How do i make c threads do cyclyc sum? - c

Question

So i have this program im working on, and the guist of it is that i need to do some operations with threads, following the next shcheme: The j-th thread Hj calculates a group of 100 consecutive iterations of the sum, making a cyclic distribution of the groups among all the threads. For example, if H = 4, the
thread H2 does the calculation of iterations [100..199, 500..599, 900..999, ...].
To ensure no data races occur, the threads must work each on a different sum variable.
Then compare after joining the threads the result achieved by the threads and the one done sequentally.
Here is the code:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <math.h>
#include <pthread.h>
#include <sys/time.h>
#define H 4
double res[H] = {0};
//Time function
float restar_tiempo(struct timeval *inicio, struct timeval *fin) {
return (fin->tv_sec - inicio->tv_sec) + 1e-6 * (fin->tv_usec - inicio->tv_usec);
}
//Thread function
void *_hilo(void *arg) {
int a = * ((int*)arg);
double pi = 0;
double n = 100 * a;
while (n < 10000000) {
res[a] += (pow(-1, n) / pow(4, n)) * ((2 / (4 * n + 1)) + (2 / (4 * n + 2)) + (1 / (4 * n + 3)));
pi++;
n++;
if ((int) n % 100 == 0)
n += (H - 1)*100;
}
printf("Result on thread[%d]: %f\n", a, res[a]);
pthread_exit(NULL);
}
int main() {
pthread_t hilo[H];
struct timeval in, mid, fin;
gettimeofday(&in, NULL);
for (int i = 0; i < H; i++) {
int* p = malloc(sizeof (int));
*p = i;
printf("Esto es i: %d\n", i);
res[i] = 0;
if (pthread_create(&hilo[i], NULL, _hilo, p) != 0) {
perror(" Error creando hilo");
exit(EXIT_FAILURE);
}
free(p);
}
//Join
for (int i = 0; i < H; i++)
pthread_join(hilo[i], NULL);
//Partial sum
double f = 0;
for (int i = 0; i < H; i++){
printf("Resultado parcial de hilo %d: %f\n", i, res[i]);
f += res[i];
}
//Total partial sum
printf("Resultado total: %lf\n", f);
//printf("Hola/n");
gettimeofday(&mid, NULL);
//Secuential sum
double s = 0;
for (double n = 0; n < 10000000; n++)
s += (pow(-1, n) / pow(4, n)) * ((2 / (4 * n + 1)) + (2 / (4 * n + 2)) + (1 / (4 * n + 3)));
//Print secuential
printf("Resultado secuencial: %f\n", s);
gettimeofday(&fin, NULL);
//Result diff
printf("Diferencia resultados: %f\n", fabs(f - s));
//Time threads
printf("Tiempo por hilos: %f\n", restar_tiempo(&in, &mid));
//Secuential time
printf("Tiempo secuencial: %f\n", restar_tiempo(&mid, &fin));
//Time diff
printf("Diferencia tiempos: %f\n", restar_tiempo(&in, &mid) - restar_tiempo(&mid, &fin));
return 0;
}
I can compile everything without warnings, but when i execute the program, the result provided by the first thread is erratic, as it changes between executions (the rest of threads display 0 because they work with very little values).
Example with some added prints inside the thread function and after doing the join:
First execution:
This is i:0
This is i:1
This is i:2
This is i:3
//Inside thread funct
Thread result[2]: 0.000000
Thread result[2]: 0.000000
Thread result[3]: 0.000000
Thread result[0]: 3.141593
//After join
Partial result of thread 0: 3.141593
Partial result of thread 1: 0.000000
Partial result of thread 2: 0.000000
Partial result of thread 3: 0.000000
Total result: 3.141593
Sequential result: 3.141593
Difference results: 0.000000
Time per threads: 0.183857
Sequential time: 0.034788
Difference times: 0.149069
Second execution:
This is i:0
This is i:1
This is i:2
This is i:3
Thread result[2]: 0.000000
Thread result[0]: 6.470162
Thread result[0]: 6.470162
Thread result[3]: 0.000000
Partial result of thread 0: 6.470162
Partial result of thread 1: 0.000000
Partial result of thread 2: 0.000000
Partial result of thread 3: 0.000000
Total result: 6.470162
Sequential result: 3.141593
Difference results: 3.328570
Time per threads: 0.189794
Sequential time: 0.374017
Difference times: -0.184223
How can i make it so the sum works properly?
I think it has something to do with arg in the function _hilo, or the subsequent int cast with int a.
(Excuse the mix in languages, i speak spanish so most of the printfs are in said language. Dont mind them, the block with the results example has the traduction)

Answer 1

Okay i solved it but i dont know why it works like this fully or why this caused issues. I just deleted the free (p) statement and now it works like a charm. If someone can enlighten me on why this happens, i´ll be grateful.