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% Manual text by Jaap Vermeulen
\section{Built-in Module \sectcode{fcntl}}
\bimodindex{fcntl}
\indexii{\UNIX{}}{file control}
\indexii{\UNIX{}}{I/O control}
This module performs file control and I/O control on file descriptors.
It is an interface to the \dfn{fcntl()} and \dfn{ioctl()} \UNIX{} routines.
File descriptors can be obtained with the \dfn{fileno()} method of a
file or socket object.
The module defines the following functions:
\renewcommand{\indexsubitem}{(in module struct)}
\begin{funcdesc}{fcntl}{fd\, op\optional{\, arg}}
Perform the requested operation on file descriptor \code{\var{fd}}.
The operation is defined by \code{\var{op}} and is operating system
dependent. Typically these codes can be retrieved from the library
module \code{FCNTL}. The argument \code{\var{arg}} is optional, and
defaults to the integer value \code{0}. When
it is 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 real \code{fcntl()}
call. When the argument is a string it represents a binary
structure, e.g.\ created by \code{struct.pack()}. The binary data is
copied to a buffer whose address is passed to the real \code{fcntl()}
call. The return value after a successful call is the contents of
the buffer, converted to a string object. In case the
\code{fcntl()} fails, an \code{IOError} will be raised.
\end{funcdesc}
\begin{funcdesc}{ioctl}{fd\, op\, arg}
This function is identical to the \code{fcntl()} function, except
that the operations are typically defined in the library module
\code{IOCTL}.
\end{funcdesc}
\begin{funcdesc}{flock}{fd\, op}
Perform the lock operation \var{op} on file descriptor \var{fd}.
See the \UNIX{} manual for details. (On some systems, this function is
emulated using \code{fcntl()}.)
\end{funcdesc}
\begin{funcdesc}{lockf}{fd\, code\, \optional{len\, \optional{start\, \optional{whence}}}}
This is a wrapper around the \code{F_SETLK} and \code{F_SETLKW}
\code{fcntl()} calls. See the \UNIX{} manual for details.
\end{funcdesc}
If the library modules \code{FCNTL} or \code{IOCTL} are missing, you
can find the opcodes in the C include files \file{sys/fcntl.h} and
\file{sys/ioctl.h}. You can create the modules yourself with the h2py
script, found in the \file{Tools/scripts} directory.
\refstmodindex{FCNTL}
\refstmodindex{IOCTL}
Examples (all on a SVR4 compliant system):
\bcode\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}\ecode
%
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{lockadata} variable is
system dependent -- therefore using the \code{flock()} call may be
better.
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