parallel and sequential dotproduct programs different result - c

Question

I've wrote this two versions of code for computing a dotproduct operation on two arrays. each length is 256. here is very simple sequential code:
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char* argv[]){
double sum;
double a[256], b[256];
int n = 256, i;
for (i=0; i<n; i++){
a[i] = i * 0.5;
b[i] = i * 2.0;
}
sum = 0;
for (i=1; i<=n; i++){
sum = sum + a[i]*b[i];
}
printf ("sum = %f\n", sum);
}//main
answer is 5559680
but the parallel code:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define NUMTHRDS 4
double sum;
double a[256], b[256];
int status;
int n=256;
pthread_t thds[NUMTHRDS];
pthread_mutex_t mutexsum;
void* dotprod(void *arg){
int myid, i, my_first, my_last;
double sum_local;
myid = (int)arg;
my_first = myid * n/NUMTHRDS;
my_last = (myid + 1) * n/NUMTHRDS;
sum_local = 0;
for (i=my_first; i<=my_last; i++){
sum_local = sum_local + a[i]*b[i];
}
pthread_mutex_lock(&mutexsum);
sum = sum + sum_local;
pthread_mutex_unlock(&mutexsum);
pthread_exit((void*)0);
}//dotprod
int main(int argc, char* argv[]){
int i;
pthread_attr_t attr;
for (i=0; i<n; i++){
a[i] = i * 0.5;
b[i] = i * 2.0;
}
pthread_mutex_init(&mutexsum, NULL);
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
for (i=0; i<NUMTHRDS; i++){
pthread_create(&thds[i], &attr, dotprod, (void*)i);
}
pthread_attr_destroy(&attr);
for(i=0; i<NUMTHRDS; i++){
pthread_join(thds[i], (void **)&status);
}
printf("sum = %f \n", sum);
pthread_mutex_destroy(&mutexsum);
pthread_exit(NULL);
return 0;
}//main
answer is 5617024
i totally confused what is this difference for?

Answer 1

off by one error.
for (i=1; i<=n; i++){
for (i=0; i<n; i++) {
and
for (i=my_first; i<=my_last; i++){
for (i=my_first; i<my_last; i++){
In the first program, you are adding in a[256] and b[256], which is off the end of the array.
most likely those values were 0, so you got the right answer.
In the second program, you are counting some parts of the array twice: 64, 128, 192, and
still adding in index 256.
Always check the boundary conditions of your loops, especially with array accesses.