Kernel Module Functions Undefined

Basically, for my OS class, I'm writing a kernel module that simulates an airport shuttle with 5 terminals. So far, I've been able to implement system calls and verify that they work properly. However, I'm trying to link these system calls into a module. For each kernel Stub and function when I try to make the module for insertion, it says it's undefined. Also, as a by product, the module won't insert when I run this command: make; sudo insmod terminal.ko due to an unknown symbol in the module, which is an error I've found little or none documentation on.

Here's the output when I try to make and insert it:

make -C /lib/modules/`uname -r`/build/ M=`pwd` modules
make[1]: Entering directory `/home/taylor/OPSYS_P2/linux-3.16.4'
  Building modules, stage 2.
  MODPOST 1 modules
WARNING: "STUB_issue_request" [/home/taylor/OPSYS_P2/linux-3.16.4/opsyscall/module/terminal.ko] undefined!
WARNING: "issue_request" [/home/taylor/OPSYS_P2/linux-3.16.4/opsyscall/module/terminal.ko] undefined!
WARNING: "STUB_stop_shuttle" [/home/taylor/OPSYS_P2/linux-3.16.4/opsyscall/module/terminal.ko] undefined!
WARNING: "stop_shuttle" [/home/taylor/OPSYS_P2/linux-3.16.4/opsyscall/module/terminal.ko] undefined!
WARNING: "STUB_start_shuttle" [/home/taylor/OPSYS_P2/linux-3.16.4/opsyscall/module/terminal.ko] undefined!
WARNING: "start_shuttle" [/home/taylor/OPSYS_P2/linux-3.16.4/opsyscall/module/terminal.ko] undefined!
make[1]: Leaving directory `/home/taylor/OPSYS_P2/linux-3.16.4'
insmod: ERROR: could not insert module terminal.ko: Unknown symbol in module

Primary Module Code:

#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/time.h> // Time Lib

#include "./syscall.h"

MODULE_LICENSE("GPL");

// Stubs for kernel module implementation of shuttle services
extern int ( * STUB_start_shuttle )( void );
extern int ( * STUB_stop_shuttle )( void );
extern int ( * STUB_issue_request )( char passenger_type, int initial_terminal, int destination_terminal );

static int terminal_show(struct seq_file *m, void *v) {

  seq_printf(m,"%s\n", "The Shuttle is out of service");

  return 0;
}

static int terminal_open(struct inode *inode, struct  file *file) {
  return single_open(file, terminal_show, NULL);
}

static const struct file_operations terminal_fops = {
  .owner = THIS_MODULE,
  .open = terminal_open,
  .read = seq_read,
  .release = single_release,
};

/**
 * Terminal Init
 *      Sets up a proc file, sets syscall stubs correctly
 */
static int __init terminal_init(void) {
    proc_create("terminal", 0, NULL, &terminal_fops);

    // Redirect stub syscalls to our implementation
    STUB_start_shuttle = &start_shuttle;
    STUB_stop_shuttle = &stop_shuttle;
    STUB_issue_request = &issue_request;
  return 0;
}

/**
 * Terminal Exit
 *      Removes the proc file, tears down stubs
 */
static void __exit terminal_exit(void) {
    STUB_start_shuttle = NULL;
    STUB_stop_shuttle = NULL;
    STUB_issue_request = NULL;
  remove_proc_entry("terminal", NULL);
}

module_init(terminal_init);
module_exit(terminal_exit);

Here's the syscall.h file:

#pragma once

#include <linux/err.h>
#include <linux/kthread.h>
#include <linux/list.h>
#include <linux/sched.h>
#include <linux/time.h>

// Prototype Declarations
int start_shuttle( void );
int stop_shuttle( void );
int issue_request( char passenger_type, int initial_terminal, int destination_terminal );

Here's the corresponding c file, syscall.c:

#include "syscalls.h"
/**
 * Start Shuttle - Syscall
 * Description: Starts the shuttle up in its own thread unless it is already running
 */
int start_shuttle( void ) {
    int result = 0;
    return result;
}

/**
 * Stop Shuttle - Syscall
 * Description: Tells the shuttle to stop unless it is already stopping
 */
int stop_shuttle( void ) {
    int result = 0;
    return result;
}

/**
 * Issue Request - Syscall
 * Description: Adds a passenger, if valid, into a chosen terminal with a desired
 *              destination
 */
int issue_request( char passenger_type, int initial_terminal, int destination_terminal ) {
    printk( KERN_DEBUG "Shuttle Service: Invalid issue_request - passenger_type %c initial_terminal %d destination_terminal %d\n",
        passenger_type, initial_terminal, destination_terminal );
    return 1;
}

Any and all help/advice would be awesome!

Running Ubuntu 14.04, kernel version 3.16.4