<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Writing Event Trigger Functions in C</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.6.21 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Event Triggers" HREF="event-triggers.html"><LINK REL="PREVIOUS" TITLE="Event Trigger Firing Matrix" HREF="event-trigger-matrix.html"><LINK REL="NEXT" TITLE="A Complete Event Trigger Example" HREF="event-trigger-example.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2021-02-27T18:26:08"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="4" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.6.21 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Event Trigger Firing Matrix" HREF="event-trigger-matrix.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="event-triggers.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 38. Event Triggers</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="A Complete Event Trigger Example" HREF="event-trigger-example.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="EVENT-TRIGGER-INTERFACE" >38.3. Writing Event Trigger Functions in C</A ></H1 ><P > 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" >"version 1"</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" >"context"</SPAN > pointer pointing to a <TT CLASS="STRUCTNAME" >EventTriggerData</TT > 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 <TT CLASS="LITERAL" >fcinfo->context</TT > to type <TT CLASS="LITERAL" >EventTriggerData *</TT > and make use of the pointed-to <TT CLASS="STRUCTNAME" >EventTriggerData</TT > structure. The function must <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >not</I ></SPAN > alter the <TT CLASS="STRUCTNAME" >EventTriggerData</TT > structure or any of the data it points to. </P ><P > <TT CLASS="STRUCTNAME" >struct EventTriggerData</TT > is defined in <TT CLASS="FILENAME" >commands/event_trigger.h</TT >: </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 ></P ></P><DIV CLASS="VARIABLELIST" ><DL ><DT ><TT CLASS="STRUCTFIELD" >type</TT ></DT ><DD ><P > Always <TT CLASS="LITERAL" >T_EventTriggerData</TT >. </P ></DD ><DT ><TT CLASS="STRUCTFIELD" >event</TT ></DT ><DD ><P > Describes the event for which the function is called, one of <TT CLASS="LITERAL" >"ddl_command_start"</TT >, <TT CLASS="LITERAL" >"ddl_command_end"</TT >, <TT CLASS="LITERAL" >"sql_drop"</TT >, <TT CLASS="LITERAL" >"table_rewrite"</TT >. See <A HREF="event-trigger-definition.html" >Section 38.1</A > for the meaning of these events. </P ></DD ><DT ><TT CLASS="STRUCTFIELD" >parsetree</TT ></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 ><TT CLASS="STRUCTFIELD" >tag</TT ></DT ><DD ><P > The command tag associated with the event for which the event trigger is run, for example <TT CLASS="LITERAL" >"CREATE FUNCTION"</TT >. </P ></DD ></DL ></DIV ><P> </P ><P > An event trigger function must return a <TT CLASS="SYMBOL" >NULL</TT > pointer (<SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >not</I ></SPAN > an SQL null value, that is, do not set <TT CLASS="PARAMETER" >isNull</TT > true). </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="event-trigger-matrix.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="event-trigger-example.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Event Trigger Firing Matrix</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="event-triggers.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >A Complete Event Trigger Example</TD ></TR ></TABLE ></DIV ></BODY ></HTML >