Doc/lib/libfcntl.tex

   1 \section{\module{fcntl} ---
   2          The \function{fcntl()} and \function{ioctl()} system calls}
   3
   4 \declaremodule{builtin}{fcntl}
   5   \platform{Unix}
   6 \modulesynopsis{The \function{fcntl()} and \function{ioctl()} system calls.}
   7 \sectionauthor{Jaap Vermeulen}{}
   8
   9 \indexii{UNIX@\UNIX{}}{file control}
  10 \indexii{UNIX@\UNIX{}}{I/O control}
  11
  12 This module performs file control and I/O control on file descriptors.
  13 It is an interface to the \cfunction{fcntl()} and \cfunction{ioctl()}
  14 \UNIX{} routines.  File descriptors can be obtained with the
  15 \method{fileno()} method of a file or socket object.
  16
  17 The module defines the following functions:
  18
  19
  20 \begin{funcdesc}{fcntl}{fd, op\optional{, arg}}
  21   Perform the requested operation on file descriptor \var{fd}.
  22   The operation is defined by \var{op} and is operating system
  23   dependent.  Typically these codes can be retrieved from the library
  24   module \module{FCNTL}\refstmodindex{FCNTL}. The argument \var{arg}
  25   is optional, and defaults to the integer value \code{0}.  When
  26   present, it can either be an integer value, or a string.  With
  27   the argument missing or an integer value, the return value of this
  28   function is the integer return value of the C \cfunction{fcntl()}
  29   call.  When the argument is a string it represents a binary
  30   structure, e.g.\ created by \function{struct.pack()}. The binary
  31   data is copied to a buffer whose address is passed to the C
  32   \cfunction{fcntl()} call.  The return value after a successful call
  33   is the contents of the buffer, converted to a string object.  The length
  34   of the returned string will be the same as the length of the \var{arg}
  35   argument.  This is limited to 1024 bytes.  If the information returned
  36   in the buffer by the operating system is larger than 1024 bytes,
  37   this is most likely to result in a segmentation violation or a more
  38   subtle data corruption.
  39
  40   If the \cfunction{fcntl()} fails, an \exception{IOError} is
  41   raised.
  42 \end{funcdesc}
  43
  44 \begin{funcdesc}{ioctl}{fd, op, arg}
  45   This function is identical to the \function{fcntl()} function, except
  46   that the operations are typically defined in the library module
  47   \module{IOCTL}.
  48 \end{funcdesc}
  49
  50 \begin{funcdesc}{flock}{fd, op}
  51 Perform the lock operation \var{op} on file descriptor \var{fd}.
  52 See the \UNIX{} manual \manpage{flock}{3} for details.  (On some
  53 systems, this function is emulated using \cfunction{fcntl()}.)
  54 \end{funcdesc}
  55
  56 \begin{funcdesc}{lockf}{fd, code, \optional{len, \optional{start, \optional{whence}}}}
  57 This is a wrapper around the \constant{FCNTL.F_SETLK} and
  58 \constant{FCNTL.F_SETLKW} \function{fcntl()} calls.  See the \UNIX{}
  59 manual for details.
  60 \end{funcdesc}
  61
  62 If the library modules \module{FCNTL}\refstmodindex{FCNTL} or
  63 \module{IOCTL}\refstmodindex{IOCTL} are missing, you can find the
  64 opcodes in the C include files \code{<sys/fcntl.h>} and
  65 \code{<sys/ioctl.h>}.  You can create the modules yourself with the
  66 \program{h2py} script, found in the \file{Tools/scripts/} directory.
  67
  68
  69 Examples (all on a SVR4 compliant system):
  70
  71 \begin{verbatim}
  72 import struct, fcntl, FCNTL
  73
  74 file = open(...)
  75 rv = fcntl(file.fileno(), FCNTL.O_NDELAY, 1)
  76
  77 lockdata = struct.pack('hhllhh', FCNTL.F_WRLCK, 0, 0, 0, 0, 0)
  78 rv = fcntl.fcntl(file.fileno(), FCNTL.F_SETLKW, lockdata)
  79 \end{verbatim}
  80
  81 Note that in the first example the return value variable \code{rv} will
  82 hold an integer value; in the second example it will hold a string
  83 value.  The structure lay-out for the \var{lockdata} variable is
  84 system dependent --- therefore using the \function{flock()} call may be
  85 better.