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<title>28.5.Cancelling Queries in Progress</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="libpq-cancel"></a>28.5.Cancelling Queries in Progress</h2></div></div></div>
<a name="id683047"></a><p>A client application can request cancellation of
a command that is still being processed by the
server, using the functions described in this section.
</p>
<div class="variablelist"><dl>
<dt><span class="term"><code class="function">PQgetCancel</code><a name="id683074"></a></span></dt>
<dd>
<p> Creates a data structure containing the information needed to cancel
a command issued through a particular database connection.
</p>
<pre class="synopsis">PGcancel *PQgetCancel(PGconn *conn);</pre>
<p><code class="function">PQgetCancel</code> creates a
<code class="structname">PGcancel</code><a name="id683103"></a> object given
a <code class="structname">PGconn</code> connection object. It will return NULL if the
given <em class="parameter"><code>conn</code></em> is NULL or an invalid connection. The
<code class="structname">PGcancel</code> object is an opaque structure that is not meant
to be accessed directly by the application; it can only be passed to
<code class="function">PQcancel</code> or <code class="function">PQfreeCancel</code>.</p>
</dd>
<dt><span class="term"><code class="function">PQfreeCancel</code><a name="id683150"></a></span></dt>
<dd>
<p> Frees a data structure created by <code class="function">PQgetCancel</code>.
</p>
<pre class="synopsis">void PQfreeCancel(PGcancel *cancel);</pre>
<p><code class="function">PQfreeCancel</code> frees a data object previously created
by <code class="function">PQgetCancel</code>.</p>
</dd>
<dt><span class="term"><code class="function">PQcancel</code><a name="id683196"></a></span></dt>
<dd>
<p> Requests that the server abandon
processing of the current command.
</p>
<pre class="synopsis">int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);</pre>
<p>The return value is 1 if the cancel request was successfully
dispatched and 0 if not. If not, <em class="parameter"><code>errbuf</code></em> is filled with an error
message explaining why not. <em class="parameter"><code>errbuf</code></em> must be a char array of size
<em class="parameter"><code>errbufsize</code></em> (the recommended size is 256 bytes).</p>
<p>Successful dispatch is no guarantee that the request will have any effect,
however. If the cancellation is effective, the current command will terminate
early and return an error result. If the cancellation fails (say, because the
server was already done processing the command), then there will be no visible
result at all.</p>
<p><code class="function">PQcancel</code> can safely be invoked from a signal handler,
if the <em class="parameter"><code>errbuf</code></em> is a local variable in the signal handler. The
<code class="structname">PGcancel</code> object is read-only as far as
<code class="function">PQcancel</code> is concerned, so it can also be invoked from a
thread that is separate from the one manipulating the <code class="structname">PGconn</code>
object.</p>
</dd>
</dl></div>
<p>
</p>
<div class="variablelist"><dl>
<dt><span class="term"><code class="function">PQrequestCancel</code><a name="id683291"></a></span></dt>
<dd>
<p> Requests that the server abandon
processing of the current command.
</p>
<pre class="synopsis">int PQrequestCancel(PGconn *conn);</pre>
<p><code class="function">PQrequestCancel</code> is a deprecated variant of
<code class="function">PQcancel</code>. It operates directly on the
<code class="structname">PGconn</code> object, and in case of failure stores the
error message in the <code class="structname">PGconn</code> object (whence it can be
retrieved by <code class="function">PQerrorMessage</code>). Although the
functionality is the same, this approach creates hazards for multiple-thread
programs and signal handlers, since it is possible that overwriting the
<code class="structname">PGconn</code>'s error message will mess up the operation currently
in progress on the connection.</p>
</dd>
</dl></div>
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