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

Please find the detailed explanation of each steps as inline comment. Also see an example at the end of the program. I hope this explanation helps you to understand the flow properly.

Please provide your feedback

Thanks and Happy learning!

#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[])

{
    //Declare the pthread variables which is used to uniquely
    //identify a thread.Thai will be used later while creating
        //the threads using pthread_create
    pthread_t tid1, tid2;
        
    //Declare the required attribute for the thread
    pthread_attr_t attr;

    //Below two lines will be reading the input(int) and storing
        //in the integer array a[] and b[] respectively declared in 
        //the global section 
    readData(a);
    readData(b);

        //Below line initializes the attributes required for a thread.
        //Here the attribute is initialized with default values.
    pthread_attr_init(&attr);
        
        //Below two linws create two threads with id tid1 and tid2 respectively.
        //1. Here the first argument(ie tid1) is to hold the id of the new thread created.
        //2. Second argument (ie attr) will be used to initialise the attributes of the new thread .
        //3. Third argument(ie runner1) is the function that will be get invoked by the thread once it is created.
        //4. Fourth argument(ie argv[1]) is the argument that will be passed to the function mentioned in step#3
        //   (ie function runner1) when it is incoked by the thread. That means when the thread invokes the function
        //    runner1, it will be as if its is calling runner1(argv[1])
    pthread_create(&tid1, &attr, runner1, argv[1]);
    pthread_create(&tid2, &attr, runner2, argv[1]);

        //Below two statements will make the parent thread(ie the thread which is executing the main()
        //function) to wait for the two threads (with id tid1 and tid2) until it is completed. This means
        //that the main thread will be waiting until the execution of functions runner1() and runner2()
        //is completed.
    pthread_join(tid1, NULL);
    pthread_join(tid2, NULL);

        //Print the sum of result calculated in the runner1() and runner2()
    printf("result = %d\n", res1 + res2);

}

void *runner1(void *param)

{
    //Here the program converts the argument passed to the
        //function into integer using atoi() function call.
    int i, upper = atoi(param);

        //Below loop executes 'upper'(calculated above using atoi()) number of times
        //and adds up the numbers in the input array a[100] (which is read in the main() function
        //readData()).
        //
        //In simple terms the below loop adds up the first 'upper' number of numbers in the
        //a[100] array and stores the result in the res1 variable.
    res1 = 0;
    for (i = 0; i < upper; i++)
        res1 += a[i];

    //Exit the thread function
    pthread_exit(0);

}

void *runner2(void *param)

{
    //Here the program converts the argument passed to the
        //function into integer using atoi() function call.
    int i, upper = atoi(param);

    res2 = 0;

    //The below loop adds up the first 'upper' number of numbers in the
        //b[100] array and stores the result in the res2 variable.
    for (i = 0; i < upper; i++)
        res2 += b[i];

    //Exit the thread function
    pthread_exit(0);

}

For example, suppose
The input array a[100] contains numbers {1,2,3,4}
The input array b[100] contains numbers {5,6,7,8}
And if argv[1](ie the first command line argument passed while executing the program) is 3

Then runner1() function(invoked by first thread tid1) will calculate rs1 as the sum of first 3 numbers from a[100]
ie rs1 = 1 + 2 + 3 that means rs1 = 6

Then runner2() function (invoked by second thread tid2) will calculate rs2 as the sum of first 3 numbers from b[100]
ie rs2 = 5 + 6 + 7 that means rs1 = 18

And in the main() function, the main thread will wait until the above two functions are finished, then it will print the
result as res1 + res2, that means result will be 6 + 18 = 24