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

int main(void)  {
  const char *nameShm = "/ELO330_C1_2s24_SHM";	/* file name */
  const int SIZE = sizeof(double);		/* file 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 *price;	/* base address, used with mmap()  */
  double newPrice;
  sem_t * free_sem, *new_sem;

  /* create the shared memory segment as if it were a file */
  shm_fd = shm_open(nameShm, O_CREAT | O_RDWR, 0666);
  /* configure the size of the shared memory segment */
  ftruncate(shm_fd, SIZE);
  /* map the shared memory segment to the address space of the process */
  price = mmap(0, SIZE, PROT_WRITE, MAP_SHARED, shm_fd, 0);  // just for writing
  /** Create two named semaphores
  /* first remove the semaphore if each one already exists */
  sem_unlink(nameFree);
  sem_unlink(nameNew);
  /* create and initialize the semaphore  */
  free_sem = sem_open(nameFree, O_CREAT, 0666, 1);
  new_sem = sem_open(nameNew, O_CREAT, 0666, 0);
  do {
    printf("New price: ");
    scanf("%lf", &newPrice);
    sem_wait(free_sem);
   /** Write to the mapped shared memory region.  */
    *price=newPrice;
    sem_post(new_sem);
    sem_post(free_sem);
  } while (newPrice > 0);
  sem_close(free_sem);
  sem_close(new_sem);
  /* remove the mapped memory segment from the address space of the process */
  munmap(price, SIZE);
  /* close the shared memory segment as if it was a file */
  close(shm_fd);
  exit(0);
}