C++ Class Pointer Deletion (Segmentation Fault Issue)

Question

SOLVED

Well, it seems that I'm receiving a segmentation fault when I try deleting my class pointer. I've tried many things, but yet haven't got it to work. Anyways, here is my main.cpp: (this segmentation fault is occurring when I delete 'm')

    /*
 * Copyright(c)Fellixombc 2010
 *
 * TextToSqlRs is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * TextToSqlRs is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with TextToSqlRs.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <ctime>
#include <iostream>
#include <unistd.h>
#include <string>
#include "eMysql.h"
#include "manage.h"

using namespace std;

int main() {
    time_t start, stop;
    double TIME;
    string host, user, password, database;
    string path;
    bool run = true;
    int runtime;
    int MAX;
    int max;


    cout << "Please enter the required information." << endl << "Absolute path to character folder: ";
    cin >> path;
    cout << "MySql Database Host: ";
    cin >> host;
    cout << "MySql Username: ";
    cin >> user;
    cout << "MySql Password: ";
    cin >> password;
    cout << "Database name: ";
    cin >> database;

    start = clock();
    eMysql* m = new eMysql;
    Manage* manage = new Manage;
    m->connect(host.c_str(), user.c_str(), password.c_str(), database.c_str());
    manage->ReadDir(path);
    manage->TextToSql(m);

    delete manage;
    delete m;

    stop = clock();

    TIME = ((double)stop - (double)start)/CLOCKS_PER_SEC;
    cout << "Finished in " << TIME << " seconds" << endl; }

eMysql deconstructor:

eMysql::~eMysql() {
mysql_free_result(result);
mysql_close(&mysql);

if(lengths) delete lengths;
if(result) delete result; }

mysql/result header:

#ifndef _MYSQL_H
#define _MYSQL_H

#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <fstream>
#include <mysql/mysql.h>
#include <string>

using namespace std;

class eMysql {
public:
    eMysql();
    int connect(const string, const string, const string, const string);
    int query(string query);
    int strip(string&);
    int rows();
    virtual ~eMysql();
private:
    MYSQL mysql;
    MYSQL_RES *result;
    MYSQL_ROW row;
    unsigned int i;
};

#endif  /* _MYSQL_H */

mysql initializer

eMysql::eMysql() {
    mysql_init(&mysql);
}

result initializer

int eMysql::query(string query) {
    if(mysql_query(&mysql, query.c_str())) {
        cout << "Failed to run query: " << mysql_error(&mysql) << endl;
        exit(1);
    }
    result = mysql_store_result(&mysql);
    query.erase();
    return 1;
} 

Answer 1

Since you never call query, you're operating on an invalid result pointer. In the eMysql constructor, initialize result to NULL:

eMysql::eMysql() : result(NULL) {
  // ...
}

Then, in the destructor:

eMysql::~eMysql() {
  if (result) mysql_free_result(result);
  mysql_close(&mysql);
}

One thing looks wrong in the destructor:

eMysql::~eMysql() {
  // You use "result" here, but later check if it's not NULL.
  // You should only free if it's not null.
  mysql_free_result(result);
  mysql_close(&mysql);

  if(lengths) delete lengths;
  // Are you sure you need to delete this?  Isn't that what the
  // mysql_free_result is for?  This will still be a non-NULL ptr
  // that has already been free'd.  This might be a double-delete
  // which would likely manifest as a segmentation fault.
  if(result) delete result;
}

But there could be other problems.

• Does TextToSQL take ownership of m?
• Are result and mysql valid in the ~eMysql?
• Is lengths allocated as an array? Delete with delete [] lengths;
• I assume these are class members, are they initialized at all?

Here are a couple methods for debugging these types of problems (on Linux):

• Run valgrind to check for invalid memory references.
• Run gdb, type bt when it crashes.

Answer 2

I don't see any code that initializes lengths or result, so you might be deleting wild pointers.