db2 external function ends with SQL0444N, Reason code 6, SQLSTATE 42724 - db2

Question

I'll develop new (external) functions for DB2. My first test:
db2crypt.h
#ifndef DB2CRYPT_H
#define DB2CRYPT_H
#include <string.h>
#include <stdlib.h>
char *encryptAes(const char *source, const char *key);
#endif /* DB2CRYPT_H */
db2crypt.cpp
#include "db2crypt.h"
#include <string>
char *encryptAes(const char *source, const char *key) {
std::string test("abc");
return (char *)test.c_str();
}
is compiled without an error.
g++ -fPIC -c db2crypt.cpp -std=c++14
g++ -shared -o db2crypt db2crypt.o -L$DB2PATH -ldb2
I also copied the new file into $DB2PATH/function and made a softlink in $DB2PATH/function/unfenced.
Then I created the function with
create function aes(VARCHAR(4096), VARCHAR(4096))
SPECIFIC encryptAes
RETURNS VARCHAR(4069)
NOT FENCED
DETERMINISTIC
NO SQL
NO EXTERNAL ACTION
LANGUAGE C
RETURNS NULL ON NULL
INPUT PARAMETER STYLE SQL
EXTERNAL NAME "db2crypt!encryptAes"
which was also ok.
But when I do select db2inst1.aes('a', 'b') from SYSIBM.SYSDUMMY1
I get the error
SQL0444N Die Routine "DB2INST1.AES" (spezifischer Name "ENCRYPTAES") ist
durch Code in Bibliothek oder Pfad ".../sqllib/function/db2crypt", Funktion
"encryptAes" implementiert, auf die kein Zugriff möglich ist. Ursachencode:
"6". SQLSTATE=42724
(sorry, I don't know how to change the error output into english)
What I made wrong?

Answer 1

Ok, I got the answer.
Thank you at #mao, you did help me. But I also needed some other help. If someone searches for an answer:
First you have to compile with some important parameters:
g++ -m64 -fPIC -c <yourfile>.cpp -std=c++14 -I/opt/ibm/db2/V11.1/include/ -D_REENTRANT
g++ -m64 -shared -o <yourfile> <yourfile>.o -L$DB2PATH -ldb2 -Wl,-rpath,$DB2PATH/$LIB -lpthread
Second: The function declaration, you also have to add parameters for null values AND the return value can't be a function return, it has to be a parameter. Also you have to use the types which are defined in sqludf.h:
void SQL_API_FN encryptAes(SQLUDF_CHAR *source,
SQLUDF_CHAR *key,
SQLUDF_CHAR out[4096],
SQLUDF_SMALLINT *sourcenull,
SQLUDF_SMALLINT *keynull,
SQLUDF_SMALLINT *outnull,
SQLUDF_TRAIL_ARGS) {
...
}
Also, when you do C++ instead of C, you have to tell the script that it has to handle the function as C:
#ifdef __cplusplus
extern "C"
#endif
void SQL_API_FN ...