// to compile: $gcc -o display display.c -lrt -lpthread
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <sys/mman.h>
#include <semaphore.h> 

int main(void) {
  const char *nameShm = "/ELO330_C1_2s24_SHM";	/* file name  */
  const int SIZE = sizeof(double);		/* share memory size  */
  const char *nameFree = "/ELO330_C1_2s24_FREE";
  const char *nameNew = "/ELO330_C1_2s24_NEW";

  int shm_fd;		  // file descriptor, used with shm_open()
  double *newPrice;  /* base address, used with mmap()  */
  sem_t * free_sem, *new_sem;

  /* open the shared memory segment as if it were a file */
  shm_fd = shm_open(nameShm, O_RDONLY, 0666);
  /* map the shared memory segment to the address space of the process */
  newPrice = mmap(0, SIZE, PROT_READ, MAP_SHARED, shm_fd, 0);
  /** Open the two named semaphores   */
  /* create and initialize the semaphore  */
  free_sem = sem_open(nameFree, 0);
  new_sem = sem_open(nameNew, 0);
  do {
    sem_wait(new_sem);
    sem_wait(free_sem);
    /* read from the mapped shared memory segment */
    printf("%.2f\n", *newPrice);
    sem_post(free_sem);
  } while (*newPrice > 0);
  sem_close(free_sem);
  sem_unlink(nameFree);
  sem_close(new_sem);  
  sem_unlink(nameNew);
  /* remove the mapped shared memory segment from the address space of the process */
  munmap(newPrice, SIZE);
  /* close the shared memory segment as if it was a file */
  close(shm_fd);
  exit(0);
}