mysql C connector functions not working when defined within a class

Question

I was trying to implement the functions in mysql c-connector from the class "Server" in my project. Here is how the class constructor looks like: (DATABASE and SOCKET are #defined).

Server::Server(MYSQL& conn)
{
    mysql_init(&conn);
    mysql_real_connect(&conn,"localhost","username","passwd",DATABASE,0,SOCKET,0);
    if (mysql_real_connect(&conn,"localhost","santu","hello",DATABASE,0,SOCKET,0) == NULL) {
        cout << mysql_error(&conn) << endl;
    }
    else
    {
        cout << "Connected." << endl;
    }
}

When I try calling the class from "main.cpp" with the connection handle, it causes an error.

Cannot connect twice. Already connected.

but if the methods were written outside the class, it runs flawlessly. Basically, this works.

#include "Server.hxx"

MYSQL san;

int main(int argc, char** argv)
{
    mysql_init(&san);
    mysql_real_connect(&san,"localhost","santu","hello", "test",0,"/tmp/mysql.sock",0);
    cout << mysql_error(&san) << endl;
    return 0;
}

But this doesn't and fails with aforementioned error.

#include "Server.hxx"

MYSQL san;

int main(int argc, char** argv)
{
    Server S0(san);
    return 0;
}

Nick · Accepted Answer · 2019-04-18 03:24:52Z

1

In your class function definition you are calling mysql_real_connect twice:

mysql_real_connect(&conn,"localhost","username","passwd",DATABASE,0,SOCKET,0);
if (mysql_real_connect(&conn,"localhost","santu","hello",DATABASE,0,SOCKET,0) == NULL) {

Just remove the first line and it will work fine.

answered Apr 18, 2019 at 3:24

Nick

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2 Comments

Vasanth Over a year ago

But why can't it ignore the second connection though ? previously, I couldn't execute mysql_close(&san). it gave me bus error. Could you please explain it ?

Nick Over a year ago

@Vasanth hard to say without looking at the internals of the MySQL API code, but I guess that's just not the way the interface is designed.

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