Question

Please change this 2 thread program to a 4 thread program.

/*compile as "gcc thread.c -pthread" */
#include <pthread.h>
#include <stdio.h>
#include <time.h>

#define SIZE 100000000 //100 M
//#define SIZE 10000
#define NUMBER_OF_TIMES 100

float a[SIZE];

typedef struct {
       int start;
       int end;
       double sum;
       int pid;
   } range;

void *thread_function(void *arg) //define a function that will be executed from
//within a thread
{
   range *incoming = (range *) arg;
   int i, j;
   double sum;
   int start, end, pid;

   start = incoming->start;
   end = incoming->end;
   pid = incoming->pid;

//   printf ("Array entries (start, end) = (%d, %d)\n", start,end);

   if(pid != 0)
   {
       printf("2T: PID %d started: time = %f\n", pid, (double) time(NULL));
   }

   sum = 0;
   for(j = 0; j <NUMBER_OF_TIMES; j++)
       for (i=start; i<end; i++)
           sum=sum + a[i];

//   printf("Pid = %d: sum = %f\n", pid, sum);
   incoming->sum = sum;   //save the result from the PE

   if(pid != 1)
   {
       printf("2T: PID %d ended: time = %f\n", pid, (double) time(NULL));
   }
   return NULL;
}

int main (void)
{
   pthread_t threadID;
   void *exit_status;
   range one;
   range two;
   int i;
   double sum;

   for (i=0; i<SIZE; i++) //initialize array
       a[i] = 1;

   printf("2T: PID 0 started: time = %f\n", (double) time(NULL));

   //create a thread that calls thread_function, and pass the indeces
//for the 1st half of the array
    one.start = 0;
   one.end = SIZE/2-1;
   one.pid = 1; //PE is 1
   pthread_create(&threadID, NULL, thread_function, &one); //create a thread and run thread_function

   //main will become the 2nd thread and will call thread_function
//Pass the indeces to the 2nd half of the array
two.start = SIZE/2;   //main computes the sum for the second half of the array elements
   two.end = SIZE;
   two.pid = 0;
   thread_function(&two);

   pthread_join(threadID, &exit_status); //wait for the 1st thread to end
   sum = one.sum + two.sum; //PE 2 combines the two partial sums

   printf("2T: PID 0 ended: time = %f\n", (double) time(NULL));

   printf("2T: Final sum = %e\n", sum);
  
   two.start = SIZE/2;   //main computes the sum for the second half of the array elements
   two.end = SIZE;
   two.pid = 0;
   thread_function(&two);

   pthread_join(threadID, &exit_status); //wait for the 1st thread to end
   sum = one.sum + two.sum; //PE 2 combines the two partial sums

   printf("2T: PID 0 ended: time = %f\n", (double) time(NULL));

   printf("2T: Final sum = %e\n", sum);

   return 0;
}

Answer 1

I have created not only for 4 threads but for N number of threads although you can set a limit to how many threads user can enter using MAX_THREADS .

HOPE THIS HELPS ...

/*compile as "gcc thread.c -pthread" */
#include <pthread.h>
#include <stdio.h>
#include <time.h>

#define SIZE 100000000 //100 M
//#define SIZE 10000
#define MAX_THREADS 10
#define NUMBER_OF_TIMES 100

float a[SIZE];

typedef struct {
int start;
int end;
double sum;
int pid;
} range;

void *thread_function(void *arg) //define a function that will be executed from
//within a thread
{
range *incoming = (range *) arg;
int i, j;
double sum;
int start, end, pid;

start = incoming->start;
end = incoming->end;
pid = incoming->pid;

printf ("Array entries (start, end) = (%d, %d)\n", start,end);

if(pid != 0)
{
printf("2T: PID %d started: time = %f\n", pid, (double) time(NULL));
}

sum = 0;
for(j = 0; j <NUMBER_OF_TIMES; j++)
{
for (i=start; i<end; i++)
{
sum=sum + a[i];
}
  
}
// printf("Pid = %d: sum = %f\n", pid, sum);
incoming->sum = sum; //save the result from the PE

if(pid != 1)
{
printf("2T: PID %d ended: time = %f\n", pid, (double) time(NULL));
}
return NULL;
}

int main (void)
{
pthread_t threadID;
void *exit_status;
range one;
range two;
int i;
double sum;
int noOfThreads, result , chunk;
range threads[MAX_THREADS];

for (i=0; i<SIZE; i++) //initialize array
a[i] = 1;

// printf("2T: PID 0 started: time = %f\n", (double) time(NULL));
printf("how many threads do you want ? \t");
scanf("%d",&noOfThreads);

  if(noOfThreads > MAX_THREADS)
{
printf("My CPU does not have this many threads\nPlease enter again ....\nThreads you want : ");
scanf("%d",&noOfThreads);
}

//if we are using N no of threads we need to divide the array into equal chunks ...
chunks = SIZE/noOfThreads ;
// loop to initialize all the threads with values and call thread_function
for(i=0 ; i<noOfThreads ; i++)
{
threads[i].start = (i*chunk)+1;
threads[i].end = (threads[i].start + chunk)-1 ;
threads[i].pid = i+1 ;
pthread_create(&threadID, NULL , thread_function, &threads[i])

}

for(i=0 ; i<noOfThreads ; i++)
{
pthread_join(threadID, &exit_status); //wait for the 1st thread to end
}

for(i=0 ; i<noOfThreads ; i++)
{
sum = sum + threads[i].sum;
}

printf("2T: PID 0 ended: time = %f\n", (double) time(NULL));

printf("2T: Final sum = %e\n", sum);

return 0;
}