#include <stdio.h>          /* printf()                 */
#include <stdlib.h>         /* exit(), malloc(), free() */
#include <sys/types.h>      /* key_t, sem_t, pid_t      */
#include <sys/shm.h>        /* shmat(), IPC_RMID        */
#include <errno.h>          /* errno, ECHILD            */
#include <semaphore.h>      /* sem_open(), sem_destroy(), sem_wait().. */
#include <fcntl.h>          /* O_CREAT, O_EXEC          */
#include <unistd.h>
#include <sys/wait.h>
#include <sys/shm.h>


typedef sem_t Semaphore;


/* semaphores are usually declared global variables */
/* synch semaphore                  */
Semaphore *LP1;
Semaphore *LP2;

int main ()
{
    int   i;                        /*      loop variables  */
    pid_t pid;                   /*      fork pid        */
    int child_status;

    /* Declaring Shared memory variables */
    key_t shmkey;
    int shmid;
    int *x;

    /* Initializing a shared variable in shared memory */
     shmkey = ftok ("/dev/null", 5);
     shmid = shmget(shmkey, sizeof (int), 0644 | IPC_CREAT);

    /* initialize semaphores for shared processes           */
    LP1 = sem_open ("LefterisP1", O_CREAT | O_EXCL, 0644, 0/*Initial value */);
    LP2 = sem_open ("LefterisP2", O_CREAT | O_EXCL, 0644, 0/*Initial value */);

    /* fork child processes                                 */
    for (i=0; i<1; i++)
    {
        pid = fork ();

        if (pid == 0)
    {
           break;
        }
    }

    /* 1st child */
    if (pid == 0)
    {

      sem_wait(LP2);
      printf("I timi tou x prin tin diergasia 1: %d\n", *x);
      *x = *x * 2;
      printf("I timi tou x meta tin diergasia 1: %d\n", *x);
      wait(NULL);

   }
   /* 2nd child */
   else if (pid != 0 && pid == 0)
   {

       printf("I timi tou x prin tin diergasia 2: %d\n", *x);
       *x = *x * (*x);
       printf("I timi tou x meta tin diergasia 2:%d\n", *x);
       sem_post(LP1);
       wait(NULL);

   }
    else
      {
       sem_wait(LP1);
       printf("I timi prin tin diergasia 3:%d\n", *x);
       *x = *x + 3;
       printf("I timi meta tin diergasia 3:%d\n", *x);
       sem_post(LP2);


           printf(" Parent unlinks the semaphores\n Bye\n");

           /* unlink prevents the semaphore existing forever */
           /* if a crash occurs during the execution         */
           sem_unlink ("LefterisP1");
           sem_close(LP1);
           sem_unlink ("LefterisP2");
           sem_close(LP2);
       }
       exit (0);
   }