/* From Learning C by example: Producer-consumer problem with semaphores
   https://jlmedina123.wordpress.com/2014/04/08/255/
 */
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <time.h>
 
#define SIZE 5
#define NUMB_THREADS 6
#define PRODUCER_LOOPS 2
 
typedef int buffer_t;
buffer_t buffer[SIZE];
int buffer_index;
 
pthread_mutex_t buffer_mutex;
/* initially buffer will be empty.  full_sem
   will be initialized to buffer SIZE, which means
   SIZE number of producer threads can write to it.
   And empty_sem will be initialized to 0, so no
   consumer can read from buffer until a producer
   thread posts to empty_sem */
sem_t full_sem;  /* when 0, buffer is full */
sem_t empty_sem; /* when 0, buffer is empty. Kind of
                    like an index for the buffer */
 
/* sem_post algorithm:
    mutex_lock  sem_t->mutex
    sem_t->value++
    mutex_unlock sem_t->mutex
 
   sem_wait algorithn:
    mutex_lock sem_t->mutex
    while (sem_t->value > 0) {
        mutex_unlock sem_t->mutex
        sleep... wake up
        mutex_lock sem_t->mutex
    }
    sem_t->value--
    mutex_unlock sem_t->mutex
*/
 
 
void insertbuffer(buffer_t value) {
  if (buffer_index < SIZE) {
    buffer[buffer_index++] = value;
  } else {
    printf("Buffer overflow\n");
  }
}
 
buffer_t dequeuebuffer() {
  if (buffer_index > 0) {
    return buffer[--buffer_index]; // buffer_index-- would be error!
  } else {
    printf("Buffer underflow\n");
  }
  return 0;
}
 
 
void *producer(void *thread_n) {
  int thread_numb = *(int *)thread_n;
  buffer_t value;
  int i=0;
  unsigned int seed;
  time_t now=time(NULL);
  seed = (unsigned int)now;

  while (i++ < PRODUCER_LOOPS) {
    sleep(rand_r(&seed) % 10);
    value = rand_r(&seed) % 100;
    sem_wait(&full_sem); // sem=0: wait. sem>0: go and decrement it
    /* possible race condition here. After this thread wakes up,
       another thread could aqcuire mutex before this one, and add to list.
       Then the list would be full again
       and when this thread tried to insert to buffer there would be
       a buffer overflow error */
    pthread_mutex_lock(&buffer_mutex); /* protecting critical section */
    insertbuffer(value);
    pthread_mutex_unlock(&buffer_mutex);
    sem_post(&empty_sem); // post (increment) emptybuffer semaphore
    printf("Producer %d added %d to buffer\n", thread_numb, value);
  }
  pthread_exit(0);
}
 
void *consumer(void *thread_n) {
  int thread_numb = *(int *)thread_n;
  buffer_t value;
  int i=0;
  while (i++ < PRODUCER_LOOPS) {
    sem_wait(&empty_sem);
    /* there could be race condition here, that could cause
       buffer underflow error */
    pthread_mutex_lock(&buffer_mutex);
    value = dequeuebuffer(value);
    pthread_mutex_unlock(&buffer_mutex);
    sem_post(&full_sem); // post (increment) fullbuffer semaphore
    printf("Consumer %d dequeue %d from buffer\n", thread_numb, value);
  }
  pthread_exit(0);
}
 
int main(int argc, int **argv) {
  buffer_index = 0;
 
  pthread_mutex_init(&buffer_mutex, NULL);
  sem_init(&full_sem, // sem_t *sem
	   0, // int pshared. 0 = shared between threads of process,  1 = shared between processes
	   SIZE); // unsigned int value. Initial value
  sem_init(&empty_sem,
	   0,
	   0);
  /* full_sem is initialized to buffer size because SIZE number of
     producers can add one element to buffer each. They will wait
     semaphore each time, which will decrement semaphore value.
     empty_sem is initialized to 0, because buffer starts empty and
     consumer cannot take any element from it. They will have to wait
     until producer posts to that semaphore (increments semaphore
     value) */
  pthread_t thread[NUMB_THREADS];
  int thread_numb[NUMB_THREADS];
  int i;
  for (i = 0; i < NUMB_THREADS; ) {
    thread_numb[i] = i;
    pthread_create(thread + i, // pthread_t *t
		   NULL,       // const pthread_attr_t *attr
		   producer,   // void *(*start_routine) (void *)
		   thread_numb + i);  // void *arg
    i++;
    thread_numb[i] = i;
    // playing a bit with thread and thread_numb pointers...
    pthread_create(&thread[i], // pthread_t *t
		   NULL, // const pthread_attr_t *attr
		   consumer, // void *(*start_routine) (void *)
		   &thread_numb[i]);  // void *arg
    i++;
  }
 
  for (i = 0; i < NUMB_THREADS; i++)
    pthread_join(thread[i], NULL);
 
  pthread_mutex_destroy(&buffer_mutex);
  sem_destroy(&full_sem);
  sem_destroy(&empty_sem);
 
  return 0;
}