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https://github.com/sigmasternchen/php-doc-en
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git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@308182 c90b9560-bf6c-de11-be94-00142212c4b1
260 lines
8.7 KiB
XML
260 lines
8.7 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<refentry xml:id="function.stream-select" xmlns="http://docbook.org/ns/docbook">
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<refnamediv>
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<refname>stream_select</refname>
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<refpurpose>Runs the equivalent of the select() system call on the given
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arrays of streams with a timeout specified by tv_sec and tv_usec </refpurpose>
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</refnamediv>
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<refsect1 role="description">
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&reftitle.description;
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<methodsynopsis>
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<type>int</type><methodname>stream_select</methodname>
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<methodparam><type>array</type><parameter role="reference">read</parameter></methodparam>
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<methodparam><type>array</type><parameter role="reference">write</parameter></methodparam>
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<methodparam><type>array</type><parameter role="reference">except</parameter></methodparam>
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<methodparam><type>int</type><parameter>tv_sec</parameter></methodparam>
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<methodparam choice="opt"><type>int</type><parameter>tv_usec</parameter><initializer>0</initializer></methodparam>
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</methodsynopsis>
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<para>
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The <function>stream_select</function> function accepts arrays of streams and
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waits for them to change status. Its operation is equivalent to that of
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the <function>socket_select</function> function except in that it acts on streams.
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</para>
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</refsect1>
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<refsect1 role="parameters">
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&reftitle.parameters;
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<para>
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<variablelist>
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<varlistentry>
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<term><parameter>read</parameter></term>
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<listitem>
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<para>
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The streams listed in the <parameter>read</parameter> array will be watched to
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see if characters become available for reading (more precisely, to see if
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a read will not block - in particular, a stream resource is also ready on
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end-of-file, in which case an <function>fread</function> will return
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a zero length string).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>write</parameter></term>
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<listitem>
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<para>
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The streams listed in the <parameter>write</parameter> array will be
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watched to see if a write will not block.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>except</parameter></term>
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<listitem>
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<para>
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The streams listed in the <parameter>except</parameter> array will be
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watched for high priority exceptional ("out-of-band") data arriving.
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</para>
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<note>
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<para>
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When <function>stream_select</function> returns, the arrays
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<parameter>read</parameter>, <parameter>write</parameter> and
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<parameter>except</parameter> are modified to indicate which stream
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resource(s) actually changed status.
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</para>
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</note>
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<simpara>
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You do not need to pass every array to
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<function>stream_select</function>. You can leave it out and use an
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empty array or &null; instead. Also do not forget that those arrays are
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passed <emphasis>by reference</emphasis> and will be modified after
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<function>stream_select</function> returns.
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</simpara>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>tv_sec</parameter></term>
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<listitem>
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<para>
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The <parameter>tv_sec</parameter> and <parameter>tv_usec</parameter>
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together form the <emphasis>timeout</emphasis> parameter,
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<parameter>tv_sec</parameter> specifies the number of seconds while
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<parameter>tv_usec</parameter> the number of microseconds.
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The <parameter>timeout</parameter> is an upper bound on the amount of time
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that <function>stream_select</function> will wait before it returns.
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If <parameter>tv_sec</parameter> and <parameter>tv_usec</parameter> are
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both set to <literal>0</literal>, <function>stream_select</function> will
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not wait for data - instead it will return immediately, indicating the
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current status of the streams.
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</para>
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<para>
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If <parameter>tv_sec</parameter> is &null; <function>stream_select</function>
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can block indefinitely, returning only when an event on one of the
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watched streams occurs (or if a signal interrupts the system call).
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</para>
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<warning>
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<para>
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Using a timeout value of <literal>0</literal> allows you to
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instantaneously poll the status of the streams, however, it is NOT a
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good idea to use a <literal>0</literal> timeout value in a loop as it
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will cause your script to consume too much CPU time.
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</para>
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<para>
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It is much better to specify a timeout value of a few seconds, although
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if you need to be checking and running other code concurrently, using a
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timeout value of at least <literal>200000</literal> microseconds will
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help reduce the CPU usage of your script.
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</para>
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<para>
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Remember that the timeout value is the maximum time that will elapse;
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<function>stream_select</function> will return as soon as the
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requested streams are ready for use.
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</para>
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</warning>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>tv_usec</parameter></term>
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<listitem>
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<para>
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See <parameter>tv_sec</parameter> description.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect1>
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<refsect1 role="returnvalues">
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&reftitle.returnvalues;
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<para>
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On success <function>stream_select</function> returns the number of
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stream resources contained in the modified arrays, which may be zero if
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the timeout expires before anything interesting happens. On error &false;
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is returned and a warning raised (this can happen if the system call is
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interrupted by an incoming signal).
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</para>
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</refsect1>
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<refsect1 role="examples">
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&reftitle.examples;
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<para>
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<example>
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<title><function>stream_select</function> Example</title>
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<para>
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This example checks to see if data has arrived for reading on either
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<parameter>$stream1</parameter> or <parameter>$stream2</parameter>.
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Since the timeout value is <literal>0</literal> it will return
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immediately:
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</para>
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<programlisting role="php">
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<![CDATA[
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<?php
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/* Prepare the read array */
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$read = array($stream1, $stream2);
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$write = NULL;
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$except = NULL;
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if (false === ($num_changed_streams = stream_select($read, $write, $except, 0))) {
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/* Error handling */
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} elseif ($num_changed_streams > 0) {
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/* At least on one of the streams something interesting happened */
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}
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?>
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]]>
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</programlisting>
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</example>
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</para>
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</refsect1>
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<refsect1 role="notes">
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&reftitle.notes;
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<note>
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<para>
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Due to a limitation in the current Zend Engine it is not possible to pass a
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constant modifier like &null; directly as a parameter to a function
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which expects this parameter to be passed by reference. Instead use a
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temporary variable or an expression with the leftmost member being a
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temporary variable:
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<programlisting role="php">
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<![CDATA[
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<?php
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$e = NULL;
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stream_select($r, $w, $e, 0);
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?>
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]]>
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</programlisting>
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</para>
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</note>
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<note>
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<para>
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Be sure to use the <literal>===</literal> operator when checking for an
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error. Since the <function>stream_select</function> may return 0 the
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comparison with <literal>==</literal> would evaluate to &true;:
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<programlisting role="php">
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<![CDATA[
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<?php
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$e = NULL;
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if (false === stream_select($r, $w, $e, 0)) {
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echo "stream_select() failed\n";
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}
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?>
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]]>
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</programlisting>
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</para>
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</note>
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<note>
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<para>
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If you read/write to a stream returned in the arrays be aware that
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they do not necessarily read/write the full amount of data you have
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requested. Be prepared to even only be able to read/write a single
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byte.
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</para>
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</note>
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<note>
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<para>
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Some streams (like <literal>zlib</literal>) cannot be selected by this
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function.
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</para>
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</note>
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<note>
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<para>
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Windows compatibility: <function>stream_select</function> used on a
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pipe returned from <function>proc_open</function> may cause data loss
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under Windows 98.
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</para>
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<para>
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Use of <function>stream_select</function> on
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file descriptors returned by <function>proc_open</function> will fail
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and return &false; under Windows.
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</para>
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</note>
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</refsect1>
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<refsect1 role="seealso">
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&reftitle.seealso;
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<simplelist>
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<member><function>stream_set_blocking</function></member>
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</simplelist>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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indent-tabs-mode:nil
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sgml-parent-document:nil
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sgml-default-dtd-file:"~/.phpdoc/manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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