Question

so in this code, it computes the sum 1+2+....+n but i want it to compute

2*(1+2+....+n)

using semaphores implement solution to the critical section problem

#include
#include

int sum; /* this data is shared by the thread(s) */
void *runner(void *param); /* threads call this function */
int main(int argc, char *argv[])
{
pthread_t tid; /* the thread identifier */
pthread_attr_t attr; /* set of thread attributes */
if (argc != 2) {
fprintf(stderr,"usage: a.out \n");
return -1;
}
if (atoi(argv[1]) < 0) {
fprintf(stderr,"%d must be >= 0\n",atoi(argv[1]));
return -1;
}
/* get the default attributes */
pthread_attr_init(&attr);
/* create the thread */
pthread_create(&tid,&attr,runner,argv[1]);
/* wait for the thread to exit */
pthread_join(tid,NULL);
printf("sum = %d\n",sum);
}
/* The thread will begin control in this function */
void *runner(void *param)
{
int i, upper = atoi(param);
sum = 0;
for (i = 1; i <= upper; i++)
sum += i;
pthread_exit(0);
}

Answer 1

Please find the program using semaphore to protect the critical section when calculating sum in two threads.

Program:

#include <stdio.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>

/*initialize here itself */
int sum =0; /* this data is shared by the thread(s) */

sem_t mutex;

void *runner(void *param); /* threads call this function */
void *runner_1(void *param); /* threads call this function */

int main(int argc, char *argv[])
{
   pthread_t tid; /* the thread identifier */
   pthread_t tid_1; /* the thread identifier */
   pthread_attr_t attr; /* set of thread attributes */

   /* semaphore initilized to protect multiple threads of process */
   sem_init(&mutex, 0, 1);

if (argc != 2) {
      fprintf(stderr,"usage: thread_sync.exe <integer> \n");
      return -1;
   }
   if (atoi(argv[1]) < 0) {
      fprintf(stderr,"%d must be >= 0\n",atoi(argv[1]));
      return -1;
   }
   /* get the default attributes */
   pthread_attr_init(&attr);

   /* create the thread */
   pthread_create(&tid,&attr,runner,argv[1]);
   pthread_create(&tid_1,&attr,runner_1,argv[1]);

   /* wait for the thread to exit */
   pthread_join(tid,NULL);
   pthread_join(tid_1,NULL);

   sem_destroy(&mutex);
   printf("sum = %d\n",sum);

}

/* The thread will begin control in this function */
void *runner(void *param)
{
   int i, upper = atoi(param);

   /* Wait/Lock using a mutex */
   sem_wait(&mutex);

   for (i = 1; i <= upper; i++)
      sum += i;

   printf("Sum computed in 1st thread is %d\n", sum);

   /* signal/Unlock the mutex */
   sem_post(&mutex);
   pthread_exit(0);
}

/* The thread will begin control in this function */
void *runner_1(void *param)
{
   int i, upper = atoi(param);

   /* Wait/Lock using a mutex */
   sem_wait(&mutex);

   for (i = 1; i <= upper; i++)
      sum += i;

   printf("Sum computed in 2nd thread is %d\n", sum);

   /* signal/Unlock the mutex */
   sem_post(&mutex);
   pthread_exit(0);
}

Output:

USER 1>./thread_sync.exe 10
Sum computed in 1st thread is 55
Sum computed in 2nd thread is 110
sum = 110