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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>40.3. Writing Event Trigger Functions in C</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="event-trigger-matrix.html" title="40.2. Event Trigger Firing Matrix" /><link rel="next" href="event-trigger-example.html" title="40.4. A Complete Event Trigger Example" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">40.3. Writing Event Trigger Functions in C</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="event-trigger-matrix.html" title="40.2. Event Trigger Firing Matrix">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="event-triggers.html" title="Chapter 40. Event Triggers">Up</a></td><th width="60%" align="center">Chapter 40. Event Triggers</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="event-trigger-example.html" title="40.4. A Complete Event Trigger Example">Next</a></td></tr></table><hr /></div><div class="sect1" id="EVENT-TRIGGER-INTERFACE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">40.3. Writing Event Trigger Functions in C <a href="#EVENT-TRIGGER-INTERFACE" class="id_link">#</a></h2></div></div></div><a id="id-1.8.5.7.2" class="indexterm"></a><p>3    This section describes the low-level details of the interface to an4    event trigger function. This information is only needed when writing5    event trigger functions in C. If you are using a higher-level language6    then these details are handled for you. In most cases you should7    consider using a procedural language before writing your event triggers8    in C. The documentation of each procedural language explains how to9    write an event trigger in that language.10   </p><p>11    Event trigger functions must use the <span class="quote">“<span class="quote">version 1</span>”</span> function12    manager interface.13   </p><p>14    When a function is called by the event trigger manager, it is not passed15    any normal arguments, but it is passed a <span class="quote">“<span class="quote">context</span>”</span> pointer16    pointing to a <code class="structname">EventTriggerData</code> structure. C functions can17    check whether they were called from the event trigger manager or not by18    executing the macro:19</p><pre class="programlisting">20CALLED_AS_EVENT_TRIGGER(fcinfo)21</pre><p>22    which expands to:23</p><pre class="programlisting">24((fcinfo)-&gt;context != NULL &amp;&amp; IsA((fcinfo)-&gt;context, EventTriggerData))25</pre><p>26    If this returns true, then it is safe to cast27    <code class="literal">fcinfo-&gt;context</code> to type <code class="literal">EventTriggerData28    *</code> and make use of the pointed-to29    <code class="structname">EventTriggerData</code> structure.  The function must30    <span class="emphasis"><em>not</em></span> alter the <code class="structname">EventTriggerData</code>31    structure or any of the data it points to.32   </p><p>33    <code class="structname">struct EventTriggerData</code> is defined in34    <code class="filename">commands/event_trigger.h</code>:35 36</p><pre class="programlisting">37typedef struct EventTriggerData38{39    NodeTag     type;40    const char *event;      /* event name */41    Node       *parsetree;  /* parse tree */42    CommandTag  tag;        /* command tag */43} EventTriggerData;44</pre><p>45 46    where the members are defined as follows:47 48    </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="structfield">type</code></span></dt><dd><p>49        Always <code class="literal">T_EventTriggerData</code>.50       </p></dd><dt><span class="term"><code class="structfield">event</code></span></dt><dd><p>51        Describes the event for which the function is called, one of52        <code class="literal">"ddl_command_start"</code>, <code class="literal">"ddl_command_end"</code>,53        <code class="literal">"sql_drop"</code>, <code class="literal">"table_rewrite"</code>.54        See <a class="xref" href="event-trigger-definition.html" title="40.1. Overview of Event Trigger Behavior">Section 40.1</a> for the meaning of these55        events.56       </p></dd><dt><span class="term"><code class="structfield">parsetree</code></span></dt><dd><p>57        A pointer to the parse tree of the command.  Check the PostgreSQL58        source code for details.  The parse tree structure is subject to change59        without notice.60       </p></dd><dt><span class="term"><code class="structfield">tag</code></span></dt><dd><p>61        The command tag associated with the event for which the event trigger62        is run, for example <code class="literal">"CREATE FUNCTION"</code>.63       </p></dd></dl></div><p>64   </p><p>65    An event trigger function must return a <code class="symbol">NULL</code> pointer66    (<span class="emphasis"><em>not</em></span> an SQL null value, that is, do not67    set <em class="parameter"><code>isNull</code></em> true).68   </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="event-trigger-matrix.html" title="40.2. Event Trigger Firing Matrix">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="event-triggers.html" title="Chapter 40. Event Triggers">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="event-trigger-example.html" title="40.4. A Complete Event Trigger Example">Next</a></td></tr><tr><td width="40%" align="left" valign="top">40.2. Event Trigger Firing Matrix </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"> 40.4. A Complete Event Trigger Example</td></tr></table></div></body></html>