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NAME

       fcntl - manipulate file descriptor

SYNOPSIS

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

       int fcntl(int fd, int cmd, ... /* arg */ );

DESCRIPTION

       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  long,  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 (long)
              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 (long; 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 (long)
              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)
              Read the file status flags; arg is ignored.

       F_SETFL (long)
              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.  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
       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 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.

       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
       processes.

       Mandatory  locks are enforced for all processes.  If a process tries to
       perform an incompatible access (e.g., read(2) or write(2))  on  a  file
       region that has an incompatible mandatory lock, then the result depends
       upon  whether  the  O_NONBLOCK  flag  is  enabled  for  its  open  file
       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 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_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 (long)
              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".)

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

       F_GETSIG (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 (long)
              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.

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

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

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

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

   Leases
       F_SETLEASE  and  F_GETLEASE (Linux 2.4 onwards) are used (respectively)
       to establish 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 (long)
              Set  or  remove a file lease according to which of the following
              values is specified in the integer arg:

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

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

              F_UNLCK
                     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
       closed.

       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.

       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
       F_RDLCK.

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

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

       If the lease breaker’s blocked open(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 (long)
              (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,
                          utime[s]).

              (In  order  to obtain these definitions, the _GNU_SOURCE feature
              test macro must be defined.)

              Directory  notifications  are  normally  "one-shot",   and   the
              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
              directories).

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

              NOTE:   New   applications  should  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).

RETURN VALUE

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

       F_DUPFD  The new descriptor.

       F_GETFD  Value of flags.

       F_GETFL  Value of flags.

       F_GETLEASE
                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
                Zero.

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

ERRORS

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

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

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

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

       EFAULT lock is outside your accessible address space.

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

CONFORMING TO

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

       F_DUPFD_CLOEXEC is specified in POSIX.1-2008.

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

NOTES

       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 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.

BUGS

       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.

SEE ALSO

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

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

COLOPHON

       This  page  is  part of release 3.21 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.