| Sean Coates 2005-08-31, 6:56 pm |
| sean Wed Aug 31 16:01:40 2005 EDT
Modified files:
/phpdoc/en/reference/var/functions debug-zval-dump.xml
Log:
change example and add (big) note on refcount
http://cvs.php.net/diff.php/phpdoc/...1.3&r2=1.4&ty=u
Index: phpdoc/en/reference/var/functions/debug-zval-dump.xml
diff -u phpdoc/en/reference/var/functions/debug-zval-dump.xml:1.3 phpdoc/en/reference/var/functions/debug-zval-dump.xml:1.4
--- phpdoc/en/reference/var/functions/debug-zval-dump.xml:1.3 Mon Apr 11 04:27:31 2005
+++ phpdoc/en/reference/var/functions/debug-zval-dump.xml Wed Aug 31 16:01:36 2005
@@ -1,5 +1,5 @@
<?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.3 $ -->
+<!-- $Revision: 1.4 $ -->
<refentry id="function.debug-zval-dump">
<refnamediv>
<refname>debug_zval_dump</refname>
@@ -49,18 +49,106 @@
$var2 =& $var1;
-debug_zval_dump($var1);
+debug_zval_dump(&$var1);
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
-string(11) "Hello World" refcount(1)
+string(11) "Hello World" refcount(3)
]]>
</screen>
</example>
</para>
+ <note>
+ <title>Beware the <literal>refcount</literal></title>
+ <para>
+ The <literal>refcount</literal> value returned by this function is
+ non-obvious in certain circumstances. For example, a developer might
+ expect the above example to indicate a <literal>refcount</literal> of
+ <literal>2</literal>. The third reference is created when actually
+ calling <function>debug_zval_dump</function>.
+ </para>
+ <para>
+ This behavior is further compounded when a variable is not passed to
+ <function>debug_zval_dump</function> by reference. To illustrate, consider
+ a slightly modified version of the above example:
+ </para>
+ <para>
+ <example>
+ <title/>
+ <programlisting role="php">
+<![CDATA[
+<?php
+$var1 = 'Hello World';
+$var2 = '';
+
+$var2 =& $var1;
+
+debug_zval_dump($var1); // not passed by reference, this time
+?>
+]]>
+ </programlisting>
+ &example.outputs;
+ <screen>
+<![CDATA[
+string(11) "Hello World" refcount(1)
+]]>
+ </screen>
+ </example>
+ </para>
+ <para>
+ Why <literal>refcount(1)</literal>? Because a copy of <literal>$var1</literal> is
+ being made, when the function is called.
+ </para>
+ <para>
+ This function becomes even <emphasis>more</emphasis> confusing when a
+ variable with a <literal>refcount</literal> of <literal>1</literal> is
+ passed (by copy/value):
+ </para>
+ <para>
+ <example>
+ <title/>
+ <programlisting role="php">
+<![CDATA[
+<?php
+$var1 = 'Hello World';
+
+debug_zval_dump($var1);
+?>
+]]>
+ </programlisting>
+ &example.outputs;
+ <screen>
+<![CDATA[
+string(11) "Hello World" refcount(2)
+]]>
+ </screen>
+ </example>
+ </para>
+ <para>
+ A <literal>refcount</literal> of <literal>2</literal>, here, is extremely
+ non-obvious. Especially considering the above examples. So what's
+ happening?
+ </para>
+ <para>
+ When a variable has a single reference (as did <literal>$var1</literal>
+ before it was used as an argument to <function>debug_zval_dump</function> ),
+ PHP's engine optimizes the manner in which it is passed to a function.
+ Internally, PHP treats <literal>$var1</literal> like a reference (in that
+ the <literal>refcount</literal> is increased for the scope of this
+ function, with the caveat that <emphasis>if</emphasis> the passed reference
+ happens to be written to, a copy is made, but only at the moment of
+ writing. This is known as "copy on write."
+ </para>
+ <para>
+ So, if <function>debug_zval_dump</function> happened to write to its sole
+ parameter (and it doesn't), then a copy would be made. Until then, the
+ parameter remains a reference, causing the <literal>refcount</literal> to
+ be incremented to <literal>2</literal> for the scope of the function call.
+ </para>
+ </note>
</refsect1>
<refsect1 role="seealso">
&reftitle.seealso;
@@ -69,6 +157,7 @@
<member><function>var_dump</function></member>
<member><function>debug_backtrace</function></member>
<member><link linkend="language.references">References Explained</link></member>
+ <member><ulink url="http://www.zend.com/zend/art/ref-count.php">Reference Counting and Aliasing (by Andi Gutmans)</ulink></member>
</simplelist>
</para>
</refsect1>
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