title: Getiing Unknown symbol ***_******(err 0) while reading or writing from linux kernel module
body: <p>I know that reading or writing(i/o) from kernel is a bad thing but under some circumstances I have to do that and while reading many sites I got the code from <a href="http://www.linuxjournal.com/node/8110/print" rel="nofollow noreferrer">things you should never do in kernel</a> so thought of trying it. While making module.while compiling it was giving me </p>
  
  <blockquote>
    <p>WARNING: "sys_write" [/path/to/file/file.ko] undefined!
    WARNING:"sys_open" [/path/to/file/file.ko] undefined!</p>
  </blockquote>
  
  <p>but I thought of inserting the module while inserting the module I was getting </p>
  
  <blockquote>
    <p>insmod: ERROR: could not insert module file.ko: Unknown symbol in
    module</p>
  </blockquote>
  
  <p>To get an answer for that I crawled through some sites(<a href="https://stackoverflow.com/questions/29578931/unknown-symbol-class-create-err-0">c-unkown symbol class create</a>) but I had </p>
  
  <blockquote>
    <p>MODULE_LICENSE("GPL");</p>
  </blockquote>
  
  <p>in my module code and mean while searching I got the way to
  make available the unknown symbols while making the modules. which is from
  <strong>Documenats/kbuild/modules.txt.</strong></p>
  
  <p>Then I made  </p>
  
  <blockquote>
    <p>make KBUILD_EXTRA_SYMBOLS='/home/lunako/Documents/ker_write/Module.symvers'</p>
  </blockquote>
  
  <p>still no result. What I should do now ? Thanks</p>