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<!--
$PostgreSQL: pgsql/doc/src/sgml/plhandler.sgml,v 1.5 2005/01/22 22:06:17 momjian Exp $
-->
<chapter id="plhandler">
<title>Writing A Procedural Language Handler</title>
<indexterm zone="plhandler">
<primary>procedural language</primary>
<secondary>handler for</secondary>
</indexterm>
<para>
All calls to functions that are written in a language other than
the current <quote>version 1</quote> interface for compiled
languages (this includes functions in user-defined procedural languages,
functions written in SQL, and functions using the version 0 compiled
language interface), go through a <firstterm>call handler</firstterm>
function for the specific language. It is the responsibility of
the call handler to execute the function in a meaningful way, such
as by interpreting the supplied source text. This chapter outlines
how a new procedural language's call handler can be written.
</para>
<para>
The call handler for a procedural language is a
<quote>normal</quote> function that must be written in a compiled
language such as C, using the version-1 interface, and registered
with <productname>PostgreSQL</productname> as taking no arguments
and returning the type <type>language_handler</type>. This
special pseudotype identifies the function as a call handler and
prevents it from being called directly in SQL commands.
</para>
<para>
The call handler is called in the same way as any other function:
It receives a pointer to a
<structname>FunctionCallInfoData</structname> <type>struct</> containing
argument values and information about the called function, and it
is expected to return a <type>Datum</type> result (and possibly
set the <structfield>isnull</structfield> field of the
<structname>FunctionCallInfoData</structname> structure, if it wishes
to return an SQL null result). The difference between a call
handler and an ordinary callee function is that the
<structfield>flinfo->fn_oid</structfield> field of the
<structname>FunctionCallInfoData</structname> structure will contain
the OID of the actual function to be called, not of the call
handler itself. The call handler must use this field to determine
which function to execute. Also, the passed argument list has
been set up according to the declaration of the target function,
not of the call handler.
</para>
<para>
It's up to the call handler to fetch the entry of the function from the
system table
<classname>pg_proc</classname> and to analyze the argument
and return types of the called function. The <literal>AS</> clause from the
<command>CREATE FUNCTION</command> command for the function will be found
in the <literal>prosrc</literal> column of the
<classname>pg_proc</classname> row. This is commonly source
text in the procedural language, but in theory it could be something else,
such as a path name to a file, or anything else that tells the call handler
what to do in detail.
</para>
<para>
Often, the same function is called many times per SQL statement.
A call handler can avoid repeated lookups of information about the
called function by using the
<structfield>flinfo->fn_extra</structfield> field. This will
initially be <symbol>NULL</>, but can be set by the call handler to point at
information about the called function. On subsequent calls, if
<structfield>flinfo->fn_extra</structfield> is already non-<symbol>NULL</>
then it can be used and the information lookup step skipped. The
call handler must make sure that
<structfield>flinfo->fn_extra</structfield> is made to point at
memory that will live at least until the end of the current query,
since an <structname>FmgrInfo</structname> data structure could be
kept that long. One way to do this is to allocate the extra data
in the memory context specified by
<structfield>flinfo->fn_mcxt</structfield>; such data will
normally have the same lifespan as the
<structname>FmgrInfo</structname> itself. But the handler could
also choose to use a longer-lived memory context so that it can cache
function definition information across queries.
</para>
<para>
When a procedural-language function is invoked as a trigger, no arguments
are passed in the usual way, but the
<structname>FunctionCallInfoData</structname>'s
<structfield>context</structfield> field points at a
<structname>TriggerData</structname> structure, rather than being <symbol>NULL</>
as it is in a plain function call. A language handler should
provide mechanisms for procedural-language functions to get at the trigger
information.
</para>
<para>
This is a template for a procedural-language handler written in C:
<programlisting>
#include "postgres.h"
#include "executor/spi.h"
#include "commands/trigger.h"
#include "fmgr.h"
#include "access/heapam.h"
#include "utils/syscache.h"
#include "catalog/pg_proc.h"
#include "catalog/pg_type.h"
PG_FUNCTION_INFO_V1(plsample_call_handler);
Datum
plsample_call_handler(PG_FUNCTION_ARGS)
{
Datum retval;
if (CALLED_AS_TRIGGER(fcinfo))
{
/*
* Called as a trigger procedure
*/
TriggerData *trigdata = (TriggerData *) fcinfo->context;
retval = ...
}
else
{
/*
* Called as a function
*/
retval = ...
}
return retval;
}
</programlisting>
Only a few thousand lines of code have to be added instead of the
dots to complete the call handler.
</para>
<para>
After having compiled the handler function into a loadable module
(see <xref linkend="dfunc">), the following commands then
register the sample procedural language:
<programlisting>
CREATE FUNCTION plsample_call_handler() RETURNS language_handler
AS '<replaceable>filename</replaceable>'
LANGUAGE C;
CREATE LANGUAGE plsample
HANDLER plsample_call_handler;
</programlisting>
</para>
<para>
The procedural languages included in the standard distribution
are good references when trying to write your own call handler.
Look into the <filename>src/pl</> subdirectory of the source tree.
</para>
</chapter>
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