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);

}

Answer 1

#Could you please leave a THUMBS Up for my work...

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.
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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 :

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