Codeigniter验证无效

I am new in codeigniter .I have done validation for my project .But it is not working fine .I have written my all code here .First is view page and second is my controller page.Please help anyone

<?php $this->load->helper('form');  
  echo validation_errors();  
  echo form_open('SM_in_controller/sm_login_action');
?>

  <input class="login_input" type="text" placeholder="Username" name="username" id="username"/>
  <input type="submit" value="Login" class="login_button" id="login_button"/>
</form>  

and my controller

public function sm_login_action() {
    $this->load->helper(array('form', 'url'));
    $this->load->library('form_validation'); 
    $this->form_validation->set_rules('username', 'Username', 'required');
}

In your controller, when you call the "run" method is when the proccess is done:

public function sm_login_action() {
    $this->load->helper(array('form', 'url'));
    $this->load->library('form_validation'); 
    $this->form_validation->set_rules('username', 'Username', 'required');

   if( $this->form_validation->run() ) { // Return TRUE on success
        // Success
   } else {
      // Failure
  }
}

<?php

class Form extends CI_Controller {

    public function index()
    {
        $this->load->helper(array('form', 'url'));

        $this->load->library('form_validation');

        $this->form_validation->set_rules('username', 'Username', 'callback_username_check');
        $this->form_validation->set_rules('password', 'Password', 'required');
        $this->form_validation->set_rules('passconf', 'Password Confirmation', 'required');
        $this->form_validation->set_rules('email', 'Email', 'required|is_unique[users.email]');

        if ($this->form_validation->run() == FALSE)
        {
            $this->load->view('myform');
        }
        else
        {
            $this->load->view('formsuccess');
        }
    }

    public function username_check($str)
    {
        if ($str == 'test')
        {
            $this->form_validation->set_message('username_check', 'The %s field can not be the word "test"');
            return FALSE;
        }
        else
        {
            return TRUE;
        }
    }

}
?>

try this for more details. https://www.codeigniter.com/user_guide/libraries/form_validation.html

Try this

<input class="login_input" type="text" placeholder="Username" name="username" id="username" required/>

Hope this helps