Squaring numbers w/ multiple threads - c

Question

I'm trying to write a program that squares numbers 1-10,000 by creating 8 threads and each thread will take turns squaring ONE NUMBER EACH. Meaning that one thread will square 1, another will square 2, etc until all threads square a number. Then one thread will square 9, etc, all the way to 10,000. My code is below:
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <pthread.h>
#include <sys/types.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++){
if ((i % NUMBER_OF_THREADS) == pthread_self()){ //if i%8 == thread id
fprintf(f, "%lu squared = %lu\n", i, i*i); //then that thread should do this
}
}
}
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
}
I am not sure where my error is. I create my 8 threads in main, and then depending on their thread id (tid), I want that thread to square a number. As of right now, nothing is being printed into the output file and I can't figure out why. Is my tid comparison not doing anything? Any tips are appreciated. Thanks guys.

Answer 1

First, you intentionally pass a parameter to each thread so it know which thread it is (from 0 to 7) That is good, but you then don't use it anymore inside the thread (this leads to one of the possible confussions you have)
Second, as you say in the explanation of how the algorithm should go, you say each thread must square a different set of numbers, but all of them do square the same set of numbers (indeed the whole set of numbers)
You have two approaches to this: Let each thread square the number, and go for the next, eight places further (so the algorithm is the one described in your explanation) or you give different sets (each 1250 consecutive numbers) and let each thread act on is own separate interval.
That said, you have to reconstruct your for loop to do one of two:
for (i = parameter; i < MAX; i += 8) ...
or
for (i = 1250*parameter; i < 1250*(parameter+1); i++) ...
that way, you'll get each thread run with a different set of input numbers.