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<title>Runs the select() system call on the given arrays of sockets with a specified timeout</title>
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<div class="refnamediv">
<h1 class="refname">socket_select</h1>
<p class="verinfo">(PHP 4 >= 4.1.0, PHP 5)</p><p class="refpurpose"><span class="refname">socket_select</span> — <span class="dc-title">Runs the select() system call on the given arrays of sockets with a specified timeout</span></p>
</div>
<div class="refsect1 description" id="refsect1-function.socket-select-description">
<h3 class="title">Description</h3>
<div class="methodsynopsis dc-description">
<span class="type">int</span> <span class="methodname"><strong>socket_select</strong></span>
( <span class="methodparam"><span class="type">array</span> <code class="parameter reference">&$read</code></span>
, <span class="methodparam"><span class="type">array</span> <code class="parameter reference">&$write</code></span>
, <span class="methodparam"><span class="type">array</span> <code class="parameter reference">&$except</code></span>
, <span class="methodparam"><span class="type">int</span> <code class="parameter">$tv_sec</code></span>
[, <span class="methodparam"><span class="type">int</span> <code class="parameter">$tv_usec</code><span class="initializer"> = 0</span></span>
] )</div>
<p class="para rdfs-comment">
<span class="function"><strong>socket_select()</strong></span> accepts arrays of sockets and waits for
them to change status. Those coming with BSD sockets background will
recognize that those socket resource arrays are in fact the so-called file
descriptor sets. Three independent arrays of socket resources are watched.
</p>
</div>
<div class="refsect1 parameters" id="refsect1-function.socket-select-parameters">
<h3 class="title">Parameters</h3>
<p class="para">
<dl>
<dt>
<span class="term"><em><code class="parameter">read</code></em></span>
<dd>
<p class="para">
The sockets listed in the <em><code class="parameter">read</code></em> array will be
watched to see if characters become available for reading (more
precisely, to see if a read will not block - in particular, a socket
resource is also ready on end-of-file, in which case a
<span class="function"><a href="function.socket-read.html" class="function">socket_read()</a></span> will return a zero length string).
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">write</code></em></span>
<dd>
<p class="para">
The sockets listed in the <em><code class="parameter">write</code></em> array will be
watched to see if a write will not block.
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">except</code></em></span>
<dd>
<p class="para">
The sockets listed in the <em><code class="parameter">except</code></em> array will be
watched for exceptions.
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">tv_sec</code></em></span>
<dd>
<p class="para">
The <em><code class="parameter">tv_sec</code></em> and <em><code class="parameter">tv_usec</code></em>
together form the <em>timeout</em> parameter. The
<em>timeout</em> is an upper bound on the amount of time
elapsed before <span class="function"><strong>socket_select()</strong></span> return.
<em><code class="parameter">tv_sec</code></em> may be zero , causing
<span class="function"><strong>socket_select()</strong></span> to return immediately. This is useful
for polling. If <em><code class="parameter">tv_sec</code></em> is <strong><code>NULL</code></strong> (no timeout),
<span class="function"><strong>socket_select()</strong></span> can block indefinitely.
</p>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">tv_usec</code></em></span>
<dd>
<p class="para">
</p>
</dd>
</dt>
</dl>
</p>
<div class="warning"><strong class="warning">Warning</strong>
<p class="para">
On exit, the arrays are modified to indicate which socket resource
actually changed status.
</p>
</div>
<p class="para">
You do not need to pass every array to
<span class="function"><strong>socket_select()</strong></span>. You can leave it out and use an
empty array or <strong><code>NULL</code></strong> instead. Also do not forget that those arrays are
passed <em class="emphasis">by reference</em> and will be modified after
<span class="function"><strong>socket_select()</strong></span> returns.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Due a limitation in the current Zend Engine it is not possible to pass a
constant modifier like <strong><code>NULL</code></strong> directly as a parameter to a function
which expects this parameter to be passed by reference. Instead use a
temporary variable or an expression with the leftmost member being a
temporary variable:
<div class="example" id="example-4597">
<p><strong>Example #1 Using <strong><code>NULL</code></strong> with <span class="function"><strong>socket_select()</strong></span></strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$e </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">socket_select</span><span style="color: #007700">(</span><span style="color: #0000BB">$r</span><span style="color: #007700">, </span><span style="color: #0000BB">$w</span><span style="color: #007700">, </span><span style="color: #0000BB">$e</span><span style="color: #007700">, </span><span style="color: #0000BB">0</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</p></blockquote>
</div>
<div class="refsect1 returnvalues" id="refsect1-function.socket-select-returnvalues">
<h3 class="title">Return Values</h3>
<p class="para">
On success <span class="function"><strong>socket_select()</strong></span> returns the number of
socket resources contained in the modified arrays, which may be zero if
the timeout expires before anything interesting happens. On error <strong><code>FALSE</code></strong>
is returned. The error code can be retrieved with
<span class="function"><a href="function.socket-last-error.html" class="function">socket_last_error()</a></span>.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Be sure to use the <em>===</em> operator when checking for an
error. Since the <span class="function"><strong>socket_select()</strong></span> may return 0 the
comparison with <em>==</em> would evaluate to <strong><code>TRUE</code></strong>:
<div class="example" id="example-4598">
<p><strong>Example #2 Understanding <span class="function"><strong>socket_select()</strong></span>'s result</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$e </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">;<br />if (</span><span style="color: #0000BB">false </span><span style="color: #007700">=== </span><span style="color: #0000BB">socket_select</span><span style="color: #007700">(</span><span style="color: #0000BB">$r</span><span style="color: #007700">, </span><span style="color: #0000BB">$w</span><span style="color: #007700">, </span><span style="color: #0000BB">$e</span><span style="color: #007700">, </span><span style="color: #0000BB">0</span><span style="color: #007700">)) {<br /> echo </span><span style="color: #DD0000">"socket_select() failed, reason: " </span><span style="color: #007700">.<br /> </span><span style="color: #0000BB">socket_strerror</span><span style="color: #007700">(</span><span style="color: #0000BB">socket_last_error</span><span style="color: #007700">()) . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</p></blockquote>
</div>
<div class="refsect1 examples" id="refsect1-function.socket-select-examples">
<h3 class="title">Examples</h3>
<p class="para">
<div class="example" id="example-4599">
<p><strong>Example #3 <span class="function"><strong>socket_select()</strong></span> example</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">/* Prepare the read array */<br /></span><span style="color: #0000BB">$read </span><span style="color: #007700">= array(</span><span style="color: #0000BB">$socket1</span><span style="color: #007700">, </span><span style="color: #0000BB">$socket2</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">$write </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$except </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$num_changed_sockets </span><span style="color: #007700">= </span><span style="color: #0000BB">socket_select</span><span style="color: #007700">(</span><span style="color: #0000BB">$read</span><span style="color: #007700">, </span><span style="color: #0000BB">$write</span><span style="color: #007700">, </span><span style="color: #0000BB">$except</span><span style="color: #007700">, </span><span style="color: #0000BB">0</span><span style="color: #007700">);<br /><br />if (</span><span style="color: #0000BB">$num_changed_sockets </span><span style="color: #007700">=== </span><span style="color: #0000BB">false</span><span style="color: #007700">) {<br /> </span><span style="color: #FF8000">/* Error handling */<br /></span><span style="color: #007700">} else if (</span><span style="color: #0000BB">$num_changed_sockets </span><span style="color: #007700">> </span><span style="color: #0000BB">0</span><span style="color: #007700">) {<br /> </span><span style="color: #FF8000">/* At least at one of the sockets something interesting happened */<br /></span><span style="color: #007700">}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</div>
<div class="refsect1 notes" id="refsect1-function.socket-select-notes">
<h3 class="title">Notes</h3>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Be aware that some socket implementations need to be handled very
carefully. A few basic rules:
<ul class="itemizedlist">
<li class="listitem">
<span class="simpara">
You should always try to use <span class="function"><strong>socket_select()</strong></span>
without timeout. Your program should have nothing to do if there is
no data available. Code that depends on timeouts is not usually
portable and difficult to debug.
</span>
</li>
<li class="listitem">
<span class="simpara">
No socket resource must be added to any set if you do not intend to
check its result after the <span class="function"><strong>socket_select()</strong></span> call,
and respond appropriately. After <span class="function"><strong>socket_select()</strong></span>
returns, all socket resources in all arrays must be checked. Any
socket resource that is available for writing must be written to, and
any socket resource available for reading must be read from.
</span>
</li>
<li class="listitem">
<span class="simpara">
If you read/write to a socket returns in the arrays be aware that
they do not necessarily read/write the full amount of data you have
requested. Be prepared to even only be able to read/write a single
byte.
</span>
</li>
<li class="listitem">
<span class="simpara">
It's common to most socket implementations that the only exception
caught with the <em><code class="parameter">except</code></em> array is out-of-bound
data received on a socket.
</span>
</li>
</ul>
</p>
</p></blockquote>
</div>
<div class="refsect1 seealso" id="refsect1-function.socket-select-seealso">
<h3 class="title">See Also</h3>
<p class="para">
<ul class="simplelist">
<li class="member"> <span class="function"><a href="function.socket-read.html" class="function" rel="rdfs-seeAlso">socket_read()</a> - Reads a maximum of length bytes from a socket</span></li>
<li class="member"> <span class="function"><a href="function.socket-write.html" class="function" rel="rdfs-seeAlso">socket_write()</a> - Write to a socket</span></li>
<li class="member"> <span class="function"><a href="function.socket-last-error.html" class="function" rel="rdfs-seeAlso">socket_last_error()</a> - Returns the last error on the socket</span></li>
<li class="member"> <span class="function"><a href="function.socket-strerror.html" class="function" rel="rdfs-seeAlso">socket_strerror()</a> - Return a string describing a socket error</span></li>
</ul>
</p>
</div>
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