Creating a very simple PostgreSQL Extension in C

gistfile1.txt

sudo apt-get update
sudo apt install postgresql-server-dev-12

C File
=========
#include "postgres.h"
#include <string.h>
#include "fmgr.h"


#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

/* by value */

PG_FUNCTION_INFO_V1(add_one);

Datum
add_one(PG_FUNCTION_ARGS)
{
    int32   arg = PG_GETARG_INT32(0);

    PG_RETURN_INT32(arg + 1);
}


Makefile
==========

MODULES = funcs

PG_CONFIG = pg_config
PGXS = $(shell $(PG_CONFIG) --pgxs)
INCLUDEDIR = $(shell $(PG_CONFIG) --includedir-server)
include $(PGXS)

funcs.so: funcs.o
	cc  -shared -o funcs.so funcs.o

funcs.o: funcs.c
	cc -o funcs.o -c funcs.c $(CFLAGS) -I$(INCLUDEDIR)
  
Build
==============

sudo make && sudo make install

SQL
================

drop function if exists add_one(integer);

CREATE or replace FUNCTION add_one(integer) RETURNS integer
     AS '/home/ranjan/funcs.so', 'add_one'
     LANGUAGE C STRICT;