FCNTL(2)                   Linux Programmer's Manual                  FCNTL(2)

       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  various  miscellaneous operations on fd.  The
       operation in question is determined by cmd.

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

              The old and new descriptors may be  used  interchangeably.  They
              share  locks,  file position pointers and flags; for example, if
              the file position is modified by  using  lseek  on  one  of  the
              descriptors, the position is also changed for the other.

              The  two  descriptors  do not share the close-on-exec flag, how-
              ever.  The close-on-exec flag of the copy is off,  meaning  that
              it will not be closed on exec.

              On success, the new descriptor is returned.

              Read  the  close-on-exec  flag.  If the FD_CLOEXEC bit is 0, the
              file will remain open across exec, otherwise it will be  closed.

              Set  the  close-on-exec  flag  to  the  value  specified  by the
              FD_CLOEXEC bit of arg.

   The file status flags
       A file descriptor has certain associated flags, initialized by  open(2)
       and possibly modified by fcntl(2).  The flags are shared between copies
       (made with dup(2), fork(2), etc.) of the same file descriptor.

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

              Read the file descriptor's flags.

              Set the file status flags part of the descriptor's flags to  the
              value  specified by arg.  Remaining bits (access mode, file cre-
              ation flags) in arg are ignored.  On Linux this command can only
              change the O_APPEND, O_NONBLOCK, O_ASYNC, and O_DIRECT 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_WDLCK) 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 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 pre-
       cisely 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 sig-
              nal 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 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.  To make use of mandatory locks,
       mandatory locking must be  enabled  (using  the  "-o  mand"  option  to
       mount(8))  for  the  file  system  containing the file to be locked and
       enabled on the file itself (by disabling group  execute  permission  on
       the file and enabling the set-GID permission bit).

       Advisory locks are not enforced and are useful only between cooperating
       processes. Mandatory locks are enforced for all processes.

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

              Get  the  process  ID or process group currently receiving SIGIO
              and SIGURG signals for events on file  descriptor  fd.   Process
              groups are returned as negative values.

              Set  the process ID or process group that will receive SIGIO and
              SIGURG signals for events on file descriptor fd.  Process groups
              are  specified  using negative values.  (F_SETSIG can be used to
              specify a different signal instead of SIGIO).

              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.

              The  process  or  process  group  to  receive  the signal can be
              selected by using the F_SETOWN command to  the  fcntl  function.
              If the file descriptor is a socket, this also selects the recip-
              ient 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 con-
              dition".)   If  the  file  descriptor  corresponds to a terminal
              device, then SIGIO signals are sent to  the  foreground  process
              group of the terminal.

              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.

              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  POSIX.1b 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 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 another process (the "contestant")
       tries to open(2) or truncate(2) that file.

              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 us to be notified
                     when another process opens the file for writing or  trun-
                     cates it.

                     Take  out  a write lease.  This will cause us to be noti-
                     fied when another process opens the file (for reading  or
                     writing) or truncates it.  A write lease may be placed on
                     a file only if no other process currently  has  the  file

                     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.

              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

       When  the  contestant  performs  an open() or truncate() that conflicts
       with a lease established via F_SETLEASE, the system call is blocked  by
       the  kernel  (unless  the  O_NONBLOCK  flag was specified to open(), in
       which case it returns immediately with  the  error  EWOULDBLOCK).   The
       kernel  then 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
       remove  its lease by performing an F_SETLEASE command specifying arg as

       If the lease holder fails to release the lease  within  the  number  of
       seconds specified in /proc/sys/fs/lease-break-time and the contestant's
       system call remains blocked  (i.e.,  the  contestant  did  not  specify
       O_NONBLOCK  on its open() call, and the system call was not interrupted
       by a signal  handler)  then  the  kerrnel  forcibly  breaks  the  lease
       holder's lease.

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

       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  sructure 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
              (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  macro
              must be defined before including <fcntl.h>.)

              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  calls specifying DN_MULTISHOT 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  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 directo-

              Especially  when using DN_MULTISHOT, a POSIX.1b real time signal
              should be used for notication, so  that  multiple  notifications
              can be queued.

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

       F_DUPFD  The new descriptor.

       F_GETFD  Value of flag.

       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

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

              Operation is prohibited by locks held by other  processes.   Or,
              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 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

       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.

       The errors returned by  dup2  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(2).

       POSIX 1003.1-2001 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.)  However, for current kernels  the  Linux  system
       call returns EINVAL in this situation.

       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.

       SVr4,  SVID,  POSIX,  X/OPEN,  BSD  4.3.   Only the operations F_DUPFD,
       specified  in POSIX.1.  F_GETOWN and F_SETOWN are BSDisms not supported
       in SVr4; F_GETSIG  and  F_SETSIG  are  specific  to  Linux.   F_NOTIFY,
       F_GETLEASE, and F_SETLEASE are Linux specific.  (Define the _GNU_SOURCE
       macro before including <fcntl.h> to  obtain  these  definitions.)   The
       flags legal for F_GETFL/F_SETFL are those supported by open(2) and vary
       between these systems; O_APPEND, O_NONBLOCK, O_RDONLY, and  O_RDWR  are
       specified  in  POSIX.1.   SVr4 supports several other options and flags
       not documented here.

       SVr4 documents additional EIO, ENOLINK and EOVERFLOW error  conditions.

       dup2(2), flock(2), lockf(3), open(2), socket(2)

       See    also    locks.txt,    mandatory.txt,    and    dnotify.txt    in

Linux-2.5.18                      2002-04-24                          FCNTL(2)