problem (session) : Cannot modify header information

[eluser]Unknown[/eluser]
hi
this message appear in top of all pages:

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\jaz\application\controllers\posts.php:1)

Filename: libraries/Session.php

Line Number: 671

the login function:

Code:

function login(){
        $this->load->model('member_model');
        $email = $this->input->post('email');
        $password = $this->input->post('password');
         $user = $this->member_model->user_exist($email,$password);      
        $newdata = array(
                   'name'  => $user->name,
                   'class'=> $user->class,
                   'id' => $user->id,
                   'logged_in' => TRUE
               );

        $this->session->set_userdata($newdata);
        $session_id = $this->session->userdata('id');
        redirect('posts/');      
    }

log out function:

Code:

function logout(){
            $this->session->sess_destroy();
            redirect('posts');
            }

( *notice: I can't log out)

can you please help me to know why the message appear ? and how to solve this problem