/* This program shows how a pair of processes can come to an agreement on their incoming-outgoing pipes. 
*  To run, first compile the program using 
*
*   g++ -lpthread agreement.cpp -o agreement
*
*  Then run two instances of the program using ./agreement in two separate terminals.
*
*/

#include <iostream>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
#include <errno.h>
#include <fcntl.h>
#include <string>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/wait.h>

#define FIRST_TO_SECOND       "FIFO1"
#define SECOND_TO_FIRST       "FIFO2"

using namespace std;

string outgoing, incoming;

int main()
{

    char c;
    int shmid;
    key_t key;
    static int *b;

    key = 5678;

    /*
    * Create one-byte shared memory segment. Each process reads this bit, if it is 1,
    * changes it to zero and if it is zero changes it to one. Depending on the initial 
    * value, the incoming and outgoing pipes are created, to ensure the two processes use different pipes for incoming/outgoing communication.
    */
    if ((shmid = shmget(key, 1, IPC_CREAT | 0666)) < 0) {
        cerr<<"shmget";
        exit(1);
    }

    /* A binary semaphore to be used for mutual exclusion */	
    sem_t * sem_id;
    sem_id=sem_open("mysem2",  O_CREAT, 0, 1);
    if(sem_id == SEM_FAILED) {
        perror("sem_open");
        return 1;
    }
    /* Semaphore is used for mutual exclusion in access to the shared bit*/
    if(sem_wait(sem_id) < 0) {
        perror("sem_wait");
    }

   /*
    * Now we attach the segment to our data space.
    */
    if ((b = ( int *)shmat(shmid, 0, 0)) == (int *) -1) 
    {
        cerr<<"shmat";
        exit(1);

    }
    cout<<getpid() << ": the bit is " << *b <<endl;
    if(*b == 0){
	*b = 1;
	outgoing = FIRST_TO_SECOND;
	incoming = SECOND_TO_FIRST;
        cout << "Process #" << getpid() << ": changed it to " << *b <<endl;
    }else{
	*b = 0;
	incoming = FIRST_TO_SECOND;
	outgoing = SECOND_TO_FIRST;
        cout <<"Process #" << getpid() << ": changed it to " << *b <<endl;
    }

    if(sem_post(sem_id) < 0) {
        perror("sem_post");
    }
    sem_close(sem_id);
    sem_unlink("mysem2"); /* Removes the semaphore when all the processes using it exit */

    cout <<  "I will read from the pipe '" << incoming << "' and write to the pipe '" << outgoing <<"'."<<endl; 

    /* Here the sender and receiver threads must start ... */
   // sleep(20); /* */
}