MySQL User Defined Functions in C++

Question

I am trying to run MySQL UDF written in C++. It compiles fine and generates the correct output, but it generates a lot of junk after the output. I would like to know the reason for this junk and how I can resolve this problem? I have attached my code and a screenshot of the output.

#include <iostream>
#include <algorithm>
#include <string>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include "mysql-connector-c-6.1.9-linux-glibc2.5-x86_64/include/mysql.h"
#include "mysql-connector-c++-1.1.8-linux-ubuntu16.04-x86-64bit/include/mysql_connection.h"
#include "mysql-connector-c++-1.1.8-linux-ubuntu16.04-x86-64bit/include/cppconn/sqlstring.h"
#include "mysql-connector-c++-1.1.8-linux-ubuntu16.04-x86-64bit/include/cppconn/resultset.h"
#include "mysql-connector-c++-1.1.8-linux-ubuntu16.04-x86-64bit/include/cppconn/datatype.h"
#include "mysql-connector-c++-1.1.8-linux-ubuntu16.04-x86-64bit/include/cppconn/resultset.h"
#include "mysql-connector-c++-1.1.8-linux-ubuntu16.04-x86-64bit/include/cppconn/resultset_metadata.h"
#include "mysql-connector-c++-1.1.8-linux-ubuntu16.04-x86-64bit/include/cppconn/exception.h"

using namespace std;

extern "C" {
    char *hello_world (UDF_INIT *initid, UDF_ARGS *args,char *result, unsigned long length,char *is_null, char *error);
    my_bool hello_world_init (UDF_INIT *initid, UDF_ARGS *args, char *message);
    void hello_world_deinit (UDF_INIT *initid);
    //template <typename T>  T adder (UDF_INIT *initid, UDF_eARGS *args,string result, unsigned long length,string is_null, string error,T v);
}

char *hello_world (UDF_INIT *initid, UDF_ARGS *args, char *result, unsigned long length, char *is_null, char *error)
{
    string res;
    res = args->args[0];
    res.append(" hello");
    char *c = new char[res.size()];
    strcpy(c, res.c_str());
    return c;
}

my_bool hello_world_init (UDF_INIT *initid, UDF_ARGS *args, char *message)
{
    return 0;
    //cout<<"success"<<endl;
}

void hello_world_deinit (UDF_INIT *initid)
{
    return;
}

enter image description here

The array you allocate as result is too small. You need to add one char for the terminating zero: new char[res.size() + 1]. — Marvin Sielenkemper
– Marvin Sielenkemper, Commented Apr 11, 2017 at 9:51
The buffer you allocate for the result will cause a memory leak. You need to manage the memory in the xxx_init and xxx_deinit functions: pre-allocate a buffer to store the result and free it afterwards. — Marvin Sielenkemper
– Marvin Sielenkemper, Commented Apr 11, 2017 at 10:06

Marvin Sielenkemper · Accepted Answer · 2017-04-11 10:07:37Z

4

The signature of the hello_world function is wrong. The fourth parameter should be

unsigned long *length

The value pointed to by that pointer has to be set to the length of the returned string.

char *c = new char[res.size() + 1];
strcpy(c, res.c_str());
*length = res.size();
return c;

edited Apr 11, 2017 at 10:07

answered Apr 11, 2017 at 9:47

Marvin Sielenkemper

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