Add documentation about calling version-1 C functions from C.

This topic wasn't really covered before, so fill in some details.

Author: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/90853055-5BBD-493D-91E5-721677C7C59B@gmail.com

2 files changed, 39 insertions, 5 deletions

diff --git a/src/tutorial/funcs.c b/src/tutorial/funcs.c
index f597777a1ff..4a61177567c 100644
--- a/src/tutorial/funcs.c
+++ b/src/tutorial/funcs.c
@@ -11,6 +11,7 @@
 #include "postgres.h"			/* general Postgres declarations */
 
 #include "executor/executor.h"	/* for GetAttributeByName() */
+#include "utils/fmgrprotos.h"	/* for text_starts_with() */
 #include "utils/geo_decls.h"	/* for point type */
 
 PG_MODULE_MAGIC;
@@ -102,6 +103,25 @@ concat_text(PG_FUNCTION_ARGS)
 	PG_RETURN_TEXT_P(new_text);
 }
 
+/* A wrapper around starts_with(text, text) */
+
+PG_FUNCTION_INFO_V1(t_starts_with);
+
+Datum
+t_starts_with(PG_FUNCTION_ARGS)
+{
+	text	   *t1 = PG_GETARG_TEXT_PP(0);
+	text	   *t2 = PG_GETARG_TEXT_PP(1);
+	Oid			collid = PG_GET_COLLATION();
+	bool		result;
+
+	result = DatumGetBool(DirectFunctionCall2Coll(text_starts_with,
+												  collid,
+												  PointerGetDatum(t1),
+												  PointerGetDatum(t2)));
+	PG_RETURN_BOOL(result);
+}
+
 /* Composite types */
 
 PG_FUNCTION_INFO_V1(c_overpaid);
diff --git a/src/tutorial/funcs.source b/src/tutorial/funcs.source
index 542b5c81ec9..eb561537542 100644
--- a/src/tutorial/funcs.source
+++ b/src/tutorial/funcs.source
@@ -123,16 +123,25 @@ SELECT * FROM EMP;
 -----------------------------
 
 CREATE FUNCTION add_one(integer) RETURNS integer
-   AS '_OBJWD_/funcs' LANGUAGE C;
+   AS '_OBJWD_/funcs' LANGUAGE C STRICT;
+
+CREATE FUNCTION add_one(double precision) RETURNS double precision
+   AS '_OBJWD_/funcs' LANGUAGE C STRICT;
 
 CREATE FUNCTION makepoint(point, point) RETURNS point
-   AS '_OBJWD_/funcs' LANGUAGE C;
+   AS '_OBJWD_/funcs' LANGUAGE C STRICT;
 
 CREATE FUNCTION copytext(text) RETURNS text
-   AS '_OBJWD_/funcs' LANGUAGE C;
+   AS '_OBJWD_/funcs' LANGUAGE C STRICT;
+
+CREATE FUNCTION concat_text(text, text) RETURNS text
+   AS '_OBJWD_/funcs' LANGUAGE C STRICT;
+
+CREATE FUNCTION t_starts_with(text, text) RETURNS boolean
+   AS '_OBJWD_/funcs' LANGUAGE C STRICT;
 
 CREATE FUNCTION c_overpaid(EMP, integer) RETURNS boolean
-   AS '_OBJWD_/funcs' LANGUAGE C;
+   AS '_OBJWD_/funcs' LANGUAGE C STRICT;
 
 SELECT add_one(3) AS four;
 
@@ -140,6 +149,8 @@ SELECT makepoint('(1,2)'::point, '(3,4)'::point ) AS newpoint;
 
 SELECT copytext('hello world!');
 
+SELECT t_starts_with('foobar', 'foo');
+
 SELECT name, c_overpaid(EMP, 1500) AS overpaid
 FROM EMP
 WHERE name = 'Bill' or name = 'Sam';
@@ -147,10 +158,13 @@ WHERE name = 'Bill' or name = 'Sam';
 -- remove functions that were created in this file
 
 DROP FUNCTION c_overpaid(EMP, integer);
+DROP FUNCTION t_starts_with(text, text);
+DROP FUNCTION concat_text(text, text);
 DROP FUNCTION copytext(text);
 DROP FUNCTION makepoint(point, point);
+DROP FUNCTION add_one(double precision);
 DROP FUNCTION add_one(integer);
---DROP FUNCTION clean_EMP();
+DROP FUNCTION clean_EMP();
 DROP FUNCTION high_pay();
 DROP FUNCTION new_emp();
 DROP FUNCTION add_em(integer, integer);