Could not allocate memory

Question

In my C code I am allocating memory for 2d array double E[2000][2000]; but when I run it gets a runtime error Segmentation fault(core dumped) and when

Answer 1

Are you declaring E as a local variable ? If so, you're running out of stack memory.

void func()
{
    double E[2000][2000]; /// definitely an overflow
}

Use dynamic allocation:

double* E = malloc(2000 * 2000 * sizeof(double));
/// don't forget to "free(E);"  later

Or if you need the 2D array, use a zig-zag:

double** E = malloc(2000 * sizeof(double*));

/* check that the memory is allocated */
if(!E)
{
    /* do something like exit(some_error_code);  to terminate your program*/
}

for(i = 0 ; i < 2000 ; i)
{
      E[i] = malloc(2000 * sizeof(double));

     /* check that the memory for this row is allocated */
     if(!E[i])
     {
        /* do something like exit(some_error_code);  to terminate your program*/
     }
}

Then the deallocation is a little more complicated:

for(i = 0 ; i < 2000 ; i)
{
      free(E[i]);
}

free(E);

P.S. If you want to keep all of you data in a continuous way, there's a trick (code from Takuya Ooura's FFT Package)

double **alloc_2d(int n1, int n2)
{
    double **ii, *i;
    int j;

    /* pointers to rows */
    ii = (double **) malloc(sizeof(double *) * n1);

    /* some error checking */
    alloc_error_check(ii);

    /* one big memory block */
    i = (double *) malloc(sizeof(double) * n1 * n2);

    /* some error checking */
    alloc_error_check(i);

    ii[0] = i;
    for (j = 1; j < n1; j++) {
        ii[j] = ii[j - 1] + n2;
    }
    return ii;
}

void free_2d(double **ii)
{
    free(ii[0]);
    free(ii);
}

The you just call

double** E = alloc2d(2000, 2000);

and

free_2d(E);