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<title>Runs the equivalent of the select() system call on the given
arrays of streams with a timeout specified by tv_sec and tv_usec</title>
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<div class="refnamediv">
<h1 class="refname">stream_select</h1>
<p class="verinfo">(PHP 4 >= 4.3.0, PHP 5)</p><p class="refpurpose"><span class="refname">stream_select</span> — <span class="dc-title">Runs the equivalent of the select() system call on the given
arrays of streams with a timeout specified by tv_sec and tv_usec </span></p>
</div>
<div class="refsect1 description" id="refsect1-function.stream-select-description">
<h3 class="title">Description</h3>
<div class="methodsynopsis dc-description">
<span class="type">int</span> <span class="methodname"><strong>stream_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">
The <span class="function"><strong>stream_select()</strong></span> function accepts arrays of streams and
waits for them to change status. Its operation is equivalent to that of
the <span class="function"><a href="function.socket-select.html" class="function">socket_select()</a></span> function except in that it acts on streams.
</p>
</div>
<div class="refsect1 parameters" id="refsect1-function.stream-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 streams 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 stream resource is also ready on
end-of-file, in which case an <span class="function"><a href="function.fread.html" class="function">fread()</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 streams 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 streams listed in the <em><code class="parameter">except</code></em> array will be
watched for high priority exceptional ("out-of-band") data arriving.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
When <span class="function"><strong>stream_select()</strong></span> returns, the arrays
<em><code class="parameter">read</code></em>, <em><code class="parameter">write</code></em> and
<em><code class="parameter">except</code></em> are modified to indicate which stream
resource(s) actually changed status.
</p>
</p></blockquote>
<span class="simpara">
You do not need to pass every array to
<span class="function"><strong>stream_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>stream_select()</strong></span> returns.
</span>
</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 class="emphasis">timeout</em> parameter,
<em><code class="parameter">tv_sec</code></em> specifies the number of seconds while
<em><code class="parameter">tv_usec</code></em> the number of microseconds.
The <em><code class="parameter">timeout</code></em> is an upper bound on the amount of time
that <span class="function"><strong>stream_select()</strong></span> will wait before it returns.
If <em><code class="parameter">tv_sec</code></em> and <em><code class="parameter">tv_usec</code></em> are
both set to <em>0</em>, <span class="function"><strong>stream_select()</strong></span> will
not wait for data - instead it will return immediately, indicating the
current status of the streams.
</p>
<p class="para">
If <em><code class="parameter">tv_sec</code></em> is <strong><code>NULL</code></strong> <span class="function"><strong>stream_select()</strong></span>
can block indefinitely, returning only when an event on one of the
watched streams occurs (or if a signal interrupts the system call).
</p>
<div class="warning"><strong class="warning">Warning</strong>
<p class="para">
Using a timeout value of <em>0</em> allows you to
instantaneously poll the status of the streams, however, it is NOT a
good idea to use a <em>0</em> timeout value in a loop as it
will cause your script to consume too much CPU time.
</p>
<p class="para">
It is much better to specify a timeout value of a few seconds, although
if you need to be checking and running other code concurrently, using a
timeout value of at least <em>200000</em> microseconds will
help reduce the CPU usage of your script.
</p>
<p class="para">
Remember that the timeout value is the maximum time that will elapse;
<span class="function"><strong>stream_select()</strong></span> will return as soon as the
requested streams are ready for use.
</p>
</div>
</dd>
</dt>
<dt>
<span class="term"><em><code class="parameter">tv_usec</code></em></span>
<dd>
<p class="para">
See <em><code class="parameter">tv_sec</code></em> description.
</p>
</dd>
</dt>
</dl>
</p>
</div>
<div class="refsect1 returnvalues" id="refsect1-function.stream-select-returnvalues">
<h3 class="title">Return Values</h3>
<p class="para">
On success <span class="function"><strong>stream_select()</strong></span> returns the number of
stream 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 and a warning raised (this can happen if the system call is
interrupted by an incoming signal).
</p>
</div>
<div class="refsect1 examples" id="refsect1-function.stream-select-examples">
<h3 class="title">Examples</h3>
<p class="para">
<div class="example" id="example-4030">
<p><strong>Example #1 <span class="function"><strong>stream_select()</strong></span> Example</strong></p>
<div class="example-contents"><p>
This example checks to see if data has arrived for reading on either
<em><code class="parameter">$stream1</code></em> or <em><code class="parameter">$stream2</code></em>.
Since the timeout value is <em>0</em> it will return
immediately:
</p></div>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">/* Prepare the read array */<br /></span><span style="color: #0000BB">$read </span><span style="color: #007700">= array(</span><span style="color: #0000BB">$stream1</span><span style="color: #007700">, </span><span style="color: #0000BB">$stream2</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 />if (</span><span style="color: #0000BB">false </span><span style="color: #007700">=== (</span><span style="color: #0000BB">$num_changed_streams </span><span style="color: #007700">= </span><span style="color: #0000BB">stream_select</span><span style="color: #007700">(</span><span style="color: #0000BB">$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 /> </span><span style="color: #FF8000">/* Error handling */<br /></span><span style="color: #007700">} elseif (</span><span style="color: #0000BB">$num_changed_streams </span><span style="color: #007700">> </span><span style="color: #0000BB">0</span><span style="color: #007700">) {<br /> </span><span style="color: #FF8000">/* At least on one of the streams something interesting happened */<br /></span><span style="color: #007700">}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</div>
<div class="refsect1 notes" id="refsect1-function.stream-select-notes">
<h3 class="title">Notes</h3>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Due to a limitation in the current Zend Engine it is not possible to pass a
constant modifier like <strong><code>NULL</code></strong> directly as a parameter to a function
which expects this parameter to be passed by reference. Instead use a
temporary variable or an expression with the leftmost member being a
temporary variable:
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$e </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">stream_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">$e</span><span style="color: #007700">, </span><span style="color: #0000BB">0</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</p>
</p></blockquote>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Be sure to use the <em>===</em> operator when checking for an
error. Since the <span class="function"><strong>stream_select()</strong></span> may return 0 the
comparison with <em>==</em> would evaluate to <strong><code>TRUE</code></strong>:
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$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">stream_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">"stream_select() failed\n"</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</p>
</p></blockquote>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
If you read/write to a stream returned 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.
</p>
</p></blockquote>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Some streams (like <em>zlib</em>) cannot be selected by this
function.
</p>
</p></blockquote>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Windows compatibility: <span class="function"><strong>stream_select()</strong></span> used on a
pipe returned from <span class="function"><a href="function.proc-open.html" class="function">proc_open()</a></span> may cause data loss
under Windows 98.
</p>
<p class="para">
Use of <span class="function"><strong>stream_select()</strong></span> on
file descriptors returned by <span class="function"><a href="function.proc-open.html" class="function">proc_open()</a></span> will fail
and return <strong><code>FALSE</code></strong> under Windows.
</p>
</p></blockquote>
</div>
<div class="refsect1 seealso" id="refsect1-function.stream-select-seealso">
<h3 class="title">See Also</h3>
<ul class="simplelist">
<li class="member"> <span class="function"><a href="function.stream-set-blocking.html" class="function" rel="rdfs-seeAlso">stream_set_blocking()</a> - Set blocking/non-blocking mode on a stream</span></li>
</ul>
</div>
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