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This section describes the low-level details of the interface to an
event trigger function. This information is only needed when writing
event trigger functions in C. If you are using a higher-level language
then these details are handled for you. In most cases you should
consider using a procedural language before writing your event triggers
in C. The documentation of each procedural language explains how to
write an event trigger in that language.
</p><p>
Event trigger functions must use the <span class="quote">“<span class="quote">version 1</span>”</span> function
manager interface.
</p><p>
When a function is called by the event trigger manager, it is not passed
any normal arguments, but it is passed a <span class="quote">“<span class="quote">context</span>”</span> pointer
pointing to a <code class="structname">EventTriggerData</code> structure. C functions can
check whether they were called from the event trigger manager or not by
executing the macro:
</p><pre class="programlisting">
CALLED_AS_EVENT_TRIGGER(fcinfo)
</pre><p>
which expands to:
</p><pre class="programlisting">
((fcinfo)->context != NULL && IsA((fcinfo)->context, EventTriggerData))
</pre><p>
If this returns true, then it is safe to cast
<code class="literal">fcinfo->context</code> to type <code class="literal">EventTriggerData
*</code> and make use of the pointed-to
<code class="structname">EventTriggerData</code> structure. The function must
<span class="emphasis"><em>not</em></span> alter the <code class="structname">EventTriggerData</code>
structure or any of the data it points to.
</p><p>
<code class="structname">struct EventTriggerData</code> is defined in
<code class="filename">commands/event_trigger.h</code>:
</p><pre class="programlisting">
typedef struct EventTriggerData
{
NodeTag type;
const char *event; /* event name */
Node *parsetree; /* parse tree */
const char *tag; /* command tag */
} EventTriggerData;
</pre><p>
where the members are defined as follows:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="structfield">type</code></span></dt><dd><p>
Always <code class="literal">T_EventTriggerData</code>.
</p></dd><dt><span class="term"><code class="structfield">event</code></span></dt><dd><p>
Describes the event for which the function is called, one of
<code class="literal">"ddl_command_start"</code>, <code class="literal">"ddl_command_end"</code>,
<code class="literal">"sql_drop"</code>, <code class="literal">"table_rewrite"</code>.
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 these
events.
</p></dd><dt><span class="term"><code class="structfield">parsetree</code></span></dt><dd><p>
A pointer to the parse tree of the command. Check the PostgreSQL
source code for details. The parse tree structure is subject to change
without notice.
</p></dd><dt><span class="term"><code class="structfield">tag</code></span></dt><dd><p>
The command tag associated with the event for which the event trigger
is run, for example <code class="literal">"CREATE FUNCTION"</code>.
</p></dd></dl></div><p>
</p><p>
An event trigger function must return a <code class="symbol">NULL</code> pointer
(<span class="emphasis"><em>not</em></span> an SQL null value, that is, do not
set <em class="parameter"><code>isNull</code></em> true).
</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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="event-triggers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="event-trigger-example.html">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">Home</a></td><td width="40%" align="right" valign="top"> 40.4. A Complete Event Trigger Example</td></tr></table></div></body></html>
