undefined reference to pthread_create/segmentation fault - multithreading

Question

i have recently started using threads ,so pretty new to this stuff. please no hate .i was working on a program which would calculate prime numbers till a limit N provided through the command line.
i would distribute this N to num_threads i.e. 4 .lets say the number is 40 and num_threads=4 then each thread should get 10 indexes.i created a structure containing the upper and lower limits for each thread.i passed a pointer of this structure and used the "high" and "low" attributes of this structure to control the flow.
there seems to be some kind of problem.if anyone of you can point out the mistake or tell me if this is the right thing to do i would be pretty grateful :)
geany gives the the following errors:
g++ -Wall -o "prime5000" "prime5000.cpp" (in directory: /home/jarrar/operating systems/threads)
/tmp/ccAHGsIw.o: In function `main':
prime5000.cpp:(.text+0xc9): undefined reference to `pthread_create'
prime5000.cpp:(.text+0xf6): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
Compilation failed.
if executed from the terminal using the following command:
g++ prime5000.cpp -lpthread -o 5prime
./5prime
it will give :
Segmentation fault(Core dump)
#include<stdio.h>
#include<stdlib.h>
#include<pthread.h>
#define num_thread 4
void* calc(void* param);
struct fml
{
long low;
long high;
};
fml* x;
int main(int argc,char* argv[])
{
pthread_t threadArray[num_thread];
int thread;
for(thread=0;thread<num_thread;thread++)
{
x->low=thread*atoi(argv[1])/num_thread;
x->high=x->low+(thread+1)*atoi(argv[1])/num_thread;
pthread_create(&threadArray[thread],NULL,calc,&x);
}
int i;
for(i=0;i<num_thread;i++)
{
pthread_join(threadArray[i],NULL);
}
return 0;
}
void* calc(void* param)
{
fml* temp=(fml*)param;
long lowerLimit=temp->low;
long upperLimit=temp->high;
int i;
int j;
for(i=lowerLimit;i<upperLimit;i++)
{
for(j=2;j<i;j++)
{ if(i!=1||i!=0)
{
if(!(i%j==0))
{
printf("%d ",i);
}
}
}
}
pthread_exit(NULL);
}