Problem with fragmentation(Dynamic matrix Allocation)

[Tinjak Mirza]

Hi,

i have one example with dynamic allocating 2-d arrays. The this generic function must return a pointer on pointer. Type is type of element in 2-d matrix given by the parameter. 2-d matrix can be deque, vector, but must support size(). I have problem with segmentation. It fails when i for example send this:

std::vector<std::deque<double>> m {{1,2,3}, {1,2,3}, {1,2,3}};

It crash when:

Dynamic [0] = new decltype(Matrix.at(0).at(0)+ Matrix.at(0).at(0))[size];

It display segmentation failed that means that i enter part of memory which is not allocated. But when i try to set this to extra try{}catch(){} construction it did'nt catch any exception.


I am glad that problem is well defined.
I use continuous memoory allocation, and with one for loop I set pointers to imitate 2-d arrays that can be used

for(int i = 0; i < n; i++)  // rows
{
   for(int j = 0; j < m; j++) //columns
     // something
}

template<typename tip>
tip** Function(tip a)
{
    return a;
}
template<typename Container>
auto MakeDynamicCopy2D(Container &Matrix) -> decltype(Matrix.at(0).at(0) + Matrix.at(0).at(0))**
{
	decltype(Function(Matrica.at(0).at(0) + Matrix.at(0).at(0)))Dynamic = nullptr;
	 try {
		
     Dynamic = new decltype(Matrix.at(0).at(0)+ Matrix.at(0).at(0))*[Matrix.size()];
	 }
     catch(std::bad_alloc) {
         delete [] Dynamic; 
         throw;
     }
	
	try {
     int size = 0;
     int k = 0;
     while (k < Matrica.size()) size+=Matrix.at(k++).size();
     Dynamic [0] = nullptr;
     Dynamic [0] = new decltype(Matrix.at(0).at(0)+ Matrix.at(0).at(0))[size];

	for(int i = 1; i < Matrix.size();i++) {
	  Dynamic [i] = Dynamic [i-1] + Matrix.at(i-1).size();
	}
	for(int i = 0; i < Matrix.size();i++)
	{
		for(int j = 0; j < Matrix.at(i).size();j++)
		Dynamic [i][j] = Matrix.at(i).at(j);
	}
	}
	catch(std::bad_alloc) {
	    if(Dynamic ) delete[] Dynamic [0];
	    delete[] Dynamic ;
	} 
	return Dynamic ;	
}

Thanks forward. Greetings

[dhayden]

Please use code tags. Edit your post, highlight the code, and then click the <> button to the right of the edit window.

template < typename tip > tip ** Function(tip a) {
    return a;
}

First of all, why?? If Function(a) returns a, then why have the function at all?
Second, you declare the function as returning a tip** but you actually return a tip. That's never going to work. What are you trying to accomplish here?

template < typename Container >
auto MakeDynamicCopy2D(Container & Matrix)->
    decltype(Matrix.at(0).at(0)+Matrix.at(0).at(0)) **
{
    typedef decltype(Matrix.at(0).at(0)+Matrix.at(0).at(0)) BaseType;
    BaseType **Dynamic = new BaseType *[Matrix.size()];

    int size = 0;
    for (unsigned k = 0;k < Matrix.size(); ++k) {
        size += Matrix.at(k).size();
    }
    Dynamic[0] = new BaseType[size];

    for (unsigned i = 1; i < Matrix.size(); i++) {
        Dynamic[i] = Dynamic[i - 1] + Matrix.at(i - 1).size();
    }
    for (unsigned i = 0; i < Matrix.size(); i++) {
        for (unsigned j = 0; j < Matrix.at(i).size(); j++)
            Dynamic[i][j] = Matrix.at(i).at(j);
    }
    return Dynamic;
}

[Tinjak Mirza]

Main function is function named MakeDynamicCopy2D. I use ' Function' to detect type of variable. It can be without it.


Problem is in MakeDynamicCopy2D. I get segmentation failed when i send

std::vector<std::deque<double>> m {{1,2,3}, {1,2,3}, {1,2,3}};

On this line:

Dynamic [0] = new decltype(Matrix.at(0).at(0)+ Matrix.at(0).at(0))[size];

It didn't catch any exception. But i have warning memory error and memory leak.

[ne555]

Function() doesn't need a body (in fact, it is never instantiated because it wouldn't compile)
it's just there for the decltype crap, to avoid writing **


can't reproduce your crash
provide a testcase, and the debug backtrace
may also want to run through valgrind to check for invalid access

by the way, Dynamic [0] = nullptr; would be invalid if the matrix is empty

[dutch]

What does this have to do with fragmentation?

[ne555]

l'I1 blame bad translation
segmentation fault, memory segment, memory segmentation, fragmentation

[Tinjak Mirza]

I have found solution. Ne555 thanks you

[dhayden]

Your problem is somewhere else. You're probably corrupting the heap somewhere and the problem doesn't show up until you execute this code. This works fine for me:

#include <vector>
#include <deque>
#include <iostream>

using std::vector;
using std::deque;
using std::cout;

template < typename Container >
auto MakeDynamicCopy2D(Container & Matrix)->
    decltype(Matrix.at(0).at(0)+Matrix.at(0).at(0)) **
{
    typedef decltype(Matrix.at(0).at(0)+Matrix.at(0).at(0)) BaseType;
    BaseType **Dynamic = new BaseType *[Matrix.size()];

    int size = 0;
    for (unsigned k = 0;k < Matrix.size(); ++k) {
	size += Matrix.at(k).size();
    }
    Dynamic[0] = new BaseType[size];

    for (unsigned i = 1; i < Matrix.size(); i++) {
	Dynamic[i] = Dynamic[i - 1] + Matrix.at(i - 1).size();
    }
    for (unsigned i = 0; i < Matrix.size(); i++) {
	for (unsigned j = 0; j < Matrix.at(i).size(); j++)
	    Dynamic[i][j] = Matrix.at(i).at(j);
    }
    return Dynamic;
}

int main()
{
    std::vector<std::deque<double>> m {{1,2,3}, {1,2,3}, {1,2,3}};

    auto TwoD = MakeDynamicCopy2D(m);
    for (unsigned i=0; i<3; ++i) {
	for (unsigned j=0; j<3; ++j) {
	    cout << TwoD[i][j] << ' ';
	}
	cout << '\n';
    }
}

[Tinjak Mirza]

dhayden,


I also concluded that. I was running it on server that has corrupted autotest. It was error in autotest. Thank you.