...
     sstr << "UPDATE T_Stock SET Stock=" << amount << " WHERE Article_ID=" << artid << " and Storehouse_ID=" << sthid << ends;
      strvar = sstr.str();

      //convert string to char*
      req_book = new char[strvar.size() + 1];
      strcpy(req_book, strvar.c_str());

      // free the stringstream object
      sstr.str("");
      sstr.clear();

      state=mysql_query(mysqlconn, req_book);       // send SQL-statement to the MYSQL-Server

...