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diff --git a/doc/src/sgml/html/libpq-notify.html b/doc/src/sgml/html/libpq-notify.html new file mode 100644 index 0000000..e0021ac --- /dev/null +++ b/doc/src/sgml/html/libpq-notify.html @@ -0,0 +1,73 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 15.5 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</h2></div></div></div><a id="id-1.7.3.16.2" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span> offers asynchronous notification + via the <code class="command">LISTEN</code> and <code class="command">NOTIFY</code> + commands. A client session registers its interest in a particular + notification channel with the <code class="command">LISTEN</code> command (and + can stop listening with the <code class="command">UNLISTEN</code> command). All + sessions listening on a particular channel will be notified + asynchronously when a <code class="command">NOTIFY</code> command with that + channel name is executed by any session. A <span class="quote">“<span class="quote">payload</span>”</span> string can + be passed to communicate additional data to the listeners. + </p><p> + <span class="application">libpq</span> applications submit + <code class="command">LISTEN</code>, <code class="command">UNLISTEN</code>, + and <code class="command">NOTIFY</code> commands as + ordinary SQL commands. The arrival of <code class="command">NOTIFY</code> + messages can subsequently be detected by calling + <code class="function" id="LIBPQ-PQNOTIFIES">PQnotifies</code>.<a id="id-1.7.3.16.4.7" class="indexterm"></a> + </p><p> + The function <code class="function">PQnotifies</code> returns the next notification + from a list of unhandled notification messages received from the server. + It returns a null pointer if there are no pending notifications. Once a + notification is returned from <code class="function">PQnotifies</code>, it is considered + handled and will be removed from the list of notifications. + +</p><pre class="synopsis"> +PGnotify *PQnotifies(PGconn *conn); + +typedef struct pgNotify +{ + char *relname; /* notification channel name */ + int be_pid; /* process ID of notifying server process */ + char *extra; /* notification payload string */ +} PGnotify; +</pre><p> + + After processing a <code class="structname">PGnotify</code> object returned + by <code class="function">PQnotifies</code>, be sure to free it with + <a class="xref" href="libpq-misc.html#LIBPQ-PQFREEMEM"><code class="function">PQfreemem</code></a>. It is sufficient to free the + <code class="structname">PGnotify</code> pointer; the + <code class="structfield">relname</code> and <code class="structfield">extra</code> + fields do not represent separate allocations. (The names of these fields + are historical; in particular, channel names need not have anything to + do with relation names.) + </p><p> + <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 illustrates + the use of asynchronous notification. + </p><p> + <code class="function">PQnotifies</code> does not actually read data from the + server; it just returns messages previously absorbed by another + <span class="application">libpq</span> function. In ancient releases of + <span class="application">libpq</span>, the only way to ensure timely receipt + of <code class="command">NOTIFY</code> messages was to constantly submit commands, even + empty ones, and then check <code class="function">PQnotifies</code> after each + <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>. While this still works, it is deprecated + as a waste of processing power. + </p><p> + A better way to check for <code class="command">NOTIFY</code> messages when you have no + useful commands to execute is to call + <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code> + </a>, then check + <code class="function">PQnotifies</code>. You can use + <code class="function">select()</code> to wait for data to arrive from the + server, thereby using no <acronym class="acronym">CPU</acronym> power unless there is + something to do. (See <a class="xref" href="libpq-status.html#LIBPQ-PQSOCKET"><code class="function">PQsocket</code></a> to obtain the file + descriptor number to use with <code class="function">select()</code>.) Note that + this will work OK whether you submit commands with + <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> or + simply use <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>. You should, however, remember + to check <code class="function">PQnotifies</code> after each + <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>, to + see if any notifications came in during the processing of the command. + </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 15.5 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>
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