php - Getting file name after uploading the file in CI

I want to store file name in a database and file in a folder when I click on the button. File is successfully uploaded but I didn't get the file name. Here is my view :

<?php echo form_open_multipart('home/do_upload');?>
<input type="file" name="userfile" size="20" />
<br /><br />
<input type="submit" value="upload" />
</form>
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Here is my controller :

public function do_upload()
    {

        $this->load->helper(array('form', 'url'));
        $image_path = realpath(APPPATH . '../assets/pics');
        $config['upload_path'] = $image_path; //base_url('assets/pics'); //APPPATH . 'uploads/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size'] = '100';
        $config['max_width']  = '1024';
        $config['max_height']  = '768';
        $this->load->library('upload', $config);
         $upload_data = $this->upload->data(); 
         $file_name =   $upload_data['file_name'];
        if ( ! $this->upload->do_upload())
        {
            echo 'error';
        }
        else
        {
            echo $file_name .' uploaded...';

        }

    }
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