title: error: implicit declaration of function ‘sys_kill’
body: <p>I've created a system call to return a tag variable (that I added in the kernel) of a specific process.
  When I make I get <em>error: implicit declaration of function ‘sys_kill’</em>.  Does anyone know what might be causing this or if there are any alternative functions I can use to see if a process is running to return the tag?</p>
  
  <pre><code>#include &lt;linux/kernel.h&gt;
  #include &lt;linux/init.h&gt;
  #include &lt;linux/sched.h&gt;    
  #include &lt;linux/syscalls.h&gt;
  #include &lt;linux/signal.h&gt;
  #include "header.h"
  
  asmlinkage int sys_get_thing(int pid){  //gets the process ID
    struct task_struct *p;                   //pointer to the struct
    p = find_task_by_vpid(pid);       //p = process found by PID
  
    if(sys_kill(p-&gt;pid, 0) == 0){ //syskill sends a 0 signal to all processes matching passed PID - checks if running
      return p-&gt;tag;  //the 0 signal found has the tag we are looking for
    }
    else{
      return -1;          //if nothing is found return -1
    }
  }
  </code></pre>