Notes

/******************************************************************************
* compile with gcc -pthread *.c -o loops
* test with valgrind --tool=helgrind ./lops
*
******************************************************************************/
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define NTHREADS 4
#define ARRAYSIZE 100000000
#define ITERATIONS ARRAYSIZE / NTHREADS

double sum=0.0;
double a[ARRAYSIZE];
pthread_mutex_t sum_mutex;


void *do_work(void *tid)
{
int i, start, *mytid, end;
double mysum=0.0;

/* Initialize my part of the global array and keep local sum */
mytid = (int *) tid;
start = (*mytid * ITERATIONS);
end = start + ITERATIONS;
printf ("n[Thread %5d] Doing iterations t%10d to t %10d",*mytid,start,end-1);
for (i=start; i < end ; i++) {
a[i] = i * 1.0;
mysum = mysum + a[i];
}

/* Lock the mutex and update the global sum, then exit */
pthread_mutex_lock (&sum_mutex);
sum = sum + mysum;
pthread_mutex_unlock (&sum_mutex);
pthread_exit(NULL);
}


int main(int argc, char *argv[])
{
int i, start, tids[NTHREADS];
pthread_t threads[NTHREADS];
pthread_attr_t attr;

/* Pthreads setup: initialize mutex and explicitly create threads in a
joinable state (for portability). Pass each thread its loop offset */
pthread_mutex_init(&sum_mutex, NULL);
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
for (i=0; i<NTHREADS; i++) {
tids[i] = i;
pthread_create(&threads[i], &attr, do_work, (void *) &tids[i]);
}

/* Wait for all threads to complete then print global sum */
for (i=0; i<NTHREADS; i++) {
pthread_join(threads[i], NULL);
}
printf ("n[MAIN] Done. Sum= %e", sum);

sum=0.0;
/* for (i=0;i<ARRAYSIZE;i++){
a[i] = i*1.0;
sum = sum + a[i]; }
printf("n[MAIN] Check Sum= %e",sum);
*/
/* Clean up and exit */
pthread_attr_destroy(&attr);
pthread_mutex_destroy(&sum_mutex);
pthread_exit (NULL);
}