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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>34.9. Asynchronous Notification</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-fastpath.html" title="34.8. The Fast-Path Interface" /><link rel="next" href="libpq-copy.html" title="34.10. Functions Associated with the COPY Command" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">34.9. Asynchronous Notification</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-fastpath.html" title="34.8. The Fast-Path Interface">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 class="application">libpq</span> — C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-copy.html" title="34.10. Functions Associated with the COPY Command">Next</a></td></tr></table><hr /></div><div class="sect1" id="LIBPQ-NOTIFY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.9. Asynchronous Notification <a href="#LIBPQ-NOTIFY" class="id_link">#</a></h2></div></div></div><a id="id-1.7.3.16.2" class="indexterm"></a><p>3   <span class="productname">PostgreSQL</span> offers asynchronous notification4   via the <code class="command">LISTEN</code> and <code class="command">NOTIFY</code>5   commands.  A client session registers its interest in a particular6   notification channel with the <code class="command">LISTEN</code> command (and7   can stop listening with the <code class="command">UNLISTEN</code> command).  All8   sessions listening on a particular channel will be notified9   asynchronously when a <code class="command">NOTIFY</code> command with that10   channel name is executed by any session. A <span class="quote">“<span class="quote">payload</span>”</span> string can11   be passed to communicate additional data to the listeners.12  </p><p>13   <span class="application">libpq</span> applications submit14   <code class="command">LISTEN</code>, <code class="command">UNLISTEN</code>,15   and <code class="command">NOTIFY</code> commands as16   ordinary SQL commands.  The arrival of <code class="command">NOTIFY</code>17   messages can subsequently be detected by calling18   <code class="function" id="LIBPQ-PQNOTIFIES">PQnotifies</code>.<a id="id-1.7.3.16.4.7" class="indexterm"></a>19  </p><p>20   The function <code class="function">PQnotifies</code> returns the next notification21   from a list of unhandled notification messages received from the server.22   It returns a null pointer if there are no pending notifications.  Once a23   notification is returned from <code class="function">PQnotifies</code>, it is considered24   handled and will be removed from the list of notifications.25 26</p><pre class="synopsis">27PGnotify *PQnotifies(PGconn *conn);28 29typedef struct pgNotify30{31    char *relname;              /* notification channel name */32    int  be_pid;                /* process ID of notifying server process */33    char *extra;                /* notification payload string */34} PGnotify;35</pre><p>36 37   After processing a <code class="structname">PGnotify</code> object returned38   by <code class="function">PQnotifies</code>, be sure to free it with39   <a class="xref" href="libpq-misc.html#LIBPQ-PQFREEMEM"><code class="function">PQfreemem</code></a>.  It is sufficient to free the40   <code class="structname">PGnotify</code> pointer; the41   <code class="structfield">relname</code> and <code class="structfield">extra</code>42   fields do not represent separate allocations.  (The names of these fields43   are historical; in particular, channel names need not have anything to44   do with relation names.)45  </p><p>46   <a class="xref" href="libpq-example.html#LIBPQ-EXAMPLE-2" title="Example 34.2. libpq Example Program 2">Example 34.2</a> gives a sample program that illustrates47   the use of asynchronous notification.48  </p><p>49   <code class="function">PQnotifies</code> does not actually read data from the50   server; it just returns messages previously absorbed by another51   <span class="application">libpq</span> function.  In ancient releases of52   <span class="application">libpq</span>, the only way to ensure timely receipt53   of <code class="command">NOTIFY</code> messages was to constantly submit commands, even54   empty ones, and then check <code class="function">PQnotifies</code> after each55   <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>.  While this still works, it is deprecated56   as a waste of processing power.57  </p><p>58   A better way to check for <code class="command">NOTIFY</code> messages when you have no59   useful commands to execute is to call60   <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>61     </a>, then check62   <code class="function">PQnotifies</code>.  You can use63   <code class="function">select()</code> to wait for data to arrive from the64   server, thereby using no <acronym class="acronym">CPU</acronym> power unless there is65   something to do.  (See <a class="xref" href="libpq-status.html#LIBPQ-PQSOCKET"><code class="function">PQsocket</code></a> to obtain the file66   descriptor number to use with <code class="function">select()</code>.) Note that67   this will work OK whether you submit commands with68   <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a>/<a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> or69   simply use <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>.  You should, however, remember70   to check <code class="function">PQnotifies</code> after each71   <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> or <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>, to72   see if any notifications came in during the processing of the command.73  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-fastpath.html" title="34.8. The Fast-Path Interface">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="libpq.html" title="Chapter 34. libpq — C Library">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="libpq-copy.html" title="34.10. Functions Associated with the COPY Command">Next</a></td></tr><tr><td width="40%" align="left" valign="top">34.8. The Fast-Path Interface </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 34.10. Functions Associated with the <code class="command">COPY</code> Command</td></tr></table></div></body></html>