Squaring numbers with multiple threads - c

Question

I am trying to square the numbers 1 - 10,000 with 8 threads. To clarify, I want the 1st thread to do 1^2, 2nd thread to do 2^2, ..., 8th thread to do 8^2, first thread to do 9^2... etc. The problem I am having is that instead of the above happening, each thread computes the squares of 1-10,000.
My code is below. I have marked sections that I'd rather those answering do not modify. Thanks in advance for any tips!
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <pthread.h>
#define NUMBER_OF_THREADS 8
#define START_NUMBER 1
#define END_NUMBER 10000
FILE *f;
void *sqrtfunc(void *tid) { //function for computing squares
int i;
for (i = START_NUMBER; i<=END_NUMBER; i++){
fprintf(f, "%lu squared = %lu\n", i, i*i);
}
}
int main(){
//Do not modify starting here
struct timeval start_time, end_time;
gettimeofday(&start_time, 0);
long unsigned i;
f = fopen("./squared_numbers.txt", "w");
//Do not modify ending here
pthread_t mythreads[NUMBER_OF_THREADS]; //thread variable
long mystatus;
for (i = 0; i < NUMBER_OF_THREADS; i++){ //loop to create 8 threads
mystatus = pthread_create(&mythreads[i], NULL, sqrtfunc, (void *)i);
if (mystatus != 0){ //check if pthread_create worked
printf("pthread_create failed\n");
exit(-1);
}
}
for (i = 0; i < NUMBER_OF_THREADS; i++){
if(pthread_join(mythreads[i], NULL)){
printf("Thread failed\n");
}
}
exit(1);
//Do not modify starting here
fclose(f);
gettimeofday(&end_time, 0);
float elapsed = (end_time.tv_sec-start_time.tv_sec) * 1000.0f + \
(end_time.tv_usec-start_time.tv_usec) / 1000.0f;
printf("took %0.2f milliseconds\n", elapsed);
//Do not modify ending here
}