What is segmentation fault in C++ mysql? - c++

Question

I am using C++ to send MySQL commands to a server. Why does it say "Segmentation fault" and what does it mean?
The output of the program when I run it is:
connection Succeeded
Segmentation fault
Edited the code to remove the i+1 line but still having same problem
the code:
#include <stdlib.h>
#include <iostream>
#include <mysql.h>
#include <stdio.h>
#define SERVER "localhost"
#define USER "root"
#define PASSWORD "coursework"
int main()
{
MYSQL *connect;
connect=mysql_init(NULL);
if (!connect)
{
std::cout<<"MySQL Initialization failed";
return 1;
}
connect=mysql_real_connect(connect, SERVER, USER, PASSWORD, NULL ,0,NULL,0);
if (connect)
{
std::cout<<"connection Succeeded\n";
}
else
{
std::cout<<"connection failed\n";
exit(1);
}
mysql_query (connect, "drop database if exists TTU;");
mysql_query (connect,"create database TTU;");
mysql_query (connect, "use TTU;");
mysql_query (connect, "create table students(tnumber char(8) PRIMARY KEY,");
mysql_query (connect, "firstname varchar(20) NOT NULL,");
mysql_query (connect, "lastname varchar(20) NOT NULL,");
mysql_query (connect, "dateofbirth date,");
mysql_query (connect, "INDEX 'lastname_i' ('lastname'),");
mysql_query (connect, "ENGINE=INNODB);");
mysql_query (connect, "insert into students(tnumber, firstname, lastname, dateofbirth)");
mysql_query (connect, "values (00001234, Joe, Smith, 1950-08-12);");
MYSQL_RES *res_set;
MYSQL_ROW row;
mysql_query (connect,"select * from students;");
unsigned int i =0;
res_set = mysql_store_result(connect);
unsigned int numrows = mysql_num_rows(res_set);
while ((row= mysql_fetch_row(res_set)) != NULL )
{
std::cout << row[i] << std::endl;
}
mysql_close (connect);
std::cout << "TEST" << std::endl;
return 0;
}

Answer 1

There are multiple bugs in the shown code. There are two major bugs: Incorrect usage of mysql_query(), and repeated failures to check the return value from various MySQL library functions, resulting in a failure to detect various errors.
mysql_query (connect, "create table students(tnumber char(8) PRIMARY KEY,");
This is not valid SQL. mysql_query() takes a complete SQL statement and executes it. It doesn't take individual fragments of an SQL statement, broken down across multiple consecutive invocations, and only executes it once a complete SQL statement gets parsed.
Because the shown code fails to check the return value from mysql_query(), it fails to detect that this whole sequence of mysql_query() calls have failed, and the table did not get created.
Subsequent call also fails to check the return value from other MySQL library functions.
The shown code is likely getting a NULL back from mysql_store_result(), fails to check for it, passes a NULL pointer to mysql_num_rows(), and crashes.