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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!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>33.6. 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 V1.79.1" /><link rel="prev" href="libpq-single-row-mode.html" title="33.5. Retrieving Query Results Row-by-Row" /><link rel="next" href="libpq-fastpath.html" title="33.7. 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">33.6. Canceling Queries in Progress</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-single-row-mode.html" title="33.5. Retrieving Query Results Row-by-Row">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 33. libpq - C Library">Up</a></td><th width="60%" align="center">Chapter 33. <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 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-fastpath.html" title="33.7. 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">33.6. Canceling Queries in Progress</h2></div></div></div><a id="id-1.7.3.13.2" class="indexterm"></a><p>

  3.    A client application can request cancellation of a command that is

  4.    still being processed by the server, using the functions described in

  5.    this section.

  6. 

  7.    </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQGETCANCEL"><span class="term">

  8.       <code class="function">PQgetCancel</code>

  9.       <a id="id-1.7.3.13.3.1.1.1.2" class="indexterm"></a>

  10.      </span></dt><dd><p>

  11.        Creates a data structure containing the information needed to cancel

  12.        a command issued through a particular database connection.

  13. </p><pre class="synopsis">

  14. PGcancel *PQgetCancel(PGconn *conn);

  15. </pre><p>

  16.       </p><p>

  17.        <code class="function">PQgetCancel</code> creates a

  18.        <code class="structname">PGcancel</code><a id="id-1.7.3.13.3.1.1.2.2.3" class="indexterm"></a> object

  19.        given a <code class="structname">PGconn</code> connection object.  It will return

  20.        <code class="symbol">NULL</code> if the given <em class="parameter"><code>conn</code></em> is <code class="symbol">NULL</code> or an invalid

  21.        connection.  The <code class="structname">PGcancel</code> object is an opaque

  22.        structure that is not meant to be accessed directly by the

  23.        application; it can only be passed to <code class="function">PQcancel</code>

  24.        or <code class="function">PQfreeCancel</code>.

  25.       </p></dd><dt id="LIBPQ-PQFREECANCEL"><span class="term">

  26.       <code class="function">PQfreeCancel</code>

  27.       <a id="id-1.7.3.13.3.1.2.1.2" class="indexterm"></a>

  28.      </span></dt><dd><p>

  29.        Frees a data structure created by <code class="function">PQgetCancel</code>.

  30. </p><pre class="synopsis">

  31. void PQfreeCancel(PGcancel *cancel);

  32. </pre><p>

  33.       </p><p>

  34.        <code class="function">PQfreeCancel</code> frees a data object previously created

  35.        by <code class="function">PQgetCancel</code>.

  36.       </p></dd><dt id="LIBPQ-PQCANCEL"><span class="term">

  37.       <code class="function">PQcancel</code>

  38.       <a id="id-1.7.3.13.3.1.3.1.2" class="indexterm"></a>

  39.      </span></dt><dd><p>

  40.        Requests that the server abandon processing of the current command.

  41. </p><pre class="synopsis">

  42. int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);

  43. </pre><p>

  44.       </p><p>

  45.        The return value is 1 if the cancel request was successfully

  46.        dispatched and 0 if not.  If not, <em class="parameter"><code>errbuf</code></em> is filled

  47.        with an explanatory error message.  <em class="parameter"><code>errbuf</code></em>

  48.        must be a char array of size <em class="parameter"><code>errbufsize</code></em> (the

  49.        recommended size is 256 bytes).

  50.       </p><p>

  51.        Successful dispatch is no guarantee that the request will have

  52.        any effect, however.  If the cancellation is effective, the current

  53.        command will terminate early and return an error result.  If the

  54.        cancellation fails (say, because the server was already done

  55.        processing the command), then there will be no visible result at

  56.        all.

  57.       </p><p>

  58.        <code class="function">PQcancel</code> can safely be invoked from a signal

  59.        handler, if the <em class="parameter"><code>errbuf</code></em> is a local variable in the

  60.        signal handler.  The <code class="structname">PGcancel</code> object is read-only

  61.        as far as <code class="function">PQcancel</code> is concerned, so it can

  62.        also be invoked from a thread that is separate from the one

  63.        manipulating the <code class="structname">PGconn</code> object.

  64.       </p></dd></dl></div><p>

  65. 

  66.    </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQREQUESTCANCEL"><span class="term">

  67.       <code class="function">PQrequestCancel</code>

  68.       <a id="id-1.7.3.13.3.2.1.1.2" class="indexterm"></a>

  69.      </span></dt><dd><p>

  70.        <code class="function">PQrequestCancel</code> is a deprecated variant of

  71.        <code class="function">PQcancel</code>.

  72. </p><pre class="synopsis">

  73. int PQrequestCancel(PGconn *conn);

  74. </pre><p>

  75.       </p><p>

  76.        Requests that the server abandon processing of the current

  77.        command.  It operates directly on the

  78.        <code class="structname">PGconn</code> object, and in case of failure stores the

  79.        error message in the <code class="structname">PGconn</code> object (whence it can

  80.        be retrieved by <code class="function">PQerrorMessage</code>).  Although

  81.        the functionality is the same, this approach creates hazards for

  82.        multiple-thread programs and signal handlers, since it is possible

  83.        that overwriting the <code class="structname">PGconn</code>'s error message will

  84.        mess up the operation currently in progress on the connection.

  85.       </p></dd></dl></div><p>

  86.   </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">33.5. 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"> 33.7. The Fast-Path Interface</td></tr></table></div></body></html>
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