Change the character encode in PostgreSQL C language function - c

Question

I am using PostgreSQL 9.5 64bit version on windows server.
The character encoding of the database is set to UTF8.
I'd like to create a function that manipulates multibyte strings.
(e.g. cleansing, replace etc.)
I copied C language logic for manipulating characters from a other system,
The logic assumes that the character code is sjis.
I do not want to change C language logic, so I want to convert from UTF8 to sjis in C language function of Postgresql.
Like the convert_to function. (However, since the convert_to function returns bytea type, I want to obtain it with TEXT type.)
Please tell me how to convert from UTF 8 to sjis in C language.
Create Function Script:
CREATE FUNCTION CLEANSING_STRING(character varying)
RETURNS character varying AS
'$libdir/MyFunc/CLEANSING_STRING.dll', 'CLEANSING_STRING'
LANGUAGE c VOLATILE STRICT;
C Source:
#include <stdio.h>
#include <string.h>
#include <postgres.h>
#include <port.h>
#include <fmgr.h>
#include <stdlib.h>
#include <builtins.h>
#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif
extern PGDLLEXPORT Datum CLEANSING_STRING(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(CLEANSING_STRING);
Datum CLEANSING_STRING(PG_FUNCTION_ARGS)
{
// Get Arg
text *arg1 = (text *)PG_GETARG_TEXT_P(0);
// Text to Char[]
char *arg;
arg = text_to_cstring(arg1);
// UTF8 to Sjis
//Char *sjisChar[] = foo(arg); // something like that..
// Copied from other system.(Assumes that the character code is sjis.)
cleansingString(sjisChar);
replaceStrimg(sjisChar);
// Sjis to UTF8
//arg = bar(sjisChar); // something like that..
//Char[] to Text and Return
PG_RETURN_TEXT_P(cstring_to_text(arg));
}

Answer 1

Succeeded in the way I was taught by question comments.
#include <mb/pg_wchar.h> //Add to include.
...
Datum CLEANSING_STRING(PG_FUNCTION_ARGS)
{
// Get Arg
text *arg1 = (text *)PG_GETARG_TEXT_P(0);
// Text to Char[]
char *arg;
arg = text_to_cstring(arg1);
// UTF8 to Sjis
Char *sjisChar[] = pg_server_to_any(arg, strlen(arg), PG_SJIS);
// Copied from other system.(Assumes that the character code is sjis.)
cleansingString(sjisChar);
replaceStrimg(sjisChar);
// Sjis to UTF8
arg = pg_any_to_server(sjisChar, strlen(sjisChar), PG_SJIS); //It converts from SJIS to server (UTF 8), the third argument sets the encoding of the conversion source.
//Char[] to Text and Return
PG_RETURN_TEXT_P(cstring_to_text(arg));
}