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            <h1>perlhacktut</h1>


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<ul><li><a href="#NAME">NAME</a><li><a href="#DESCRIPTION">DESCRIPTION</a><li><a href="#EXAMPLE-OF-A-SIMPLE-PATCH">EXAMPLE OF A SIMPLE PATCH</a><ul><li><a href="#Writing-the-patch">Writing the patch</a><li><a href="#Testing-the-patch">Testing the patch</a><li><a href="#Documenting-the-patch">Documenting the patch</a><li><a href="#Submit">Submit</a></ul><li><a href="#AUTHOR">AUTHOR</a></ul><a name="NAME"></a><h1>NAME</h1>
<p>perlhacktut - Walk through the creation of a simple C code patch</p>
<a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>This document takes you through a simple patch example.</p>
<p>If you haven't read <a href="perlhack.html">perlhack</a> yet, go do that first! You might also
want to read through <a href="perlsource.html">perlsource</a> too.</p>
<p>Once you're done here, check out <a href="perlhacktips.html">perlhacktips</a> next.</p>
<a name="EXAMPLE-OF-A-SIMPLE-PATCH"></a><h1>EXAMPLE OF A SIMPLE PATCH</h1>
<p>Let's take a simple patch from start to finish.</p>
<p>Here's something Larry suggested: if a <code class="inline"><span class="w">U</span></code>
 is the first active format
during a <code class="inline"><a class="l_k" href="functions/pack.html">pack</a></code>, (for example, <code class="inline"><a class="l_k" href="functions/pack.html">pack</a> <span class="q">&quot;U3C8&quot;</span><span class="cm">,</span> <span class="i">@stuff</span></code>
) then the
resulting string should be treated as UTF-8 encoded.</p>
<p>If you are working with a git clone of the Perl repository, you will
want to create a branch for your changes. This will make creating a
proper patch much simpler. See the <a href="perlgit.html">perlgit</a> for details on how to do
this.</p>
<a name="Writing-the-patch"></a><h2>Writing the patch</h2>
<p>How do we prepare to fix this up? First we locate the code in question
- the <code class="inline"><a class="l_k" href="functions/pack.html">pack</a></code> happens at runtime, so it's going to be in one of the
<i>pp</i> files. Sure enough, <code class="inline"><span class="w">pp_pack</span></code>
 is in <i>pp.c</i>. Since we're going
to be altering this file, let's copy it to <i>pp.c~</i>.</p>
<p>[Well, it was in <i>pp.c</i> when this tutorial was written. It has now
been split off with <code class="inline"><span class="w">pp_unpack</span></code>
 to its own file, <i>pp_pack.c</i>]</p>
<p>Now let's look over <code class="inline"><span class="w">pp_pack</span></code>
: we take a pattern into <code class="inline"><span class="w">pat</span></code>
, and then
loop over the pattern, taking each format character in turn into
<code class="inline"><span class="w">datum_type</span></code>
. Then for each possible format character, we swallow up
the other arguments in the pattern (a field width, an asterisk, and so
on) and convert the next chunk input into the specified format, adding
it onto the output SV <code class="inline"><span class="w">cat</span></code>
.</p>
<p>How do we know if the <code class="inline"><span class="w">U</span></code>
 is the first format in the <code class="inline"><span class="w">pat</span></code>
? Well, if
we have a pointer to the start of <code class="inline"><span class="w">pat</span></code>
 then, if we see a <code class="inline"><span class="w">U</span></code>
 we can
test whether we're still at the start of the string. So, here's where
<code class="inline"><span class="w">pat</span></code>
 is set up:</p>
<pre class="verbatim"><ol><li>    <span class="w">STRLEN</span> <span class="w">fromlen</span><span class="sc">;</span></li><li>    <span class="w">char</span> *<span class="w">pat</span> = <span class="i">SvPVx</span><span class="s">(</span><span class="i">*+</span>+<span class="w">MARK</span><span class="cm">,</span> <span class="w">fromlen</span><span class="s">)</span><span class="sc">;</span></li><li>    <span class="w">char</span> *<span class="w">patend</span> = <span class="w">pat</span> + <span class="w">fromlen</span><span class="sc">;</span></li><li>    <span class="w">I32</span> <span class="w">len</span><span class="sc">;</span></li><li>    <span class="w">I32</span> <span class="w">datumtype</span><span class="sc">;</span></li><li>    <span class="w">SV</span> *<span class="w">fromstr</span><span class="sc">;</span></li></ol></pre><p>We'll have another string pointer in there:</p>
<pre class="verbatim"><ol><li>    <span class="w">STRLEN</span> <span class="w">fromlen</span><span class="sc">;</span></li><li>    <span class="w">char</span> *<span class="w">pat</span> = <span class="i">SvPVx</span><span class="s">(</span><span class="i">*+</span>+<span class="w">MARK</span><span class="cm">,</span> <span class="w">fromlen</span><span class="s">)</span><span class="sc">;</span></li><li>    <span class="w">char</span> *<span class="w">patend</span> = <span class="w">pat</span> + <span class="w">fromlen</span><span class="sc">;</span></li><li> +  <span class="w">char</span> *<span class="w">patcopy</span><span class="sc">;</span></li><li>    <span class="w">I32</span> <span class="w">len</span><span class="sc">;</span></li><li>    <span class="w">I32</span> <span class="w">datumtype</span><span class="sc">;</span></li><li>    <span class="w">SV</span> *<span class="w">fromstr</span><span class="sc">;</span></li></ol></pre><p>And just before we start the loop, we'll set <code class="inline"><span class="w">patcopy</span></code>
 to be the start
of <code class="inline"><span class="w">pat</span></code>
:</p>
<pre class="verbatim"><ol><li>    <span class="w">items</span> = <span class="w">SP</span> - <span class="w">MARK</span><span class="sc">;</span></li><li>    <span class="w">MARK</span>++<span class="sc">;</span></li><li>    <span class="i">SvPVCLEAR</span><span class="s">(</span><span class="w">cat</span><span class="s">)</span><span class="sc">;</span></li><li> +  <span class="w">patcopy</span> = <span class="w">pat</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/while.html">while</a> <span class="s">(</span><span class="w">pat</span> &lt; <span class="w">patend</span><span class="s">)</span> <span class="s">{</span></li></ol></pre><p>Now if we see a <code class="inline"><span class="w">U</span></code>
 which was at the start of the string, we turn on
the <code class="inline"><span class="w">UTF8</span></code>
 flag for the output SV, <code class="inline"><span class="w">cat</span></code>
:</p>
<pre class="verbatim"><ol><li> +  if (datumtype == 'U' &amp;&amp; pat==patcopy+1)</li><li> +      SvUTF8_on(cat);</li><li>    if (datumtype == '#') {</li><li>        while (pat &lt; patend &amp;&amp; *pat != '\n')</li><li>            pat++;</li></ol></pre><p>Remember that it has to be <code class="inline"><span class="w">patcopy</span>+<span class="n">1</span></code>
 because the first character of
the string is the <code class="inline"><span class="w">U</span></code>
 which has been swallowed into <code class="inline"><span class="w">datumtype</span>!</code>
</p>
<p>Oops, we forgot one thing: what if there are spaces at the start of the
pattern? <code class="inline"><a class="l_k" href="functions/pack.html">pack</a><span class="s">(</span><span class="q">&quot;  U*&quot;</span><span class="cm">,</span> <span class="i">@stuff</span><span class="s">)</span></code>
 will have <code class="inline"><span class="w">U</span></code>
 as the first active
character, even though it's not the first thing in the pattern. In this
case, we have to advance <code class="inline"><span class="w">patcopy</span></code>
 along with <code class="inline"><span class="w">pat</span></code>
 when we see
spaces:</p>
<pre class="verbatim"><ol><li>    if (isSPACE(datumtype))</li><li>        continue;</li></ol></pre><p>needs to become</p>
<pre class="verbatim"><ol><li>    if <span class="s">(</span><span class="i">isSPACE</span><span class="s">(</span><span class="w">datumtype</span><span class="s">)</span><span class="s">)</span> <span class="s">{</span></li><li>        <span class="w">patcopy</span>++<span class="sc">;</span></li><li>        <a class="l_k" href="functions/continue.html">continue</a><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>OK. That's the C part done. Now we must do two additional things before
this patch is ready to go: we've changed the behaviour of Perl, and so
we must document that change. We must also provide some more regression
tests to make sure our patch works and doesn't create a bug somewhere
else along the line.</p>
<a name="Testing-the-patch"></a><h2>Testing the patch</h2>
<p>The regression tests for each operator live in <i>t/op/</i>, and so we make
a copy of <i>t/op/pack.t</i> to <i>t/op/pack.t~</i>. Now we can add our tests
to the end. First, we'll test that the <code class="inline"><span class="w">U</span></code>
 does indeed create Unicode
strings.</p>
<p>t/op/pack.t has a sensible ok() function, but if it didn't we could use
the one from t/test.pl.</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/require.html">require</a> <span class="q">&#39;./test.pl&#39;</span><span class="sc">;</span></li><li> <span class="i">plan</span><span class="s">(</span> <span class="w">tests</span> <span class="cm">=&gt;</span> <span class="n">159</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>so instead of this:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&#39;not &#39;</span> unless <span class="q">&quot;1.20.300.4000&quot;</span> eq <a class="l_k" href="functions/sprintf.html">sprintf</a> <span class="q">&quot;%vd&quot;</span><span class="cm">,</span></li><li>                                               <a class="l_k" href="functions/pack.html">pack</a><span class="s">(</span><span class="q">&quot;U*&quot;</span><span class="cm">,</span><span class="n">1</span><span class="cm">,</span><span class="n">20</span><span class="cm">,</span><span class="n">300</span><span class="cm">,</span><span class="n">4000</span><span class="s">)</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;ok $test\n&quot;</span><span class="sc">;</span> <span class="i">$test</span>++<span class="sc">;</span></li></ol></pre><p>we can write the more sensible (see <a href="Test/More.html">Test::More</a> for a full
explanation of is() and other testing functions).</p>
<pre class="verbatim"><ol><li> <span class="i">is</span><span class="s">(</span> <span class="q">&quot;1.20.300.4000&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/sprintf.html">sprintf</a> <span class="q">&quot;%vd&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/pack.html">pack</a><span class="s">(</span><span class="q">&quot;U*&quot;</span><span class="cm">,</span><span class="n">1</span><span class="cm">,</span><span class="n">20</span><span class="cm">,</span><span class="n">300</span><span class="cm">,</span><span class="n">4000</span><span class="s">)</span><span class="cm">,</span></li><li>                                       <span class="q">&quot;U* produces Unicode&quot;</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>Now we'll test that we got that space-at-the-beginning business right:</p>
<pre class="verbatim"><ol><li> <span class="i">is</span><span class="s">(</span> <span class="q">&quot;1.20.300.4000&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/sprintf.html">sprintf</a> <span class="q">&quot;%vd&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/pack.html">pack</a><span class="s">(</span><span class="q">&quot;  U*&quot;</span><span class="cm">,</span><span class="n">1</span><span class="cm">,</span><span class="n">20</span><span class="cm">,</span><span class="n">300</span><span class="cm">,</span><span class="n">4000</span><span class="s">)</span><span class="cm">,</span></li><li>                                     <span class="q">&quot;  with spaces at the beginning&quot;</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>And finally we'll test that we don't make Unicode strings if <code class="inline"><span class="w">U</span></code>
 is
<b>not</b> the first active format:</p>
<pre class="verbatim"><ol><li> <span class="i">isnt</span><span class="s">(</span> <span class="v">v1.20.300.4000</span><span class="cm">,</span> <a class="l_k" href="functions/sprintf.html">sprintf</a> <span class="q">&quot;%vd&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/pack.html">pack</a><span class="s">(</span><span class="q">&quot;C0U*&quot;</span><span class="cm">,</span><span class="n">1</span><span class="cm">,</span><span class="n">20</span><span class="cm">,</span><span class="n">300</span><span class="cm">,</span><span class="n">4000</span><span class="s">)</span><span class="cm">,</span></li><li>                                       <span class="q">&quot;U* not first isn't Unicode&quot;</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>Mustn't forget to change the number of tests which appears at the top,
or else the automated tester will get confused. This will either look
like this:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;1..156\n&quot;</span><span class="sc">;</span></li></ol></pre><p>or this:</p>
<pre class="verbatim"><ol><li> <span class="i">plan</span><span class="s">(</span> <span class="w">tests</span> <span class="cm">=&gt;</span> <span class="n">156</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>We now compile up Perl, and run it through the test suite. Our new
tests pass, hooray!</p>
<a name="Documenting-the-patch"></a><h2>Documenting the patch</h2>
<p>Finally, the documentation. The job is never done until the paperwork
is over, so let's describe the change we've just made. The relevant
place is <i>pod/perlfunc.pod</i>; again, we make a copy, and then we'll
insert this text in the description of <code class="inline"><a class="l_k" href="functions/pack.html">pack</a></code>:</p>
<pre class="verbatim"><ol><li> =<span class="w">item</span> *</li><li></li><li> <span class="w">If</span> <span class="w">the</span> <span class="w">pattern</span> <span class="w">begins</span> <span class="w">with</span> <span class="w">a</span> <span class="w">C</span><span class="q">&lt;U&gt;</span><span class="cm">,</span> <span class="w">the</span> <span class="w">resulting</span> <span class="w">string</span> <span class="w">will</span> <span class="w">be</span> <span class="w">treated</span></li><li> <span class="w">as</span> <span class="w">UTF</span>-<span class="n">8</span>-<span class="w">encoded</span> <span class="w">Unicode</span>. <span class="w">You</span> <span class="w">can</span> <span class="w">force</span> <span class="w">UTF</span>-<span class="n">8</span> <span class="w">encoding</span> <span class="w">on</span> <span class="w">in</span> <span class="w">a</span> <span class="w">string</span></li><li> <span class="w">with</span> <span class="w">an</span> <span class="w">initial</span> <span class="w">C</span><span class="q">&lt;U0&gt;</span><span class="cm">,</span> and <span class="w">the</span> <span class="w">bytes</span> <span class="w">that</span> <span class="w">follow</span> <span class="w">will</span> <span class="w">be</span> <span class="w">interpreted</span> <span class="w">as</span></li><li> <span class="w">Unicode</span> <span class="w">characters</span>. <span class="w">If</span> <span class="w">you</span> <span class="w">don&#39;t</span> <span class="w">want</span> <span class="w">this</span> <span class="w">to</span> <span class="w">happen</span><span class="cm">,</span> <span class="w">you</span> <span class="w">can</span> <span class="w">begin</span></li><li> <span class="w">your</span> <span class="w">pattern</span> <span class="w">with</span> <span class="w">C</span><span class="q">&lt;C0&gt;</span> <span class="s">(</span>or <span class="w">anything</span> else<span class="s">)</span> <span class="w">to</span> <span class="w">force</span> <span class="w">Perl</span> not <span class="w">to</span> <span class="w">UTF</span>-<span class="n">8</span></li><li> <span class="w">encode</span> <span class="w">your</span> <span class="w">string</span><span class="cm">,</span> and <span class="w">then</span> <span class="w">follow</span> <span class="w">this</span> <span class="w">with</span> <span class="w">a</span> <span class="w">C</span><span class="q">&lt;U*&gt;</span> <span class="w">somewhere</span> <span class="w">in</span> <span class="w">your</span></li><li> <span class="w">pattern</span>.</li></ol></pre><a name="Submit"></a><h2>Submit</h2>
<p>See <a href="perlhack.html">perlhack</a> for details on how to submit this patch.</p>
<a name="AUTHOR"></a><h1>AUTHOR</h1>
<p>This document was originally written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.</p>




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