Stack overflow C++ - c++

Question

This is my code. When I access dtr array in initImg function it gives a stack overflow exception. What might be the reason?
#define W 1000
#define H 1000
#define MAX 100000
void initImg(int img[], float dtr[])
{
for(int i=0;i<W;i++)
for(int j=0;j<H;j++)
img[i*W+j]=255;
for(int j=0;j<H;j++)
{
img[j] = 0;
img[W*(W-1)+j] = 0;
}
for(int i=0;i<W;i++)
{
img[i*W] = 0;
img[i*W+H-1] = 0;
}
for(int i=0;i<W;i++)
for(int j=0;j<H;j++)
{
if(img[i*W+j]==0)
dtr[i*W+j] = 0; // <------here
else
dtr[i*W+j] = MAX; // <------here
}
}
int main()
{
int image[W*H];
float dtr[W*H];
initImg(image,dtr);
return 0;
}

Answer 1

This:
int image[W*H];
float dtr[W*H];
Creates each a 4 * 1000 * 1000 ~ 4 MB array into the stack. The stack space is limited, and usually it's less than 4 MB. Don't do that, create the arrays in the heap using new.
int *image = new int[W*H];
float *dtr = new float[W*H];

Answer 2

Your stack probably isn't big enough to hold a million ints and a million floats (8MB). So as soon as you try to access beyond your stack size, your operating system throws you an error. Objects or arrays above a certain size need to be allocated on the heap - preferably using a self-managing self-bounds-checking class such as std::vector - the specific size depends on your implementation.

Answer 3

In addition to the stack overrun, you have another problem -- one which is masked by your definitions of W and H.
for(int i=0;i<W;i++)
for(int j=0;j<H;j++)
{
if(img[i*W+j]==0)
dtr[i*W+j] = 0; // <------here
else
dtr[i*W+j] = MAX; // <------here
}
Your i loop should count from 0 to H-1, rather than W-1 (and the j loop should swap as well). Otherwise your code will only work correctly if W==H. If WH you will overrun your buffers.
This same problem exists elsewhere in your code sample as well.

Answer 4

You're creating giant arrays on the stack. Just use std::vector instead:
std::vector<int> image(W*H);
std::vector<float> dtr(W*H);

Answer 5

Your stack is full. You can allocate memory in heap or increase the stack memory. From what I know the maximum size is about 8MB, but this is not a very good idea. The best solution is to use heap allocation or some containers (vector) available in std.

Answer 6

You will eventually get to
dtr[W*W+j] = 0;   <------here
Which is much more than you have allocated.

Answer 7

Your compiler will define the stack size. A way to get around this is to dynamically allocate your arrays using std::vector array_one(W*H).

Answer 8

You are trying to allocate memory from stack. the maximum memory which can be allocated using stack is complier dependent.
So try something like this to avoid this kind of exception.
#include <stdlib.h>
#define W 1000
#define H 1000
#define MAX 100000
void initImg(int img[], float dtr[])
{
for(int i=0;i<W;i++)
for(int j=0;j<H;j++)
img[i*W+j]=255;
for(int j=0;j<H;j++)
{
img[j] = 0;
img[W*(W-1)+j] = 0;
}
for(int i=0;i<W;i++)
{
img[i*W] = 0;
img[i*W+H-1] = 0;
}
for(int i=0;i<W;i++)
for(int j=0;j<H;j++)
{
if(img[i*W+j]==0)
dtr[i*W+j] = 0; // <------here
else
dtr[i*W+j] = MAX; // <------here
}
}
int main()
{
int *image = (int*)malloc(4*W*H); //Malloc the memory....(Allocated from Heap..)
float *dtr = (float*)malloc(4*W*H);
if(image && dtr) //If none of the ptr is NULL. Means memory is allocated...
{
initImg(image,dtr);
}
return 0;
}
You can use new as well instead of using malloc to allocate memory from heap...