Best way to convert 2D vector to C type (for SGX enclave)

I'm trying to write an SGX enclave that takes in a 2D vector of int (representing a grayscale image), but edger8r accepts only C types.

I am aware of this and this, but could someone provide an example on how to transform a type like:

vector< vector <int> > ImagePixels;

into a C type so that I can pass it to an SGX enclave?

std::vector has a data() method that will give you a plain pointer to its data. However, you'll need to do something with that second layer of vectors, so you'll need to create a temporary array that will store a list of plain pointers from the inner vector.

#include <algorithm>
#include <iostream>
#include <vector>


extern "C" int my_c_function(int **double_pointer, int rows, int cols) {
    for (int i = 0; i < rows; ++i) {
        for (int j = 0; j < cols; ++j) {
            printf("%d ", double_pointer[i][j]);
        }
        printf("\n");
    }
}


int main() {
    std::vector<std::vector<int>> ImagePixels{{1, 2, 3}, {4, 5, 6}};

    // transform a vector of vectors into a vector of pointers
    std::vector<int*> ptrs(ImagePixels.size());
    std::transform(begin(ImagePixels), end(ImagePixels), begin(ptrs),
        [](std::vector<int> &inner_vec) {
            return inner_vec.data();
        });

    my_c_function(ptrs.data(), ImagePixels.size(), ImagePixels[0].size());
}

Live example: https://onlinegdb.com/BJvb3yG3f

However, if you have control of the entire code, I would suggest using a flat structure, where you just store the whole picture into a falt std::vector<int> row-by-row. You could even do a simple wrapper around std::vector to make it easier to manipulate such a structure:

class image {
public:
    image(int rows = 0; int cols = 0)
        : rows_{rows}, cols_{cols}, data_(rows * cols) {}

    int &operator ()(int row, int col) {
        return data_[row * cols_ + col];
    }

    int *data() { return data_.data(); };

    // needs a few more utilities and constant versions of the methods
private:
    std::vector<int> data_;
    int rows_;
    int cols_;
};

Then you can do:

image im(3, 2);
// set pixels
im(0, 0) = 1;
im(0, 1) = 2;
int *d = im.data(); // a plain pointer to an array of data