/**
 * Simple program demonstrating shared memory in POSIX systems.
 * This is the consumer process
 *
 * @author Agustín J. González 
 * One line at the time is read from stdin by producer and written in the shared memory.
 * Then the consumer reads each line and displays it.
 * This a variant of a very similar program by Gagne, Galvin, Silberschatz
 * Operating System Concepts - Ninth Edition
 * Copyright John Wiley & Sons - 2013
 *
 * modifications by dheller@cse.psu.edu, 31 Jan. 2014
 * modifications by Agustín J. González, 14 Oct. 2019
 *    gcc consumer.c -lpthread -lrt
* modifications by Agustín J. González, 27 Sept. 2022
 *    display function was removed to focus the code on sharing mechanism
  */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <errno.h>
#include <string.h>
#include <semaphore.h> 

void display(char *prog, char *bytes, int n);

int main(void)
{
  const char *name = "/shm-example";	/* file name  */
  const int SIZE = 4096;		/* file size  */
  const char *nameEmpty = "/EMPTY";
  const char *nameFull = "/FULL";

  int shm_fd;		  // file descriptor, used with shm_open()
  char *shm_base;	// base address, used with mmap()
  sem_t * empty_sem, *full_sem;

  /* open the shared memory segment as if it were a file */
  shm_fd = shm_open(name, O_RDONLY, 0666);
  if (shm_fd == -1) {
    printf("cons: Shared memory failed: %s\n", strerror(errno));
    exit(1);
  }

  /* map the shared memory segment to the address space of the process */
  shm_base = mmap(0, SIZE, PROT_READ, MAP_SHARED, shm_fd, 0);
  if (shm_base == MAP_FAILED) {
    printf("cons: Map failed: %s\n", strerror(errno));
    exit(1);   /* Cleanup of shm_open was not done  */
  }

    /**
     * Open the two named semaphores
     */

    /* create and initialize the semaphore  */
    if ( (empty_sem = sem_open(nameEmpty, 0)) == SEM_FAILED)
        printf("Error opening %s: %s\n",nameEmpty, strerror(errno));
    if ( (full_sem = sem_open(nameFull, 0)) == SEM_FAILED)
        printf("Error opening %s: %s\n",nameFull, strerror(errno));
    do {
      if (sem_wait(full_sem)!=0) 
          printf("Error waiting %s\n",strerror(errno));
      else {
          /* read from the mapped shared memory segment */
          printf("%s", shm_base);
          if (sem_post(empty_sem)!=0)
             printf("Error posting %s\n",strerror(errno));
       }
     } while (strlen(shm_base) > 0);
     sem_close(empty_sem);
     sem_unlink(nameEmpty);
     sem_close(full_sem);  
     sem_unlink(nameFull);

  /* remove the mapped shared memory segment from the address space of the process */
  if (munmap(shm_base, SIZE) == -1) {
    printf("cons: Unmap failed: %s\n", strerror(errno));
    exit(1);
  }

  /* close the shared memory segment as if it was a file */
  if (close(shm_fd) == -1) {
    printf("cons: Close failed: %s\n", strerror(errno));
    exit(1);
  }

  /* remove the shared memory segment from the file system */
  if (shm_unlink(name) == -1) {
    printf("cons: Error removing %s: %s\n", name, strerror(errno));
    exit(1);
  }
  return 0;
}