Modify the program to implement the strict alternation solution to achieve mutua

Question

Modify the program to implement the strict alternation solution to achieve mutual exclusion (refer back to the relevant prep work video/slides if you need to).

Code:

#include <iostream>

#include <pthread.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

  

  

  

// 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.

  

}

  

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);

}

Thread #0 count = 1 Thread #1 count = 2 Thread #0 count = 3 Thread #1 count = 4 Thread #0 count = 5 Thread #1 count = 6 Thread #0 count = 7 Thread #1 count = 8 Thread #0 count = 9 Thread #1 count = 10 Thread #0 count = 11 Thread #1 count = 12 Thread #0 count = 13 Thread #1 count = 14 Thread #0 count = 15 Thread #1 count = 16 Thread #0 count = 17 Thread #1 count = 18 Thread #0 count = 19 Thread #1 count = 20 Final count 20

Explanation / Answer

main.cpp

#include <iostream>

#include <pthread.h>

using namespace std;

int count;

int turn = 0;

void* myFunction(void* arg)

{

int actual_arg = *((int*) arg);

  

for(unsigned int i = 0; i < 10; ++i) {

  

  

while(turn != actual_arg);

  

  

count++;

std::cout << "Thread #" << actual_arg << " count = " << count << std::endl;

  

if(actual_arg == 0) {

turn = 1;

} else {

turn = 0;

}

  

}

  

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);

}

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