Why do i get a segmentation fault (core dumped) in the program below when i use N=1023 instead of 1024?

Question

Below is a simple program to plot a 2D gaussian, for N=1022 and below the program works fine, but for N=1023 and higher i get Segmentation error(core dumped), am i missing something?

I have not used pointers anywhere, except for writing the data file

#include<stdio.h>
#include<math.h>

void main()
{
    int i, j;
    int N;
    N=1022;
    //Why will i get a segmentation fault if i use 1023 instead of 1024?
    double M[N][N];
    double x[N];
    double y[N];
    double xmax,ymax;
    double dx,dy;
    FILE *fp1,*fp2;
    fp2=fopen("strtfn.txt","w+");

    xmax=10;
    dx=(2*xmax)/N;
    for(i=0; i<N; i++)
    {
        x[i]=-xmax+(i*dx);
    }
    ymax=10;
    dy=(2*ymax)/N;
    for(i=0; i<N; i++)
    {
        y[i]=-ymax+(i*dy);
    }

    for(i=0;i<N;i++)
    {
        for(j=0;j<N;j++)
        {
            M[i][j]=exp(-x[i]*x[i]/10./10.)*exp(-y[j]*y[j]/10./10.);
            fprintf(fp2,"%lf\t%lf\t%lf\n",x[i],y[j],M[i][j]);
        }
    // printf("\n");
    }

    fclose(fp2);

}

Answer 1

The problem is that you don't use pointers. When you have N as 1022 the variable M will be 8355872 bytes large, and on eg Linux the default process stack space (in which local variables are stored) is 8MB (ie just a little bit more than the space needed by your matrix M when N is 1022).

Other platforms have even smaller default stack sizes (Windows, using the VC++ compiler, defaults to only 1MB).

---

There are two ways of solving this: Either make your array(s) and matrix(es) global variables (then they will not be on the stack any more), or use pointers and allocate dynamically off the heap.