Question

Modify the included program to implement the strict alternation solution to achieve mutual exclusion. It does not matter whether Thread 0 goes first or Thread 1, but it is important that the thread strictly alternate.

PROGRAM:

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

int count;
int turn = 0; // Shared variable used to implement strict alternation

void* myFunction(void* arg)
{
   int actual_arg = *((int*) arg);
  
   for(unsigned int i = 0; i < 10; ++i) {
  
// TODO:
// Make sure that the thread waits for its turn
// before it enters the critical region.
//
// HINT: The thread ID is stored in actual_arg
  
while(turn != actual_arg);
  
  
  
// Beginning of the critical region
  
count++;
std::cout << "Thread #" << actual_arg << " count = " << count << std::endl;
  
// End of the critical region
  
  
  
// TODO:
// Make sure that the other thread gets a turn
//
// HINT: There are only two threads: 0 and 1
// You can use the C++ NOT operator (!)
// to toggle back and forth.
  
  

// Random wait - This code is just to ensure that the threads
// show data sharing problems
int max = rand() % 100000;
  
for (int x = 0; x < max; x++);
  
// End of random wait code
  
   }
  
   pthread_exit(NULL);
}

// HINT: It is not necessary to make any changes in main()
int main()
{
int rc[2];
pthread_t ids[2];
int args[2];
  
count = 0;
for(unsigned int i = 0; i < 2; ++i) {
args[i] = i;
rc[i] = pthread_create(&ids[i], NULL, myFunction, (void*) &args[i]);
}
  
for(unsigned int i = 0; i < 2; ++i) {
pthread_join(ids[i], NULL);
}
  
std::cout << "Final count = " << count << std::endl;
pthread_exit(NULL);
}

Answer 1

Hello! :)

Since the turn variable is initialized with 0, which is the value of actual_arg of the first thread, it gets to execute the part after line 22 first but the second thread is blocked at line 22 because its actual_arg value is 1. After execution, if we toggle the value of turn (assign the not of turn to turn), the first thread gets blocked at line 22 whereas the second thread gets to execute the part after line 22 since turn becomes 1 now. Similarly, after the second thread is done executing and toggling turn, it gets blocked and the first thread gets to execute. This alteration continues till both threads are done executing.

Here's the code with the modification in line 43 (marked with the comment: // ADDED):

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


int count;
int turn = 0; // Shared variable used to implement strict alternation


void* myFunction(void* arg)
{
   int actual_arg = *((int*) arg);
  
   for(unsigned int i = 0; i < 10; ++i) {
  
// TODO:
// Make sure that the thread waits for its turn
// before it enters the critical region.
//
// HINT: The thread ID is stored in actual_arg
  
while(turn != actual_arg);
  
  
  
// Beginning of the critical region
  
count++;
std::cout << "Thread #" << actual_arg << " count = " << count << std::endl;
  
// End of the critical region
  
  
  
// TODO:
// Make sure that the other thread gets a turn
//
// HINT: There are only two threads: 0 and 1
// You can use the C++ NOT operator (!)
// to toggle back and forth.
  
  
turn = !turn;                                                               // ADDED


// Random wait - This code is just to ensure that the threads
// show data sharing problems
int max = rand() % 100000;
  
for (int x = 0; x < max; x++);
  
// End of random wait code
  
   }
  
   pthread_exit(NULL);
}


// HINT: It is not necessary to make any changes in main()
int main()
{
int rc[2];
pthread_t ids[2];
int args[2];
  
count = 0;
for(unsigned int i = 0; i < 2; ++i) {
args[i] = i;
rc[i] = pthread_create(&ids[i], NULL, myFunction, (void*) &args[i]);
}
  
for(unsigned int i = 0; i < 2; ++i) {
pthread_join(ids[i], NULL);
}
  
std::cout << "Final count = " << count << std::endl;
pthread_exit(NULL);
}

Here's a screenshot of a demo run:

Hope this helps! :)