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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, ... /* arg */ );

DESCRIPTION
       fcntl() performs one of the operations described below on the open file descriptor fd.  The operation is deter-
       mined 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 pro-
       cesses  may hold a read lock (shared lock) on a file region, but only one process may hold a write lock (exclu-
       sive 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 ter-
       minates 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 func-
       tion 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  mean-
              ing: 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 imple-
              mentations, 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_SIG-
              INFO.

              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  sigac-
              tion(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_SIG-
              INFO 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 descrip-
       tor.

       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 estab-
       lished 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 pro-
       cess.  (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 moni-
              tored.  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  establish-
              ing 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 com-
              mand  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 (nega-
       tive) 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 ren-
       der  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  locks.txt,  mandatory-locking.txt,  and  dnotify.txt  in  the  kernel  source  directory  Documenta-
       tion/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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-07-25                          FCNTL(2)