\section{\module{posix} ---
         The most common \POSIX{} system calls}

\declaremodule{builtin}{posix}
  \platform{Unix}
\modulesynopsis{The most common \POSIX{} system calls (normally used
                via module \refmodule{os}).}


This module provides access to operating system functionality that is
standardized by the C Standard and the \POSIX{} standard (a thinly
disguised \UNIX{} interface).

\strong{Do not import this module directly.}  Instead, import the
module \refmodule{os}, which provides a \emph{portable} version of this
interface.  On \UNIX{}, the \refmodule{os} module provides a superset of
the \module{posix} interface.  On non-\UNIX{} operating systems the
\module{posix} module is not available, but a subset is always
available through the \refmodule{os} interface.  Once \refmodule{os} is
imported, there is \emph{no} performance penalty in using it instead
of \module{posix}.  In addition, \refmodule{os}\refstmodindex{os}
provides some additional functionality, such as automatically calling
\function{putenv()} when an entry in \code{os.environ} is changed.

The descriptions below are very terse; refer to the corresponding
\UNIX{} manual (or \POSIX{} documentation) entry for more information.
Arguments called \var{path} refer to a pathname given as a string.

Errors are reported as exceptions; the usual exceptions are given for
type errors, while errors reported by the system calls raise
\exception{error} (a synonym for the standard exception
\exception{OSError}), described below.


\subsection{Large File Support \label{posix-large-files}}
\sectionauthor{Steve Clift}{clift@mail.anacapa.net}
\index{large files}
\index{file!large files}


Several operating systems (including AIX, HPUX, Irix and Solaris)
provide support for files that are larger than 2 Gb from a C
programming model where \ctype{int} and \ctype{long} are 32-bit
values. This is typically accomplished by defining the relevant size
and offset types as 64-bit values. Such files are sometimes referred
to as \dfn{large files}.

Large file support is enabled in Python when the size of an
\ctype{off_t} is larger than a \ctype{long} and the \ctype{long long}
type is available and is at least as large as an \ctype{off_t}. Python
longs are then used to represent file sizes, offsets and other values
that can exceed the range of a Python int. It may be necessary to
configure and compile Python with certain compiler flags to enable
this mode. For example, it is enabled by default with recent versions
of Irix, but with Solaris 2.6 and 2.7 you need to do something like:

\begin{verbatim}
CFLAGS="`getconf LFS_CFLAGS`" OPT="-g -O2 $CFLAGS" \
        ./configure
\end{verbatim} % $ <-- bow to font-lock

On large-file-capable Linux systems, this might work:

\begin{verbatim}
CC="-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64"
export CC
./configure
\end{verbatim}


\subsection{Module Contents \label{posix-contents}}


Module \module{posix} defines the following data item:

\begin{datadesc}{environ}
A dictionary representing the string environment at the time the
interpreter was started. For example, \code{environ['HOME']} is the
pathname of your home directory, equivalent to
\code{getenv("HOME")} in C.

Modifying this dictionary does not affect the string environment
passed on by \function{execv()}, \function{popen()} or
\function{system()}; if you need to change the environment, pass
\code{environ} to \function{execve()} or add variable assignments and
export statements to the command string for \function{system()} or
\function{popen()}.

\strong{Note:} The \refmodule{os} module provides an alternate
implementation of \code{environ} which updates the environment on
modification.  Note also that updating \code{os.environ} will render
this dictionary obsolete.  Use of the \refmodule{os} for this is
recommended over direct access to the \module{posix} module.
\end{datadesc}

Additional contents of this module should only be accessed via the
\refmodule{os} module; refer to the documentation for that module for
further information.
