Multiplication of matrix with threads problem - c++

Question

When I try to start my program it keeps saying "undefined reference to 'pthread_create' and 'pthread_join' . The program is supposed to multiply two matrix A and B and the result should be put in matrix C, the first thread should calculate first 2 rows of matrix C, and the 2nd thread is supposed to calculate the 2nd two rows of matrix C. The program also works fine without the threads. I'm new to threads so I can't figure out where I messed up, hope someone could help me out?
This is my code:
#include <cstdlib>
#include <ctime>
#include <csignal>
#include <pthread.h>
using namespace std;
int MAX_m, MAX_n, MAX_l;
int *matA = new int[MAX_m * MAX_l];
int *matB = new int[MAX_m * MAX_l];
int *matC = new int[MAX_l * MAX_l];
int rd_i = 0;
void* umnoz(void* arg)
{
int i = rd_i++;
for (int j = 0; j < MAX_l; j++)
{
for (int k = 0; k < MAX_m; k++)
{
matC[i * MAX_l + j] += matA[i * MAX_m + k] * matB[k * MAX_l + j];
}
}
}
pthread_t *dretve;
int main(int argc, char *argv[]) {
MAX_l = atoi(argv[1]);
MAX_m = atoi(argv[2]);
MAX_n = atoi(argv[3]);
for (int i = 0; i < MAX_l; i++)
{
for (int j = 0; j < MAX_m; j++)
{
matA[i * MAX_m + j] = rand() % 10;
}
}
for (int i = 0; i < MAX_m; i++)
{
for (int j = 0; j < MAX_l; j++)
{
matB[i * MAX_l + j] = rand() % 10;
}
}
cout << endl;
cout << "Mnozenik: " << endl;
for (int i = 0; i < MAX_l; i++)
{
for (int j = 0; j < MAX_m; j++)
{
cout << matA[i * MAX_m + j] << " ";
cout << endl;
}
}
cout << endl;
cout << "Mnozitelj: " << endl;
for (int i = 0; i < MAX_m; i++)
{
for (int j = 0; j < MAX_l; j++)
{
cout << matB[i * MAX_l + j] << " ";
cout << endl;
}
}
dretve = new pthread_t [MAX_n];
for (int i = 0; i < MAX_n; i++)
{
int *p;
pthread_create(&dretve[i], NULL, umnoz, (void *) (p));
}
for (int i = 0; i < MAX_n; i++)
{
pthread_join(dretve[i], NULL);
}
cout << endl;
cout << "Umnozak: " << endl;
for (int i = 0; i < MAX_l; i++)
{
for (int j = 0; j < MAX_l; j++)
{
cout << matC[i * MAX_l + j] << " ";
cout << endl;
}
}
return 0;
}