Question

The following program uses Pthreads to create two threads. They do some work for the process and then exit. The process then outputs a result.

Assume all supporting libraries and other functions have been included.

=> Use the answer text field to describe what work (operations) the threads are doing, and what kind of result (what is it?) is output by the process.

#include <pthread.h>

#include <stdio.h>

#include <stdlib.h>

int res1, res2, a[100], b[100];

void *runner1(void *param);

void *runner2(void *param);

void readData(int []);

int main(int argc, char *argv[])

{

  pthread_t tid1, tid2;

  pthread_attr_t attr;

  readData(a);

  readData(b);

  pthread_attr_init(&attr);

  pthread_create(&tid1, &attr, runner1, argv[1]);

  pthread_create(&tid2, &attr, runner2, argv[1]);

  pthread_join(tid1,NULL);

  pthread_join(tid2,NULL);

  printf("result = %d\n", res1+res2);

}

void *runner1(void *param)

{

  int i, upper = atoi(param);

  res1 = 0;

  for (i = 0; i < upper; i++)

    res1 += a[i];

  pthread_exit(0);

}

void *runner2(void *param)

{

  int i, upper = atoi(param);

  res2 = 0;

  for (i = 0; i < upper; i++)

    res2 += b[i];

  pthread_exit(0);

}

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Answer 1

Before answering, I have made few modifications to your code. Please keep in mind these points.

• First, readData() function is not defined, so for running the code, I have initialised array a[0...99] with all 1's, i.e. a[0]=1, a[1]=1.... a[99]=1, similarly, array b[0..99] is also initialised with 1's inside main() function
• The range of values allowed for the the first argument passed, i.e, argv[1], should be in beween 0-100. Reason is, this value is used to iterate through values of array a[] and b[], since size of both array is 100, passing value more than 100 will lead to fetching garbage value.

Now, moving to the actual question of your's:

Ques1: Describe what work (operations) the threads are doing?

• Thread 1 computes the sum of values of array a[] indexing from 0 to argv[1]. Here, argv[1] is converted to integer value using atoi() function.​​​​​​​ The sum is stored in res1.
• Similarly, Thread 2 computes the sum of values of array b[] indexing from 0 to argv[1]. ​The sum is stored in res2.
• The purpose of using thread, here, is to compute the sum parallelly for both the arrays, instead of computing it linearly one-by-one. This clearly make the program run faster.

​​​​​​​

Ques2: What kind of result (what is it?) is output by the process?

• Once res1 and res2 are computed by thread1 and thread2 respectively, as explained above.
• In main() function, both values are added, and printed as output.
• The print() statement will only run after both the thread exits.
• In the code, pthread_join() enables the main(),ie, parent process, to wait for the thread to terminate.

To support the explanation, I am attaching a working code and the output. Please go through it and ask if you face any issue.

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

int res1, res2, a[100], b[100];
void *runner1(void *param);
void *runner2(void *param);
void readData(int []);

int main(int argc, char *argv[]){

  argv[1] = "57";   // Remove this statement if passing value while running the code

  pthread_t tid1, tid2;
  pthread_attr_t attr;

  int i=0;

// instead of readData(a), I have initialised array a[] here
  for(i=0; i<100; i++)
      a[i] = 1;

//  readData(b), I have initialised array b[] here
  for(i=0; i<100; i++)
      b[i] = 1;


  pthread_attr_init(&attr);
  pthread_create(&tid1, &attr, runner1, argv[1]);
  pthread_create(&tid2, &attr, runner2, argv[1]);
  pthread_join(tid1,NULL);
  pthread_join(tid2,NULL);

  printf("result = %d\n", res1+res2);
}

void *runner1(void *param){

  int i, upper = atoi(param);
  res1 = 0;

  for (i = 0; i < upper; i++)
    res1 += a[i];

  pthread_exit(0);
}

void *runner2(void *param){

  int i, upper = atoi(param);
  res2 = 0;

  for (i = 0; i < upper; i++)
    res2 += b[i];

  pthread_exit(0);
}

Output: