Boost::mpi sending array - c++

Question

Good Morning
I'm implementing a distributed image normalization algorithm an I'm using Boost::mpi with a class Pixel that contain the serialization code,
#ifndef PIXEL_H
#define PIXEL_H
#include <boost/mpi.hpp>
#include <boost/serialization/access.hpp>
class Pixel
{
private:
unsigned char m_red;
unsigned char m_green;
unsigned char m_blue;
friend class boost::serialization::access;
template <class Archive>
void serialize(Archive &ar, const unsigned int version) {
ar & m_red;
ar & m_green;
ar & m_blue;
}
public:
Pixel();
Pixel(unsigned char red,unsigned char green,unsigned char blue) : m_red(red), m_green(green), m_blue(blue) {};
virtual ~Pixel();
unsigned char getRed();
void setRed(unsigned char val);
unsigned char getGreen();
void setGreen(unsigned char val);
unsigned char getBlue();
void setBlue(unsigned char val);
void setColor (unsigned char red,unsigned char green,unsigned char blue);
};
The main.cpp is
#include <iostream>
#include <boost/mpi.hpp>
#include <vector>
#include "include/Pixel.h"
#include <cstdlib>
#include <ctime>
#define ALTEZZA 2
#define LARGHEZZA 2
namespace mpi=boost::mpi;
int main(int argc, char * argv[]) {
std::cout<<"Inizializzazione dell'ambiente MPI"<<std::endl;
mpi::environment env;
mpi::communicator world;
Pixel **vettore;
int i,j;
//Inizializzazione della matrice di test
if(world.rank() == 0){
std::cout<<"Inizializzazione matrice di test..."<<std::endl;
std::srand(std::time(0));
vettore = new Pixel *[ALTEZZA];
for (i = 0; i < ALTEZZA; i++) {
vettore[i] = new Pixel[LARGHEZZA];
}
for (i = 0; i < ALTEZZA; i++) {
for (j = 0; j < LARGHEZZA; j++) {
vettore[i][j].setColor(std::rand() % 256, std::rand() % 256, std::rand() % 256);
std::cout<<"Vettore["<<i<<"]["<<j<<"] = ("<<int(vettore[i][j].getRed())<<","<<int(vettore[i][j].getGreen())<<","<<int(vettore[i][j].getBlue())<<");"<<std::endl;
}
}
}
if (world.rank() == 0) {
std::cout<<"Invio matrice.."<<std::endl;
world.send(1, 0, vettore[0]);
}else {
Pixel *px;
world.recv(0, 0, px);
for (j = 0; j < LARGHEZZA; j++) {
std::cout<<int(px[j].getRed())<<" "<<int(px[j].getGreen())<<" "<<int(px[j].getBlue())<<std::endl;
}
}
return 0;
}
but when i run the program the cout on the receiving process print wrong value like this
Inizializzazione dell'ambiente MPI
Inizializzazione dell'ambiente MPI
Inizializzazione matrice di test...
Vettore[0][0] = (170,103,165);
Vettore[0][1] = (84,0,186);
Vettore[1][0] = (93,228,162);
Vettore[1][1] = (31,100,204);
Invio matrice..
170 103 165
217 1 0
I think that the problem is the 2d array because if I use std::vector i haven't this problem but I don't understand why.

Answer 1

I would imagine you have several problems (I can't test as I don't have a capable MPI installation..)
Firstly, your send() is wrong, currently you are triggering the overload:
template<typename T> void send(int, int, const T &) const;
But you are trying to send a raw array, I imagine the fix here has to be to pass the count, for example:
world.send(1, 0, vettore[0], 2); // 2 Pixels
Secondly, on the receiver side (this I'm not sure about), but I imagine you need to have a suitable array to read the data into.., for example:
Pixel px[LARGHEZZA];
world.recv(0, 0, px, 2);
I think this should fix your problems...