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:

       F_DUPFD  Find the lowest numbered available file  descrip­
                tor 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 inter­
                changeably.  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

                The two descriptors do not  share  the  close-on-
                exec  flag,  however.   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.

       F_GETFD  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.

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

       F_GETFL  Read the descriptor's flags (all flags (as set by
                open(2)) are returned).

       F_SETFL  Set the descriptor's flags to the value specified
                by arg.  Only O_APPEND,  O_NONBLOCK  and  O_ASYNC
                may be set; the other flags are unaffected.

                The  flags  are  shared between copies (made with
                dup(2), fork(2), etc.) of the same file  descrip­

                The  flags  and  their semantics are described in

       tionary  file locks.  The third argument lock is a pointer
       to a struct flock (that may be overwritten by this  call).

              Return  the  flock  structure that prevents us from
              obtaining the lock, or set the l_type field of  the
              lock to F_UNLCK if there is no obstruction.

              The lock is set (when l_type is F_RDLCK or F_WRLCK)
              or cleared (when it is F_UNLCK).  If  the  lock  is
              held by someone else, this call returns -1 and sets
              errno to EACCES or EAGAIN.

              Like F_SETLK, but instead of returning an error  we
              wait for the lock to be released.  If a signal that
              is to be caught is received while fcntl is waiting,
              it is interrupted and (after the signal handler has
              returned) returns immediately (with return value -1
              and errno set to EINTR).

       F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to man­
       age I/O availability 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 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 the file
              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 set­
              ting 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  associ­
              ated  with the event.  Otherwise, there is no indi­
              cation 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

              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).


       For  a  successful  call,  the return value depends on the

       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

       All other commands

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


       EACCES   Operation  is  prohibited  by locks held by other

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

       EBADF    fd is not an open file descriptor.

       EDEADLK  It  was detected that the specified F_SETLKW com­
                mand 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 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.


       The  errors  returned  by  dup2  are  different from those
       returned by F_DUPFD.
       SVr4, SVID, POSIX, X/OPEN, BSD 4.3.  Only  the  operations
       F_SETLK and F_SETLKW are specified in  POSIX.1.   F_GETOWN
       and  F_SETOWN  are BSDisms not supported in SVr4; F_GETSIG
       and F_SETSIG are specific to Linux.  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


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