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  <div class="section" id="module-posix">
<span id="posix-the-most-common-posix-system-calls"></span><h1>35.1. <a class="reference internal" href="#module-posix" title="posix: The most common POSIX system calls (normally used via module os). (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">posix</span></tt></a> &#8212; The most common POSIX system calls<a class="headerlink" href="#module-posix" title="Permalink to this headline">¶</a></h1>
<p>This module provides access to operating system functionality that is
standardized by the C Standard and the POSIX standard (a thinly disguised Unix
interface).</p>
<p id="index-0"><strong>Do not import this module directly.</strong>  Instead, import the module <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a>,
which provides a <em>portable</em> version of this interface.  On Unix, the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a>
module provides a superset of the <a class="reference internal" href="#module-posix" title="posix: The most common POSIX system calls (normally used via module os). (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">posix</span></tt></a> interface.  On non-Unix operating
systems the <a class="reference internal" href="#module-posix" title="posix: The most common POSIX system calls (normally used via module os). (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">posix</span></tt></a> module is not available, but a subset is always
available through the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> interface.  Once <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> is imported, there is
<em>no</em> performance penalty in using it instead of <a class="reference internal" href="#module-posix" title="posix: The most common POSIX system calls (normally used via module os). (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">posix</span></tt></a>.  In addition,
<a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> provides some additional functionality, such as automatically calling
<tt class="xref py py-func docutils literal"><span class="pre">putenv()</span></tt> when an entry in <tt class="docutils literal"><span class="pre">os.environ</span></tt> is changed.</p>
<p>Errors are reported as exceptions; the usual exceptions are given for type
errors, while errors reported by the system calls raise <a class="reference internal" href="exceptions.html#exceptions.OSError" title="exceptions.OSError"><tt class="xref py py-exc docutils literal"><span class="pre">OSError</span></tt></a>.</p>
<div class="section" id="large-file-support">
<span id="posix-large-files"></span><h2>35.1.1. Large File Support<a class="headerlink" href="#large-file-support" title="Permalink to this headline">¶</a></h2>
<p id="index-1">Several operating systems (including AIX, HP-UX, Irix and Solaris) provide
support for files that are larger than 2 GB from a C programming model where
<tt class="xref c c-type docutils literal"><span class="pre">int</span></tt> and <tt class="xref c c-type docutils literal"><span class="pre">long</span></tt> 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 <em class="dfn">large files</em>.</p>
<p>Large file support is enabled in Python when the size of an <tt class="xref c c-type docutils literal"><span class="pre">off_t</span></tt> is
larger than a <tt class="xref c c-type docutils literal"><span class="pre">long</span></tt> and the <tt class="xref c c-type docutils literal"><span class="pre">long</span> <span class="pre">long</span></tt> type is available and is
at least as large as an <tt class="xref c c-type docutils literal"><span class="pre">off_t</span></tt>. 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:</p>
<div class="highlight-python"><pre>CFLAGS="`getconf LFS_CFLAGS`" OPT="-g -O2 $CFLAGS" \
        ./configure</pre>
</div>
<p>On large-file-capable Linux systems, this might work:</p>
<div class="highlight-python"><pre>CFLAGS='-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64' OPT="-g -O2 $CFLAGS" \
        ./configure</pre>
</div>
</div>
<div class="section" id="notable-module-contents">
<span id="posix-contents"></span><h2>35.1.2. Notable Module Contents<a class="headerlink" href="#notable-module-contents" title="Permalink to this headline">¶</a></h2>
<p>In addition to many functions described in the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module documentation,
<a class="reference internal" href="#module-posix" title="posix: The most common POSIX system calls (normally used via module os). (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">posix</span></tt></a> defines the following data item:</p>
<dl class="data">
<dt id="posix.environ">
<tt class="descclassname">posix.</tt><tt class="descname">environ</tt><a class="headerlink" href="#posix.environ" title="Permalink to this definition">¶</a></dt>
<dd><p>A dictionary representing the string environment at the time the interpreter
was started.  For example, <tt class="docutils literal"><span class="pre">environ['HOME']</span></tt> is the pathname of your home
directory, equivalent to <tt class="docutils literal"><span class="pre">getenv(&quot;HOME&quot;)</span></tt> in C.</p>
<p>Modifying this dictionary does not affect the string environment passed on by
<tt class="xref py py-func docutils literal"><span class="pre">execv()</span></tt>, <tt class="xref py py-func docutils literal"><span class="pre">popen()</span></tt> or <tt class="xref py py-func docutils literal"><span class="pre">system()</span></tt>; if you need to change the
environment, pass <tt class="docutils literal"><span class="pre">environ</span></tt> to <tt class="xref py py-func docutils literal"><span class="pre">execve()</span></tt> or add variable assignments and
export statements to the command string for <tt class="xref py py-func docutils literal"><span class="pre">system()</span></tt> or <tt class="xref py py-func docutils literal"><span class="pre">popen()</span></tt>.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module provides an alternate implementation of <tt class="docutils literal"><span class="pre">environ</span></tt> which
updates the environment on modification.  Note also that updating <tt class="docutils literal"><span class="pre">os.environ</span></tt>
will render this dictionary obsolete.  Use of the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module version of
this is recommended over direct access to the <a class="reference internal" href="#module-posix" title="posix: The most common POSIX system calls (normally used via module os). (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">posix</span></tt></a> module.</p>
</div>
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  <ul>
<li><a class="reference internal" href="#">35.1. <tt class="docutils literal"><span class="pre">posix</span></tt> &#8212; The most common POSIX system calls</a><ul>
<li><a class="reference internal" href="#large-file-support">35.1.1. Large File Support</a></li>
<li><a class="reference internal" href="#notable-module-contents">35.1.2. Notable Module Contents</a></li>
</ul>
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