error: undefined reference to `mysql_init'

micha_eleric

trying to understand how to use database, search for database gave mysql, search for mysql example gave https://www.codeguru.com/database/database-programming-with-c-c/

finally got mysql libraries installed.

.pro

QT += core gui
QT += sql

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++11

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0
INCLUDEPATH += /usr/include/mysql/

SOURCES += \
CBankAccount.cpp \
CBankTransaction.cpp \
main.cpp \
CMainWindow.cpp

HEADERS += \
CBankAccount.h \
CBankTransaction.h \
CMainQindow.h

FORMS += \
database.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

CBankTransaction.h

#ifndef CBANKTRANSACTION_H
#define CBANKTRANSACTION_H

#include "CBankAccount.h"
#include <string>

class CBankTransaction
{
private:
   MYSQL* db_conn;

public:
    CBankTransaction(const std::string HOST, const std::string USER,
                     const std::string PASSWORD, const std::string DATABASE);
};

#endif // CBANKTRANSACTION_H

CBankTransaction.cpp

#include "CBankTransaction.h"

CBankTransaction::CBankTransaction(const std::string HOST, const std::string USER,
                                   const std::string PASSWORD,
                                   const std::string DATABASE)
{
    db_conn = mysql_init(NULL);
    db_conn = mysql_real_connect(db_conn, HOST.c_str(), USER.c_str(),
                                 PASSWORD.c_str(), DATABASE.c_str(), 0, NULL, 0);
}

error

error: undefined reference to `mysql_init'
undefined reference to `mysql_real_connect'

what i have tried:
added "QT += sql", no change.
removed "#include <mysql.h>" from "CBankTransaction.h", because
"CBankAccount.h" already had it, no change.

someone said to a similar question, "The error is telling you that you haven't added the MySQL libs to your link-line.", but i have no clue about linker in qt creator. modify pro file, make file is auto generated, linker file? linker options?

from general undefined reference search, function not defined was an option, but mysql is a library, so function should be defined. i can find the .h file, but dont know what file the compiler is linking.

error is in the constructor. am i missing something in my code?
is it possible that library is not properly installed?

jsulm

@micha_eleric said in error: undefined reference to `mysql_init':

unix:LIBS += -L/usr/lib/x86_64-linux-gnu -llibmysqlclient

This is wrong. If you use -l you skip the lib prefix:

unix:LIBS += -L/usr/lib/x86_64-linux-gnu -lmysqlclient

Christian Ehrlicher

You forgot to link against the mysql libs.

micha_eleric

@Christian-Ehrlicher said in error: undefined reference to `mysql_init':

You forgot to link against the mysql libs.

that was my guess, but no clue how to link with qt creator.

Christian Ehrlicher

@micha_eleric you already have a pro file so click on my link I provided and add the necessary line.

micha_eleric

@Christian-Ehrlicher did not notice link was a link.
would i add

unix:LIBS += -L/usr/include/mysql -lmysql

?
and does it matter where in .pro i add it?

jsulm

@micha_eleric said in error: undefined reference to `mysql_init':

unix:LIBS += -L/usr/include/mysql -lmysql

Include folders contain header files not libraries. Libs are in /usr/lib

micha_eleric

@jsulm would i add

unix:LIBS += -L/usr/lib/mysql -lmysql

?
and does it matter where in .pro i add it?

jsulm

@micha_eleric Why don't you simply try?
It does not matter where in the pro file you add it.

micha_eleric

@jsulm :-1: error: cannot find -lmysql: No such file or directory

Christian Ehrlicher

@micha_eleric Then you should pass the correct path where the mysql library resides in.

micha_eleric

@jsulm lol, even tried

unix:LIBS += -L/usr/lib -lmysql

and

unix:LIBS += -L/usr/lib/mysql

of course it did not work, nor did i think it would, but it was a guess out of complete ignorance.

micha_eleric

@Christian-Ehrlicher path has plugin, plugins, and private folders.
i know what the header files are called, no clue what the library files are called, so dont know what to look for.

Christian Ehrlicher

It must be something like libmysql.so

micha_eleric

@Christian-Ehrlicher closest i find is libmysqlclient.so

micha_eleric

@Christian-Ehrlicher said in error: undefined reference to `mysql_init':

It must be something like libmysql.so

when i search install libmysql.so, it give links to install libmysqlclient, which i already installed.

Christian Ehrlicher

@micha_eleric So why don't you try it out?

SGaist

Hi,

When you want to use a library as you do, you need to install the corresponding dev package.

JonB

@micha_eleric
I wonder if you should take a step back and question why you are even trying to do this? You have chosen to pick example code from 2016, and one that has nothing to do with Qt. It uses direct, "low level" libraries to interact with MYSQL. But Qt provides a whole set of "higher level" classes, QSql..., which are integrated with Qt (e.g. can be used for models for Qt's views) and preferable in a host of ways.

Why don't you abandon that example's approach and just use the Qt way? My understanding is that if you are Windows you may have to(?) compile the MySQL libraries, but if you are Linux like I am your distro (e.g. Ubuntu) will offer a pre-compiled set of libraries for MySQL. My system does not have any libmysql.so files, the only stuff is libmysqlclient.... That is the route I would follow.

micha_eleric

@Christian-Ehrlicher said in error: undefined reference to `mysql_init':

@micha_eleric So why don't you try it out?

i did. even different ways.

micha_eleric

@SGaist said in error: undefined reference to `mysql_init':

Hi,

When you want to use a library as you do, you need to install the corresponding dev package.

back tracking installs
[9th] sudo apt install mariadb-client-core-10.6
[8th] sudo apt install mysql-client-core-8.0
[7th] sudo apt-get install pike7.8-mysql
[6th] sudo apt-get install libmariadb-dev
[5th] sudo apt-get install libmariadb-dev-compat
[4th] sudo apt-get install libmariadbclient-dev [i think this is what moved mysql.h to new location, which caused previous .pro to need to be changed]
[3rd] sudo apt-get install libmysqlclient-dev
[2nd] sudo apt-get install mysql-shell
[1st] sudo dpkg-reconfigure libdvd-pkg