How to mmap Eigen into shared memory?

Question

I want to use Eigen library as my shared memory data structure (by mmap ).

here is my code:

producer.cpp:

#include<sys/mman.h>
#include<sys/types.h>
#include<fcntl.h>
#include<string.h>
#include<stdio.h>
#include<unistd.h>

#include <vector>
#include <iostream>
#include <string>
#include "eigen3/Eigen/Dense"

typedef Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::ColMajor> eigen_matrix;
using namespace std;

int main(int argc,char **argv) {
  int fd = open(argv[1], O_CREAT | O_RDWR | O_TRUNC, 0777);
  size_t size = 100 * 100 * sizeof(typename eigen_matrix::Scalar);
  lseek(fd, size, SEEK_SET);
  write(fd,"",1);
  eigen_matrix* p =(eigen_matrix*)mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 
  p->resize(100, 100);
  (*p)(0, 0) = 1;
  double s;
  size_t count = 0;
  while (std::cin >> s && s != 0) {
    (*p)(count++, 0) = s;
  }
  printf("initialize over\n");
  munmap(p, size);
  close(fd);
}

consumer.cpp:

#include<sys/mman.h>
#include<sys/types.h>
#include<fcntl.h>
#include<string.h>
#include<stdio.h>
#include<unistd.h>

#include <vector>
#include <iostream>
#include <string>
#include "eigen3/Eigen/Dense"

using namespace std;
typedef Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::ColMajor> eigen_matrix;

int main(int argc,char **argv) {
  int fd = open(argv[1], O_RDWR, 0777);
  size_t size = 100 * 100 * sizeof(typename eigen_matrix::Scalar);
  lseek(fd, size, SEEK_SET);
  eigen_matrix* p = (eigen_matrix*)mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0); 
  close(fd);
  printf("%zu %zu\n", p->cols(), p->rows());
  cout << *p <<endl;  // here crashed !!
  std::string s;
  while (std::cin >> s && s != "quit") {
    cout << *p << endl;
  }
  munmap(p, size);
}

As you can see in the code, consumer crashed on

cout << *p << endl;

could you help on this? Is there anything i ignored?

Answer 1

Dynamic Eigen matrices are like std::vector. They don't hold the actual data, they contain pointers to the data plus the size information. You mmapped the object, not the actual data.

Something like this should work:

Eigen::Index rows = 100, cols = 100;
const void* space = mmap(NULL, rows * cols * sizeof(double),
                         PROT_READ, MAP_SHARED, fd, 0);
Eigen::MatrixXd::ConstMapType mapped = Eigen::MatrixXd::Map(
      static_cast<const double*>(space), rows, cols);