       EBADF  fd is not an open file descriptor, or the command was F_SETLK or
              F_SETLKW  and  the  file descriptor open mode 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.  For
              F_GETLK and F_SETLK, the command was  interrupted  by  a  signal
              before  the  lock  was  checked  or  acquired.  Most likely when
              locking  a  remote  file  (e.g.,  locking  over  NFS),  but  can
              sometimes happen locally.

       EINVAL For  F_DUPFD,  arg  is  negative  or is greater than the maximum
              allowable value.  For F_SETSIG, arg is not an  allowable  signal

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

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

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


       SVr4, 4.3BSD, POSIX.1-2001.   Only  the  operations  F_DUPFD,  F_GETFD,
       F_SETOWN are specified in POSIX.1-2001.

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


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

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

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

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


       A limitation of the Linux system call conventions on some architectures
       (notably i386) means that if  a  (negative)  process  group  ID  to  be
       returned  by  F_GETOWN  falls in the range -1 to -4095, then the return
       value is wrongly interpreted by glibc as an error in the  system  call;
       that is, the return value of fcntl() will be -1, and errno will contain
       the (positive) process group ID.

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

       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),

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


       This page is part of release 3.01 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at