#include "postgres.h"
#include "fmgr.h"
#include <winsock.h>
#include "string.h"
#include "utils/array.h" // подключаем заголовочный файл для работы с массивами
#include "catalog/pg_type.h" // INT4OID
PG_MODULE_MAGIC;
Datum array_add(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(array_add);
Datum array_add(PG_FUNCTION_ARGS)
{
ArrayType *array1;
text *res;
array1 = PG_GETARG_ARRAYTYPE_P(0);
res=(text*)"тест";
PG_RETURN_TEXT_P(res);
}

SET d1=C:/PROGRA~1/POSTGR~1/10/include
C:\mingw-w64\mingw64\bin\gcc -I%d1%/server/port/win32 -I%d1%/server -I%d1% -fpic -c vlads.c -DHAVE_LONG_INT_64
C:\mingw-w64\mingw64\bin\gcc -shared -LC:/PROGRA~1/POSTGR~1/10/lib -o vlads.so vlads.o -l postgres

CREATE OR REPLACE FUNCTION public.array_add ( t text [] )
RETURNS text AS
'vlads.so', 'array_add'
LANGUAGE 'c'
VOLATILE CALLED ON NULL INPUT SECURITY INVOKER PARALLEL UNSAFE COST 1;

SELECT array_add(array['1','2']::text[]);

server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

#include "postgres.h"
#include <string.h>
#include "fmgr.h"
#include "utils/geo_decls.h"

PG_MODULE_MAGIC;

/* 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);
}

/* by reference, fixed length */

PG_FUNCTION_INFO_V1(add_one_float8);

Datum
add_one_float8(PG_FUNCTION_ARGS)
{
/* The macros for FLOAT8 hide its pass-by-reference nature. */
float8 arg = PG_GETARG_FLOAT8(0);

PG_RETURN_FLOAT8(arg + 1.0);
}

PG_FUNCTION_INFO_V1(makepoint);

Datum
makepoint(PG_FUNCTION_ARGS)
{
/* Here, the pass-by-reference nature of Point is not hidden. */
Point *pointx = PG_GETARG_POINT_P(0);
Point *pointy = PG_GETARG_POINT_P(1);
Point *new_point = (Point *) palloc(sizeof(Point));

new_point->x = pointx->x;
new_point->y = pointy->y;

PG_RETURN_POINT_P(new_point);
}

/* by reference, variable length */

PG_FUNCTION_INFO_V1(copytext);

Datum
copytext(PG_FUNCTION_ARGS)
{
text *t = PG_GETARG_TEXT_PP(0);

/*
* VARSIZE_ANY_EXHDR is the size of the struct in bytes, minus the
* VARHDRSZ or VARHDRSZ_SHORT of its header. Construct the copy with a
* full-length header.
*/
text *new_t = (text *) palloc(VARSIZE_ANY_EXHDR(t) + VARHDRSZ);
SET_VARSIZE(new_t, VARSIZE_ANY_EXHDR(t) + VARHDRSZ);

/*
* VARDATA is a pointer to the data region of the new struct. The source
* could be a short datum, so retrieve its data through VARDATA_ANY.
*/
memcpy((void *) VARDATA(new_t), /* destination */
(void *) VARDATA_ANY(t), /* source */
VARSIZE_ANY_EXHDR(t)); /* how many bytes */
PG_RETURN_TEXT_P(new_t);
}

PG_FUNCTION_INFO_V1(concat_text);

Datum
concat_text(PG_FUNCTION_ARGS)
{
text *arg1 = PG_GETARG_TEXT_PP(0);
text *arg2 = PG_GETARG_TEXT_PP(1);
int32 arg1_size = VARSIZE_ANY_EXHDR(arg1);
int32 arg2_size = VARSIZE_ANY_EXHDR(arg2);
int32 new_text_size = arg1_size + arg2_size + VARHDRSZ;
text *new_text = (text *) palloc(new_text_size);

SET_VARSIZE(new_text, new_text_size);
memcpy(VARDATA(new_text), VARDATA_ANY(arg1), arg1_size);
memcpy(VARDATA(new_text) + arg1_size, VARDATA_ANY(arg2), arg2_size);
PG_RETURN_TEXT_P(new_text);
}