CODEWITHSUNDEEP
c++dynamicmatrixsegmentation-fault
Richard
Richard

Reputation:

Why do I get a segmentation fault (core dumped) when I try to allocate a 4D array?

I'm trying to allocate a large 4D matrix but I want to do it dynamically. When I just create the static matrix everything works fine so I know I have enough memory for now. However when I try and implement the same thing dynamically it breaks whenever I enter the third dimension and I need to get to a fourth! Can anyone tell me why this code does not work?

#include <iostream>

using namespace std;

static const int time1 = 7;
static const int tlat = 15;
static const int tlon = 17;
static const int outlev = 3;  

int main(void)
{
    //allocate four dimensional dataIn
    int ****dataIn;
    dataIn = new int ***[time1];
    if (dataIn == NULL) { return 1; }

    for(int i = 0 ; i < time1 ; i++) { 
        dataIn[i] = new int **[tlat];
        if (dataIn[i] == NULL) { return 1; }

        for(int j = 0 ; j < tlat ; j++) {
            dataIn[i][j] = new int *[tlon];
            if (dataIn[i][j] == NULL) { return 1; }

            for(int k = 0 ; k < tlon ; k++) {
                dataIn[i][j][k] = new int[outlev];
                if (dataIn[i][j][k] == NULL) { return 1; }
            }
        }
    }
    //there is more code that happens here to add values to dataIn
    //and eventually output it but I know all of that works        
    return 0;
}

I have tried many different variations on this code, and even used malloc instead of new, but I cannot get it working. Any help will be greatly appreciated.

Upvotes: 0

Views: 1259

Answers (5)

janneb
janneb

Reputation: 37208

Check out boost::multiarray, it does exactly what you want, except more efficiently by only doing a single heap allocation.

Upvotes: 0

Sebastian Mach
Sebastian Mach

Reputation: 39089

As pointed out by cmeerw, testing against NULL or 0 is not needed in std-c++.

It would help if you could add a comment to the exact spot where you get a segv.

Also: Which compiler are you using and on which OS?

Upvotes: 0

cmeerw
cmeerw

Reputation: 7356

It compiles and runs fine on my Linux machine.

BTW, with standard C++, new would throw a std::bad_alloc exception (instead of returning NULL) when it's unable to allocate the memory. So it might be worth catching that exception instead of testing for a NULL pointer.

Upvotes: 1

Tom
Tom

Reputation: 44821

You're probably best off allocating all the memory in a flat array and then calculating the indices yourself. Wrapping the whole thing in an object for encapsulation.

class Matrix {
private:
        int* data;        
        int[] sizes;
        int nDimensions;

public:
        // allocates the data pointer and copies the other parameters
        Matrix(int[] sizes, int nDimensions); 

        // frees the data and sizes arrays
        ~Matrix();

        // calculates the cell position and returns it
        int getCell(int[] coordinates);

        // calcultes the cell position and sets its value
        void setCell(int[] coordinates, int value);

private:
        // used by getCell and setCell, calculates the cell's 
        // location in the data array
        size_t calculateCellPosition(int[] coordinates);
};

Upvotes: 2

olliej
olliej

Reputation: 36783

Have you run this in a debugger? To my eyes the code looks fine, but a debugger will tell you where it's crashing which alone may be enough to help you fix it

Upvotes: 2

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