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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>34.6. Canceling Queries in Progress</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="libpq-single-row-mode.html" title="34.5. Retrieving Query Results Row-By-Row" /><link rel="next" href="libpq-fastpath.html" title="34.7. The Fast-Path Interface" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">34.6. Canceling Queries in Progress</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-single-row-mode.html" title="34.5. Retrieving Query Results Row-By-Row">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 34. libpq - C Library">Up</a></td><th width="60%" align="center">Chapter 34. <span xmlns="http://www.w3.org/1999/xhtml" class="application">libpq</span> - C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-fastpath.html" title="34.7. The Fast-Path Interface">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LIBPQ-CANCEL"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.6. Canceling Queries in Progress</h2></div></div></div><a id="id-1.7.3.13.2" class="indexterm"></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 class="variablelist"><dt id="LIBPQ-PQGETCANCEL"><span class="term">
<code class="function">PQgetCancel</code>
<a id="id-1.7.3.13.3.1.1.1.2" class="indexterm"></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>
</p><p>
<code class="function">PQgetCancel</code> creates a
<code class="structname">PGcancel</code><a id="id-1.7.3.13.3.1.1.2.2.3" class="indexterm"></a> object
given a <code class="structname">PGconn</code> connection object. It will return
<code class="symbol">NULL</code> if the given <em class="parameter"><code>conn</code></em> is <code class="symbol">NULL</code> 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 id="LIBPQ-PQFREECANCEL"><span class="term">
<code class="function">PQfreeCancel</code>
<a id="id-1.7.3.13.3.1.2.1.2" class="indexterm"></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>
</p><p>
<code class="function">PQfreeCancel</code> frees a data object previously created
by <code class="function">PQgetCancel</code>.
</p></dd><dt id="LIBPQ-PQCANCEL"><span class="term">
<code class="function">PQcancel</code>
<a id="id-1.7.3.13.3.1.3.1.2" class="indexterm"></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>
</p><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 explanatory error message. <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 class="variablelist"><dt id="LIBPQ-PQREQUESTCANCEL"><span class="term">
<code class="function">PQrequestCancel</code>
<a id="id-1.7.3.13.3.2.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
<code class="function">PQrequestCancel</code> is a deprecated variant of
<code class="function">PQcancel</code>.
</p><pre class="synopsis">
int PQrequestCancel(PGconn *conn);
</pre><p>
</p><p>
Requests that the server abandon processing of the current
command. 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><p>
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-single-row-mode.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="libpq.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="libpq-fastpath.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">34.5. Retrieving Query Results Row-By-Row </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 34.7. The Fast-Path Interface</td></tr></table></div></body></html>
