C threads

I was trying to create threads in my code. This is the first time I tried this I used these as reference
http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_create.html
and
http://www.mit.edu:8001/people/proven/IAP_2000/basic_example.html

but I always get a error message saying

engine_driver.o: In function `e_admit_to_engine':/home/shahan/engine_driver.c:39: undefined reference to `pthread_create'
:/home/shahan/engine_driver.c:43: undefined reference to `pthread_detach'
collect2: ld returned 1 exit status

it doesnt recognize the functins

can any one help me on this???

I enterd the code here (only the part I use threads. I return NULL in engine_ex_comm_write )

Code:

 
#include <pthread.h>
#include "engine.h"

void * engine_ex_comm_read(profile_t *data);
void * engine_ex_comm_write(profile_t *data);
int e_admit_engine();


int e_admit_to_engine()
{

   profile_t *this_data;
   //extern void * start_routine(void *ths_data);
   //char * this_data1;
   profile_t *new_profile;
   element_t *new_element, *new_element1;
   profile_t * loc;
   int y=0, x, ret;
   pthread_t tid;
   pthread_attr_t attr;

   while(1)
   {
      if(y!=is_buffer_emty())
      {
          this_data= read_from_buffer();

          if(this_data)
          {
             engine_ex_comm_write(this_data);
             if( pthread_create(&tid, NULL, engine_ex_comm_write, this_data))
             {
                printf("error creating thread\n");
             }
             pthread_detach(tid);
          }