Simple Postgres audit trigger written in C. Tested with PG 10.4

simple_postgres_c_audit_trigger.c

#include "postgres.h"

#include "access/htup_details.h"
#include "catalog/pg_type.h"
#include "commands/trigger.h"
#include "executor/spi.h"
#include "miscadmin.h"
#include "utils/builtins.h"
#include "utils/rel.h"
#include "utils/datetime.h"

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(set_audit_columns_if_empty);

Datum
set_audit_columns_if_empty(PG_FUNCTION_ARGS)
{
	TriggerData *trigdata = (TriggerData *) fcinfo->context;
	Trigger    *trigger;		/* to get trigger name */
	int			nargs;			/* # of arguments */
	bool		newnull = false;		/* null flag */
	char	  **args;			/* arguments */
	char	   *relname;		/* triggered relation name */
	char	   *valbuf;
	Relation	rel;			/* triggered relation */
	HeapTuple	rettuple = NULL;
	TupleDesc	tupdesc;		/* tuple description */
	int			attnum;
	Datum		newval;				/* for new timestamp */



	/* sanity checks from autoinc.c */
	if (!CALLED_AS_TRIGGER(fcinfo))
		/* internal error */
		elog(ERROR, "set_audit_columns_if_empty: not fired by trigger manager");
	if (!TRIGGER_FIRED_FOR_ROW(trigdata->tg_event))
		/* internal error */
		elog(ERROR, "set_audit_columns_if_empty: must be fired for row");
	if (!TRIGGER_FIRED_BEFORE(trigdata->tg_event))
		/* internal error */
		elog(ERROR, "set_audit_columns_if_empty: must be fired before event");

	if (TRIGGER_FIRED_BY_INSERT(trigdata->tg_event))
		rettuple = trigdata->tg_trigtuple;
	else if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
		rettuple = trigdata->tg_newtuple;
	else
		/* internal error */
		elog(ERROR, "set_audit_columns_if_empty: cannot process DELETE events");

	rel = trigdata->tg_relation;

	tupdesc = rel->rd_att;

	// audit field #1 - last_modified_on
	attnum = SPI_fnumber(tupdesc, "last_modified_on");

	if (attnum <= 0)
		ereport(ERROR,
				(errcode(ERRCODE_TRIGGERED_ACTION_EXCEPTION),
				 errmsg("relation \"%d\" has no attribute \"%s\"", rel->rd_id, "last_modified_on")));

	valbuf = (char*)SPI_getvalue(rettuple, tupdesc, attnum);
	if (valbuf == NULL) {
		newval = GetCurrentTimestamp();
		rettuple = heap_modify_tuple_by_cols(rettuple, tupdesc,
											 1, &attnum, &newval, &newnull);
	}
	
	// audit field #2 - last_modified_by
	attnum = SPI_fnumber(tupdesc, "last_modified_by");
	if (attnum <= 0)
		ereport(ERROR,
				(errcode(ERRCODE_TRIGGERED_ACTION_EXCEPTION),
				 errmsg("relation \"%d\" has no attribute \"%s\"", rel->rd_id, "last_modified_by")));

	valbuf = (char*)SPI_getvalue(rettuple, tupdesc, attnum);
	if (valbuf == NULL) {
		newval = CStringGetTextDatum(GetUserNameFromId(GetUserId(), false));
		rettuple = heap_modify_tuple_by_cols(rettuple, tupdesc,
											 1, &attnum, &newval, &newnull);
	}
	
	return PointerGetDatum(rettuple);
}