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><A
NAME="EVENT-TRIGGER-INTERFACE"
>37.3. 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)-&gt;context != NULL &amp;&amp; IsA((fcinfo)-&gt;context, EventTriggerData))</PRE
><P>
    If this returns true, then it is safe to cast
    <TT
CLASS="LITERAL"
>fcinfo-&gt;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
>.
        See <A
HREF="event-trigger-definition.html"
>Section 37.1</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
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