Simplest C function – return (a + b)

Let's start with a simple function, which takes two integer arguments and returns the sum of these. We first present the source code and then will move on to show you how to compile it, load it into PostgreSQL, and then use it as any native function.

add_func.c

A C source file implementing add(int, int) returns int function looks like the following code snippet:

#include "postgres.h"
#include "fmgr.h"

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(add_ab);

Datum
add_ab(PG_FUNCTION_ARGS)
{
    int32   arg_a = PG_GETARG_INT32(0);
    int32   arg_b = PG_GETARG_INT32(1);

    PG_RETURN_INT32(arg_a + arg_b);
}

Let's go over the code explaining the use of each segment:

• #include "postgres.h": This includes most of the basic definitions and declarations needed for writing any C code for running in PostgreSQL.

• #include "fmgr.h": This includes the definitions for PG_* macros used in this code.

• PG_MODULE_MAGIC;: This is a "magic block" defined in fmgr.h. This block is used by...