/*
 * Creating a shared memory segment with POSIX API, 
 * attaching it to the logical address space.  
 $ g++ -Wall sharedMem1a.c++ -lrt             
 $ ./a.out w                                  
 $ ./a.out r                                    
*/
#include <iostream>
using namespace std;
#include <stdlib.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>       
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#define SIZE 4

// sharedMem1a.c++       
int main(int count, char *vect[]) {
    int *p, shmD ;

    if(count < 2) {
       cerr << "No 2nd argument\n";
       exit(1) ;
    }
    shmD = shm_open("/myShm", O_CREAT | O_RDWR, 0777);
    if(shmD == -1){
       cerr << "shm_open() error\n";
       exit(1);
    } 		
    if(ftruncate(shmD, SIZE) == -1){
       cerr << "ftruncate() error\n";
       exit(1);
    }
    p = (int *)mmap(NULL, SIZE, PROT_READ | PROT_WRITE, 
                              MAP_SHARED, shmD, 0) ;
    if(p == MAP_FAILED){
       cerr << "mmap() error\n";
       exit(1); 
    }
    cout << "Attached at VA: " << p << endl;
    if(vect[1][0] == 'w') {  
       cout << "Enter an integer: ";
       cin >> *p ;  // Write data
    }
    else if(vect[1][0] == 'r') // read data
            cout << "The data is:" << *p << "\n";
        // shm_unlink("/myShm");
    return 0 ;
}