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

struct element {
  int value;
  struct element *next;
} *head = NULL, *tail = NULL;

void add_element(int v) {
  struct element *e = malloc(sizeof(struct element));
  e->next = NULL;
  e->value = v;
  if (tail == NULL)
    head = tail = e;
  else
    tail = tail->next = e;
}

int take_element() {
  int v = head->value;
  struct element *e = head;
  head = head->next;
  if (head == NULL)
    tail = NULL;
  free(e);
  return v;
}

#define DO_HEAVY_COMPUTATION (usleep(rand() % 10000000), rand() % 1000000)

void *worker_thread(void *arg) {
  srand(time(0) + (intptr_t)arg);
  while (1) {
    int v = DO_HEAVY_COMPUTATION;
    pthread_testcancel();
    add_element(v);
  }
}

int main() {
  pthread_t t[4];
  for (int i = 0; i < 4; ++i)
    pthread_create(t + i, NULL, worker_thread, NULL);

  int computed_count = 0;
  while (computed_count < 10) {
    while (head == NULL);
    int value = take_element();
    printf("%02d: %d\n", ++computed_count, value);
  }

  for (int i = 0; i < 4; ++i)
    pthread_cancel(t[i]);
  for (int i = 0; i < 4; ++i)
    pthread_join(t[i], NULL);

  return 0;
}