Question

Please explain your answers thoroughly for the following question.

1) Compile thread2.c (remember the -lpthread flag). Run it numerous times, and notice the output usually differs.

a) Why is the variable myglobal usually not 40? Be specific, be precise.

b) In what special case would myglobal be 40? Be specific, be precise.

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

/*
Purpose: Use 2 threads to increment myglobal exactly 40 times in total.
Compile: using -pthread option
*/

int myglobal = 0;

void *thread_function(void *arg)
{
   int i,j;
  
   for (i = 0; i < 20; i++)
   {
       j = myglobal;
       j = j + 1;
       printf(".");
       fflush(stdout);
       sleep(0.2);
       myglobal = j;
   }
  
   return NULL;
}

int main(void)
{
   pthread_t mythread;
   int i;

   if (pthread_create(&mythread, NULL, thread_function, NULL)) {
       printf("error creating thread.");
       abort();
   }

   for (i = 0; i < 20; i++)
   {
       myglobal = myglobal + 1;
       printf("o");
       fflush(stdout);
       sleep(0.21);
   }

   if (pthread_join(mythread, NULL)) {
       printf("error joining thread.");
       abort();
   }

   printf("\nmyglobal equals %d\n", myglobal);

   exit(0);
}

Answer 1

Note: Done accordingly. Please comment for any problem. Please Uprate. Thanks

a)

Both threads share a single attribute which is myglobal. Global variable occurs only once per process, and is typically covered by mutex in order to prevent simultaneous access. Now in our case both the threads are modifying the myglobal and then their is context switch after

j = myglobal;

then program will return back to
j = j + 1;

and will use old value and hence we will not get perfecct 40. Take example below : Here due to context switch in between lines of loop threadfunction prints old value(i.e. 7 ) instead of modified value from main(i.e 21).

b)

We will get 40 if context switch is done when whole loop is executed (i.e context switch will be done either before entering the next loop or after completion of loop)