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   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>38.2. 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-definition.html" title="38.1. Overview of Event Trigger Behavior" /><link rel="next" href="event-trigger-example.html" title="38.3. 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">38.2. Writing Event Trigger Functions in C</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="event-trigger-definition.html" title="38.1. Overview of Event Trigger Behavior">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="event-triggers.html" title="Chapter 38. Event Triggers">Up</a></td><th width="60%" align="center">Chapter 38. Event Triggers</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="event-trigger-example.html" title="38.3. 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">38.2. Writing Event Trigger Functions in C <a href="#EVENT-TRIGGER-INTERFACE" class="id_link">#</a></h2></div></div></div><a id="id-1.8.5.6.2" class="indexterm"></a><p>
   3     This section describes the low-level details of the interface to an
   4     event trigger function. This information is only needed when writing
   5     event trigger functions in C. If you are using a higher-level language
   6     then these details are handled for you. In most cases you should
   7     consider using a procedural language before writing your event triggers
   8     in C. The documentation of each procedural language explains how to
   9     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> function
  12     manager interface.
  13    </p><p>
  14     When a function is called by the event trigger manager, it is not passed
  15     any normal arguments, but it is passed a <span class="quote">“<span class="quote">context</span>”</span> pointer
  16     pointing to a <code class="structname">EventTriggerData</code> structure. C functions can
  17     check whether they were called from the event trigger manager or not by
  18     executing the macro:
  19 </p><pre class="programlisting">
  20 CALLED_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 cast
  27     <code class="literal">fcinfo-&gt;context</code> to type <code class="literal">EventTriggerData
  28     *</code> and make use of the pointed-to
  29     <code class="structname">EventTriggerData</code> structure.  The function must
  30     <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 in
  34     <code class="filename">commands/event_trigger.h</code>:
  35
  36 </p><pre class="programlisting">
  37 typedef struct EventTriggerData
  38 {
  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 of
  52         <code class="literal">"login"</code>, <code class="literal">"ddl_command_start"</code>,
  53         <code class="literal">"ddl_command_end"</code>, <code class="literal">"sql_drop"</code>,
  54         <code class="literal">"table_rewrite"</code>.
  55         See <a class="xref" href="event-trigger-definition.html" title="38.1. Overview of Event Trigger Behavior">Section 38.1</a> for the meaning of these
  56         events.
  57        </p></dd><dt><span class="term"><code class="structfield">parsetree</code></span></dt><dd><p>
  58         A pointer to the parse tree of the command.  Check the PostgreSQL
  59         source code for details.  The parse tree structure is subject to change
  60         without notice.
  61        </p></dd><dt><span class="term"><code class="structfield">tag</code></span></dt><dd><p>
  62         The command tag associated with the event for which the event trigger
  63         is run, for example <code class="literal">"CREATE FUNCTION"</code>.
  64        </p></dd></dl></div><p>
  65    </p><p>
  66     An event trigger function must return a <code class="symbol">NULL</code> pointer
  67     (<span class="emphasis"><em>not</em></span> an SQL null value, that is, do not
  68     set <em class="parameter"><code>isNull</code></em> true).
  69    </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="event-trigger-definition.html" title="38.1. Overview of Event Trigger Behavior">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="event-triggers.html" title="Chapter 38. Event Triggers">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="event-trigger-example.html" title="38.3. A Complete Event Trigger Example">Next</a></td></tr><tr><td width="40%" align="left" valign="top">38.1. Overview of Event Trigger Behavior </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 38.3. A Complete Event Trigger Example</td></tr></table></div></body></html>