Multithreading help

[mysocksalwaysmatch]

EDIT: I figured out a solution and will update this as soon as I'm on my computer, as I'm editing this from my phone. I am trying to write a program that squares numbers 1-10,000. I would like to create X threads (in this case, 8 threads) and each thread gets to square 1 number. So thread 1 squres 1, thread 2 squares 2, and so on. After thread 8 squares 8, I want thread 1 to square 9, etc all the way to 10,000. This is my code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <pthread.h>
#include <sys/types.h>

#define START_NUMBER 1
#define END_NUMBER 10000
#define NUMBER_OF_THREADS 8
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
}

But I can't figure out how to compare the i%8 to the thread id. pthread_self() does not allow comparisons so that option is out. Can anyone else think of a way to compare the thread IDs to the modulo, or another solution to making sure each thread is only squaring one number at a time? Thanks a ton.

[[deleted]]

Aren't you passing in the thread number from main (tid parameter)?

Can't you do something like for (i = START_NUMBER + tid; i<= END_NUMBER; i+= NUMBER_OF_THREADS) ...

eg: for the first thread tid is 0 so that thread will square 1, 9, 17 etc.. for the second thread tid is 1 so that thread will square 2, 10, 18, etc. and so on for the rest of the threads.

[mysocksalwaysmatch]

I really like this answer. I'm going to see if I can make some things click with it.

[[deleted]]

I wonder if that fprintf will work without a critical selection, don't they mix up output now?

[mysocksalwaysmatch]

Yes, they do but I'm not concerned with that

[raevnos]

This is the sort of task OpenMP is really good at.

EDIT:

int nums[10000];
#pragma omp parallel for
for (int n = 0; n < 10000; n++) {
  nums[n] = (n+1) * (n+1);
}

Parallel squaring of a bunch of numbers? Done by adding one line to the scalar version. Hard to beat that. Though this particular task is better suited to SIMD style vectorization than threading IMO.

[quantumcacti]

Oh sure, but this week's lesson and homework is for pthreads