Thread: mysql_use_result() returns NULL problem

Hello everybody,
I recently moved from PHP to C++ and started to learn MySQL DB connections. When I try to execute a query, mysql_use_result leads to access violation error and I can't figure out why. The code is:

Code:

#include <iostream>
#include <string>
#include <cstdlib>
#include <stdio.h>
#include <time.h>
#include <windows.h>
#include <mysql.h>
#pragma link "libmySQL.lib"

MYSQL * db_connect;
MYSQL_RES *res, *res1;
MYSQL_ROW row;

using namespace std;

char *server = "localhost";
char *user = "root";
char *password = "";
char *database = "f";

char Query[256];

char input_user_login[32];
char* user_login;
char input_user_password[32];
char* user_password;
char* user_id;
char* user_db_password;

int CHAR_STR;
int CHAR_HP;

void choose_char() {

sprintf(Query, "SELECT id, str, hp FROM characters WHERE user_id=%s",user_id);

   mysql_query(db_connect, Query);

   res = mysql_use_result(db_connect);

   while ((row = mysql_fetch_row(res)) != NULL) {
	  cout << row[0] << "      " << row[1] << "     " << row[2] << "\n";
	}

cout << "Choose a character id:\n";
char character[32];
cin >> character;

sprintf(Query, "SELECT str, hp FROM characters WHERE id=%s",character);

if (mysql_query(db_connect, Query)) {
	  cout << "Error getting the info \n";
 }

	res = mysql_use_result(db_connect);
	row = mysql_fetch_row(res);
	CHAR_STR = atoi(row[0]);
	CHAR_HP = atoi(row[1]);

   mysql_free_result(res);
   mysql_close(db_connect);

}
void user_login_screen() {


	start:

	cin >> input_user_login;
	cin >> input_user_password;

	user_login = (char*)input_user_login;
	user_password = (char*)input_user_password;

	sprintf(Query, "SELECT id, password FROM users WHERE login=%s",user_login);


	mysql_query(db_connect, Query);

   res = mysql_use_result(db_connect);

   if ((row = mysql_fetch_row(res)) == NULL) {
	   cout << "This user is not in the DB\n\n";
	   goto start;
   }

   user_id = row[0];
   user_db_password = row[1];

   if (strcmp(user_password, user_db_password) != 0) {
	   cout << "Wrong login or password.\n\n";
	   goto start;
   }

}

int main() {

	db_connect = mysql_init(NULL);

	if (!mysql_real_connect(db_connect, server,
		 user, password, database, 0, 0, 0)) {
	cout << "Could not connect to the DB";
	   }
	else {
		   cout << "Connection succesful!\n";
	}


        user_login_screen();
	cout << "Log in successful\n";
	Sleep(1000);

	choose_char();

return(0);
}

The problem is login goes alright with no problems, but when choose_char() is called, I get access violation because res = mysql_use_result(db_connect); is NULL.

Help please

Sorry, I posted it in the wrong section, chould be C++ :/

Thanks for the tips.

I've been fighting with this for 2 days and the solution was easy as pie (like always). Using mysql_store_result would fix all access violation errors.