fcntl


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 argu-
       ment  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 pro-
              gram 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, ini-
       tialized 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_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  rela-
       tive  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 posi-
       tive, 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

       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 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.

       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)).

       The Linux implementation of mandatory locking is unreliable.  See  BUGS
       below.
       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 com-
              monly, 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 han-
              dler if installed with SA_SIGINFO.  arg is ignored.

              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  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  writ-
                     ing  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,
              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 spec-
       ifies  the  O_NONBLOCK flag when calling open(2), 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 (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)

              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 directo-
              ries).

              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  (avail-
              able since kernel 2.6.13), which provides a much superior inter-
              face 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.

       EFAULT lock is outside your accessible address space.

       EINTR  For F_SETLKW, the command was interrupted by a signal; see  sig-
              nal(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-spe-
       cific.  (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 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

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

       See also locks.txt, mandatory-locking.txt, and dnotify.txt in the  ker-
       nel  source  directory  Documentation/filesystems/.  (On older kernels,
       these files are directly under the Documentation/ directory, and manda-
       tory-locking.txt is called mandatory.txt.)

COLOPHON
       This  page  is  part of release 3.23 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/.



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