#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <pthread.h>
#include <stdbool.h>
#include <time.h>

// WARNING: This example does not work, I couldn't figure out why the lock does not work correcly.
// The final counter result should not change, but it does.

struct shared_data {
  pthread_mutex_t mutex;
  int counter;
};

void run(const char *name, struct shared_data *data) {
  for (int i = 0; i < 10000; i++) {
    if (pthread_mutex_lock(&data->mutex) != 0) {
      perror("pthread_mutex_lock");
    }

    data->counter++;

    if (pthread_mutex_unlock(&data->mutex) != 0) {
      perror("pthread_mutex_unlock");
    }
  }
}

int main() {
  struct shared_data *shared_data = mmap(NULL, sizeof(struct shared_data), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
  if (shared_data == NULL) {
    perror("mmap");
    return -1;
  }

  if (pthread_mutex_init(&shared_data->mutex, NULL) < 0) {
    printf("Failed to initialise mutex\n");
    return -1;
  }

  shared_data->counter = 0;

  int pid = fork();
  if (pid == 0) {
    srand(time(NULL)+1);
    run("Child ", shared_data);
    return 0;
  } else {
    srand(time(NULL)+2);
    run("Parent", shared_data);
  }

  int child_status;
  waitpid(pid, &child_status, 0);

  printf("Counter is %d\n", shared_data->counter);
}