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% Manual text by Jaap Vermeulen
\section{Built-in Module \sectcode{fcntl}}
\label{module-fcntl}
\bimodindex{fcntl}
\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. File descriptors can be obtained with the
\method{fileno()} method of a file or socket object.
The module defines the following functions:
\begin{funcdesc}{fcntl}{fd, op\optional{, arg}}
Perform the requested operation on file descriptor \var{fd}.
The operation is defined by \var{op} and is operating system
dependent. Typically these codes can be retrieved from the library
module \module{FCNTL}\refstmodindex{FCNTL}. 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{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. In
case the \cfunction{fcntl()} fails, an \exception{IOError} is
raised.
\end{funcdesc}
\begin{funcdesc}{ioctl}{fd, op, arg}
This function is identical to the \function{fcntl()} function, except
that the operations are typically defined in the library module
\module{IOCTL}.
\end{funcdesc}
\begin{funcdesc}{flock}{fd, op}
Perform the lock operation \var{op} on file descriptor \var{fd}.
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, code, \optional{len, \optional{start, \optional{whence}}}}
This is a wrapper around the \constant{FCNTL.F_SETLK} and
\constant{FCNTL.F_SETLKW} \function{fcntl()} calls. See the \UNIX{}
manual for details.
\end{funcdesc}
If the library modules \module{FCNTL}\refstmodindex{FCNTL} or
\module{IOCTL}\refstmodindex{IOCTL} are missing, you can find the
opcodes in the \C{} include files \code{<sys/fcntl.h>} and
\code{<sys/ioctl.h>}. You can create the modules yourself with the
\program{h2py} script, found in the \file{Tools/scripts/} directory.
Examples (all on a SVR4 compliant system):
\begin{verbatim}
import struct, FCNTL
file = open(...)
rv = fcntl(file.fileno(), FCNTL.O_NDELAY, 1)
lockdata = struct.pack('hhllhh', FCNTL.F_WRLCK, 0, 0, 0, 0, 0)
rv = fcntl(file.fileno(), FCNTL.F_SETLKW, lockdata)
\end{verbatim}
Note that in the first example the return value variable \code{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.
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