undefined reference pthread_create

I am trying to make sense of a program:

#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <pthread.h>

int main(int argc, char** argv) {

volatile double fShared = 0.0;
// create pthread structures - one for each thread
pthread_t t0;
pthread_t t1;
//Arbitrary pointer variables - void* is a pointer to anything
void *thread_res0;
void *thread_res1;
int res0,res1;
//create a new thread AND run each function
// we pass the ADDRESS of the thread structure as the first argument 
// we pass a function name as the third argument - this is known as a FUNCTION pointer 
// you might want to look at the man pages for pthread_create 
 res0 = pthread_create(&t0, NULL, do_this, (void*)"");
 res1 = pthread_create(&t1, NULL, do_that, (void*)"");
// Two threads are now running independently of each other and this main function
// the function main is said to run in the main thread, so we are actually running 

// The main code can now continue while the threads run - so we can simply block
 res0 = pthread_join(t0, &thread_res0);
 res1 = pthread_join(t1, &thread_res1);

 printf ("\n\nFinal result is fShared = %f\n",fShared);
 return (EXIT_SUCCESS);
} 

It should be noted that do_this and do_that are functions created earlier in the program. I am getting the following errors:

seed@ubuntu:~/Programs$ Homework2OS.c
/tmp/cceiRtg8.O: in function 'main'
Undefined reference to 'pthread_create' 
Undefined reference to 'pthread_create'
Undefined reference to 'pthread_join' 
Undefined reference to 'pthread_join' 

We were given this bit of code to fix. I have found elsewhere the format for the pthread_create constructor. Do I need to actually define it above the main? I was under the impression it was imported with the library.

I would also venture to guess that it has something to do with the fourth parameter? I understand that the first parameter is the location of the thread (defined above), the second is NULLed, the third is the function call but I do not quite understand what the fourth parameter is supposed to be.

What's wrong?