petterson.c

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

#define NUM_THREADS 2

int turn;
int flag[2];
int shared_variable = 0;

void* thread_function(void* arg) {
    int thread_id = *((int*)arg);

    while (1) {
        flag[thread_id] = 1;
        turn = 1 - thread_id;
        while (flag[1 - thread_id] && turn == 1 - thread_id);

        // critical section
        shared_variable++;
        printf("Thread %d incremented shared variable to %d.\n", thread_id, shared_variable);
        // end critical section

        flag[thread_id] = 0;
    }

    pthread_exit(NULL);
}

int main() {
    pthread_t threads[NUM_THREADS];
    int thread_args[NUM_THREADS];
    int result_code;
    unsigned index;

    // create all threads
    for (index = 0; index < NUM_THREADS; ++index) {
        thread_args[index] = index;
        result_code = pthread_create(&threads[index], NULL, thread_function, &thread_args[index]);
        if (result_code != 0) {
            perror("Thread creation failed");
            exit(EXIT_FAILURE);
        }
    }

    // wait for each thread to complete
    for (index = 0; index < NUM_THREADS; ++index) {
        result_code = pthread_join(threads[index], NULL);
        if (result_code != 0) {
            perror("Thread join failed");
            exit(EXIT_FAILURE);
        }
    }

    printf("Program completed.\n");

    return 0;
}