FCNTL(2)                   Linux Programmer's Manual                  FCNTL(2)
NAME
       fcntl - manipulate file descriptor

SYNOPSIS
       #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);

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

   Duplicating a file descriptor
       F_DUPFD
              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.

   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
              Read the file descriptor flags.

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

   File status flags
       Each  open  file  description has certain associated status flags, ini-
       tialized by open(2) and possibly modified by fcntl(2).  Duplicated file
       descriptors  (made  with  dup(), fcntl(F_DUPFD), fork(), 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
              Read the file status flags.

       F_SETFL
              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
       grows.

       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  con-
       versions may involve splitting, shrinking, or coalescing with an exist-
       ing lock if the byte range specified by the new lock does not precisely
       coincide with the range of the existing lock.)

       F_SETLK
              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
              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 sig-
              nal handler has returned) returns immediately (with return value
              -1 and errno set to EINTR).

       F_GETLK
              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 auto-
       matically 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  manda-
       tory, 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 descrip-
       tion.  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 com-
       patible 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).  Manda-
       tory 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)).

   Managing signals
       F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O avail-
       ability signals:

       F_GETOWN
              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).

       F_SETOWN
              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 spec-
              ified as a negative value.  Most commonly, the  calling  process
              specifies itself as the owner (that is, arg is specified as get-
              pid()).

              If you set the O_ASYNC status flag on a file descriptor  (either
              by  providing  this  flag with the open(2) call, or 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() instead of getpid() to get sen-
              sible results when F_SETSIG is used.  (In current Linux  thread-
              ing  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() or getpid() in this scenario.)  Note, how-
              ever, 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
              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
              SA_SIGINFO.

       F_SETSIG
              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
              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 struc-
              ture.  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.   (Queu-
              ing  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 and retrieve the current setting of the calling  process's
       lease on the file referred to by 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) that file.

       F_SETLEASE
              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 pro-
                     cess 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 no other process currently has the file open.

              F_UNLCK
                     Remove our lease from the file.

       A process may hold only one type of lease on a file.

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

       F_GETLEASE
              Indicates what type of lease we hold on the file referred to  by
              fd by returning either F_RDLCK, F_WRLCK, or F_UNLCK, indicating,
              respectively, that the calling process holds a read, a write, or
              no  lease  on the file.  (The third argument to fcntl() is omit-
              ted.)

       When a process (the "lease breaker") performs an open()  or  truncate()
       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 we currently hold a write lease on the  file,  and  the
       lease breaker is opening the file for reading, then it is sufficient 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() or truncate() 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() or truncate(),  then  the
       other steps still occur as described above.  If the lease breaker spec-
       ifies the O_NONBLOCK flag when calling open(), then  the  call  immedi-
       ately 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  han-
       dler  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
              (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:

              Bit         Description (event in directory)
              -------------------------------------------------------------
              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  appli-
              cation   must  re-register  to  receive  further  notifications.
              Alternatively, if DN_MULTISHOT is included in arg, then  notifi-
              cation 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  noti-
              fication  of all events, make an F_NOTIFY call specifying arg as
              0.

              Notification occurs via delivery of a signal.  The default  sig-
              nal 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 struc-
              ture  contains the file descriptor which generated the notifica-
              tion  (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 consider using the inotify inter-
              face (available since kernel 2.6.13), which provides a  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_GETOWN Value of descriptor owner.

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

       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.  For
              F_GETLK and F_SETLK, the command was  interrupted  by  a  signal
              before the lock was checked or acquired.  Most likely when lock-
              ing 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.

NOTES
       The errors returned by dup2() 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(2).

       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 e.g.  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 x86) 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  unprivi-
       leged  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 pro-
       cess (group) is one that the caller has permission to send signals  to.
       Despite  this  error return, the file descriptor owner is set, and sig-
       nals will be sent to the owner.

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

SEE ALSO
       dup2(2),  flock(2), open(2), socket(2), lockf(3), capabilities(7), fea-
       ture_test_macros(7)

       See    also    locks.txt,    mandatory.txt,    and    dnotify.txt    in
       /usr/share/doc/kernel-doc-2.6.18/Documentation.

Linux 2.6.14                      2005-05-20                          FCNTL(2)