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<title>PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space</title>
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<h1><a>PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space</a></h1>
<p><a name="__index__"></a></p>
<!-- INDEX BEGIN -->
<ul>
<li><a href="#name">NAME</a></li>
<li><a href="#synopsis">SYNOPSIS</a></li>
<li><a href="#description">DESCRIPTION</a></li>
<ul>
<li><a href="#custom_layers">Custom Layers</a></li>
<li><a href="#alternatives_to_raw">Alternatives to raw</a></li>
<li><a href="#defaults_and_how_to_override_them">Defaults and how to override them</a></li>
<li><a href="#querying_the_layers_of_filehandles">Querying the layers of filehandles</a></li>
</ul>
<li><a href="#author">AUTHOR</a></li>
<li><a href="#see_also">SEE ALSO</a></li>
</ul>
<!-- INDEX END -->
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<p>
</p>
<h1><a name="name">NAME</a></h1>
<p>PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space</p>
<p>
</p>
<hr />
<h1><a name="synopsis">SYNOPSIS</a></h1>
<pre>
<span class="keyword">open</span><span class="operator">(</span><span class="variable">$fh</span><span class="operator">,</span><span class="string">"<:crlf"</span><span class="operator">,</span> <span class="string">"my.txt"</span><span class="operator">);</span> <span class="comment"># support platform-native and CRLF text files</span>
</pre>
<pre>
<span class="keyword">open</span><span class="operator">(</span><span class="variable">$fh</span><span class="operator">,</span><span class="string">"<"</span><span class="operator">,</span><span class="string">"his.jpg"</span><span class="operator">);</span> <span class="comment"># portably open a binary file for reading</span>
<span class="keyword">binmode</span><span class="operator">(</span><span class="variable">$fh</span><span class="operator">);</span>
</pre>
<pre>
Shell:
PERLIO=perlio perl ....</pre>
<p>
</p>
<hr />
<h1><a name="description">DESCRIPTION</a></h1>
<p>When an undefined layer 'foo' is encountered in an <a href="../lib/Pod/perlfunc.html#item_open"><code>open</code></a> or
<a href="../lib/Pod/perlfunc.html#item_binmode"><code>binmode</code></a> layer specification then C code performs the equivalent of:</p>
<pre>
<span class="keyword">use</span> <span class="variable">PerlIO</span> <span class="string">'foo'</span><span class="operator">;</span>
</pre>
<p>The perl code in PerlIO.pm then attempts to locate a layer by doing</p>
<pre>
<span class="keyword">require</span> <span class="variable">PerlIO::foo</span><span class="operator">;</span>
</pre>
<p>Otherwise the <code>PerlIO</code> package is a place holder for additional
PerlIO related functions.</p>
<p>The following layers are currently defined:</p>
<dl>
<dt><strong><a name="item__3aunix">:unix</a></strong>
<dd>
<p>Lowest level layer which provides basic PerlIO operations in terms of
UNIX/POSIX numeric file descriptor calls
(open(), read(), write(), lseek(), close()).</p>
</dd>
</li>
<dt><strong><a name="item__3astdio">:stdio</a></strong>
<dd>
<p>Layer which calls <code>fread</code>, <code>fwrite</code> and <code>fseek</code>/<code>ftell</code> etc. Note
that as this is "real" stdio it will ignore any layers beneath it and
got straight to the operating system via the C library as usual.</p>
</dd>
</li>
<dt><strong><a name="item__3aperlio">:perlio</a></strong>
<dd>
<p>A from scratch implementation of buffering for PerlIO. Provides fast
access to the buffer for <code>sv_gets</code> which implements perl's readline/<>
and in general attempts to minimize data copying.</p>
</dd>
<dd>
<p><a href="#item__3aperlio"><code>:perlio</code></a> will insert a <a href="#item__3aunix"><code>:unix</code></a> layer below itself to do low level IO.</p>
</dd>
</li>
<dt><strong><a name="item__3acrlf">:crlf</a></strong>
<dd>
<p>A layer that implements DOS/Windows like CRLF line endings. On read
converts pairs of CR,LF to a single "\n" newline character. On write
converts each "\n" to a CR,LF pair. Note that this layer likes to be
one of its kind: it silently ignores attempts to be pushed into the
layer stack more than once.</p>
</dd>
<dd>
<p>It currently does <em>not</em> mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.</p>
</dd>
<dd>
<p>(Gory details follow) To be more exact what happens is this: after
pushing itself to the stack, the <a href="#item__3acrlf"><code>:crlf</code></a> layer checks all the layers
below itself to find the first layer that is capable of being a CRLF
layer but is not yet enabled to be a CRLF layer. If it finds such a
layer, it enables the CRLFness of that other deeper layer, and then
pops itself off the stack. If not, fine, use the one we just pushed.</p>
</dd>
<dd>
<p>The end result is that a <a href="#item__3acrlf"><code>:crlf</code></a> means "please enable the first CRLF
layer you can find, and if you can't find one, here would be a good
spot to place a new one."</p>
</dd>
<dd>
<p>Based on the <a href="#item__3aperlio"><code>:perlio</code></a> layer.</p>
</dd>
</li>
<dt><strong><a name="item__3ammap">:mmap</a></strong>
<dd>
<p>A layer which implements "reading" of files by using <code>mmap()</code> to
make (whole) file appear in the process's address space, and then
using that as PerlIO's "buffer". This <em>may</em> be faster in certain
circumstances for large files, and may result in less physical memory
use when multiple processes are reading the same file.</p>
</dd>
<dd>
<p>Files which are not <code>mmap()</code>-able revert to behaving like the <a href="#item__3aperlio"><code>:perlio</code></a>
layer. Writes also behave like <a href="#item__3aperlio"><code>:perlio</code></a> layer as <code>mmap()</code> for write
needs extra house-keeping (to extend the file) which negates any advantage.</p>
</dd>
<dd>
<p>The <a href="#item__3ammap"><code>:mmap</code></a> layer will not exist if platform does not support <code>mmap()</code>.</p>
</dd>
</li>
<dt><strong><a name="item__3autf8">:utf8</a></strong>
<dd>
<p>Declares that the stream accepts perl's internal encoding of
characters. (Which really is UTF-8 on ASCII machines, but is
UTF-EBCDIC on EBCDIC machines.) This allows any character perl can
represent to be read from or written to the stream. The UTF-X encoding
is chosen to render simple text parts (i.e. non-accented letters,
digits and common punctuation) human readable in the encoded file.</p>
</dd>
<dd>
<p>Here is how to write your native data out using UTF-8 (or UTF-EBCDIC)
and then read it back in.</p>
</dd>
<dd>
<pre>
<span class="keyword">open</span><span class="operator">(</span><span class="variable">F</span><span class="operator">,</span> <span class="string">">:utf8"</span><span class="operator">,</span> <span class="string">"data.utf"</span><span class="operator">);</span>
<span class="keyword">print</span> <span class="variable">F</span> <span class="variable">$out</span><span class="operator">;</span>
<span class="keyword">close</span><span class="operator">(</span><span class="variable">F</span><span class="operator">);</span>
</pre>
</dd>
<dd>
<pre>
<span class="keyword">open</span><span class="operator">(</span><span class="variable">F</span><span class="operator">,</span> <span class="string">"<:utf8"</span><span class="operator">,</span> <span class="string">"data.utf"</span><span class="operator">);</span>
<span class="variable">$in</span> <span class="operator">=</span> <span class="operator"><</span><span class="variable">F</span><span class="operator">>;</span>
<span class="keyword">close</span><span class="operator">(</span><span class="variable">F</span><span class="operator">);</span>
</pre>
</dd>
</li>
<dt><strong><a name="item__3abytes">:bytes</a></strong>
<dd>
<p>This is the inverse of <a href="#item__3autf8"><code>:utf8</code></a> layer. It turns off the flag
on the layer below so that data read from it is considered to
be "octets" i.e. characters in range 0..255 only. Likewise
on output perl will warn if a "wide" character is written
to a such a stream.</p>
</dd>
</li>
<dt><strong><a name="item__3araw">:raw</a></strong>
<dd>
<p>The <a href="#item__3araw"><code>:raw</code></a> layer is <em>defined</em> as being identical to calling
<a href="../lib/Pod/perlfunc.html#item_binmode"><code>binmode($fh)</code></a> - the stream is made suitable for passing binary data
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