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+\section{\module{fcntl} ---
+         The \function{fcntl()} and \function{ioctl()} system calls}
+
+\declaremodule{builtin}{fcntl}
+  \platform{Unix}
+\modulesynopsis{The \function{fcntl()} and \function{ioctl()} system calls.}
+\sectionauthor{Jaap Vermeulen}{}
+
+\indexii{UNIX@\UNIX}{file control}
+\indexii{UNIX@\UNIX}{I/O control}
+
+This module performs file control and I/O control on file descriptors.
+It is an interface to the \cfunction{fcntl()} and \cfunction{ioctl()}
+\UNIX{} routines.
+
+All functions in this module take a file descriptor \var{fd} as their
+first argument.  This can be an integer file descriptor, such as
+returned by \code{sys.stdin.fileno()}, or a file object, such as
+\code{sys.stdin} itself, which provides a \method{fileno()} which
+returns a genuine file descriptor.
+
+The module defines the following functions:
+
+
+\begin{funcdesc}{fcntl}{fd, op\optional{, arg}}
+  Perform the requested operation on file descriptor \var{fd} (file
+  objects providing a \method{fileno()} method are accepted as well).
+  The operation is defined by \var{op} and is operating system
+  dependent.  These codes are also found in the \module{fcntl}
+  module. The argument \var{arg} is optional, and defaults to the
+  integer value \code{0}.  When present, it can either be an integer
+  value, or a string.  With the argument missing or an integer value,
+  the return value of this function is the integer return value of the
+  C \cfunction{fcntl()} call.  When the argument is a string it
+  represents a binary structure, e.g.\ created by
+  \function{\refmodule{struct}.pack()}. The binary data is copied to a buffer
+  whose address is passed to the C \cfunction{fcntl()} call.  The
+  return value after a successful call is the contents of the buffer,
+  converted to a string object.  The length of the returned string
+  will be the same as the length of the \var{arg} argument.  This is
+  limited to 1024 bytes.  If the information returned in the buffer by
+  the operating system is larger than 1024 bytes, this is most likely
+  to result in a segmentation violation or a more subtle data
+  corruption.
+
+  If the \cfunction{fcntl()} fails, an \exception{IOError} is
+  raised.
+\end{funcdesc}
+
+\begin{funcdesc}{ioctl}{fd, op\optional{, arg\optional{, mutate_flag}}}
+  This function is identical to the \function{fcntl()} function,
+  except that the operations are typically defined in the library
+  module \refmodule{termios} and the argument handling is even more
+  complicated.
+  
+  The parameter \var{arg} can be one of an integer, absent (treated
+  identically to the integer \code{0}), an object supporting the
+  read-only buffer interface (most likely a plain Python string) or an
+  object supporting the read-write buffer interface.
+  
+  In all but the last case, behaviour is as for the \function{fcntl()}
+  function.
+  
+  If a mutable buffer is passed, then the behaviour is determined by
+  the value of the \var{mutate_flag} parameter.
+  
+  If it is false, the buffer's mutability is ignored and behaviour is
+  as for a read-only buffer, except that the 1024 byte limit mentioned
+  above is avoided -- so long as the buffer you pass is as least as
+  long as what the operating system wants to put there, things should
+  work.
+  
+  If \var{mutate_flag} is true, then the buffer is (in effect) passed
+  to the underlying \function{ioctl()} system call, the latter's
+  return code is passed back to the calling Python, and the buffer's
+  new contents reflect the action of the \function{ioctl()}.  This is a
+  slight simplification, because if the supplied buffer is less than
+  1024 bytes long it is first copied into a static buffer 1024 bytes
+  long which is then passed to \function{ioctl()} and copied back into
+  the supplied buffer.
+  
+  If \var{mutate_flag} is not supplied, then from Python 2.5 it
+  defaults to true, which is a change from versions 2.3 and 2.4.
+  Supply the argument explicitly if version portability is a priority.
+
+  An example:
+
+\begin{verbatim}
+>>> import array, fcntl, struct, termios, os
+>>> os.getpgrp()
+13341
+>>> struct.unpack('h', fcntl.ioctl(0, termios.TIOCGPGRP, "  "))[0]
+13341
+>>> buf = array.array('h', [0])
+>>> fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1)
+0
+>>> buf
+array('h', [13341])
+\end{verbatim}
+\end{funcdesc}
+
+\begin{funcdesc}{flock}{fd, op}
+Perform the lock operation \var{op} on file descriptor \var{fd} (file
+  objects providing a \method{fileno()} method are accepted as well).
+See the \UNIX{} manual \manpage{flock}{3} for details.  (On some
+systems, this function is emulated using \cfunction{fcntl()}.)
+\end{funcdesc}
+
+\begin{funcdesc}{lockf}{fd, operation,
+    \optional{length, \optional{start, \optional{whence}}}}
+This is essentially a wrapper around the \function{fcntl()} locking
+calls.  \var{fd} is the file descriptor of the file to lock or unlock,
+and \var{operation} is one of the following values:
+
+\begin{itemize}
+\item \constant{LOCK_UN} -- unlock
+\item \constant{LOCK_SH} -- acquire a shared lock
+\item \constant{LOCK_EX} -- acquire an exclusive lock
+\end{itemize}
+
+When \var{operation} is \constant{LOCK_SH} or \constant{LOCK_EX}, it
+can also be bit-wise OR'd with \constant{LOCK_NB} to avoid blocking on
+lock acquisition.  If \constant{LOCK_NB} is used and the lock cannot
+be acquired, an \exception{IOError} will be raised and the exception
+will have an \var{errno} attribute set to \constant{EACCES} or
+\constant{EAGAIN} (depending on the operating system; for portability,
+check for both values).  On at least some systems, \constant{LOCK_EX}
+can only be used if the file descriptor refers to a file opened for
+writing.
+
+\var{length} is the number of bytes to lock, \var{start} is the byte
+offset at which the lock starts, relative to \var{whence}, and
+\var{whence} is as with \function{fileobj.seek()}, specifically:
+
+\begin{itemize}
+\item \constant{0} -- relative to the start of the file
+      (\constant{SEEK_SET})
+\item \constant{1} -- relative to the current buffer position
+      (\constant{SEEK_CUR})
+\item \constant{2} -- relative to the end of the file
+      (\constant{SEEK_END})
+\end{itemize}
+
+The default for \var{start} is 0, which means to start at the
+beginning of the file.  The default for \var{length} is 0 which means
+to lock to the end of the file.  The default for \var{whence} is also
+0.
+\end{funcdesc}
+
+Examples (all on a SVR4 compliant system):
+
+\begin{verbatim}
+import struct, fcntl, os
+
+f = open(...)
+rv = fcntl.fcntl(f, fcntl.F_SETFL, os.O_NDELAY)
+
+lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0)
+rv = fcntl.fcntl(f, fcntl.F_SETLKW, lockdata)
+\end{verbatim}
+
+Note that in the first example the return value variable \var{rv} will
+hold an integer value; in the second example it will hold a string
+value.  The structure lay-out for the \var{lockdata} variable is
+system dependent --- therefore using the \function{flock()} call may be
+better.
+
+\begin{seealso}
+  \seemodule{os}{If the locking flags \constant{O_SHLOCK} and
+		 \constant{O_EXLOCK} are present in the \module{os} module,
+  		 the \function{os.open()} function provides a more
+  		 platform-independent alternative to the \function{lockf()}
+  		 and \function{flock()} functions.}
+\end{seealso}