C -multidimensional array causes Segmentation Fault (GCC)

Question

Why if I change the size of one of the dimensionality of an array I get a run-time Error: "Segmentation Fault?". Example:

#include <stdio.h>
#define DIM 8 
int main(int argc, char **argv)
{
    int a[3][3][3][3][3][3][3][3][3][3][DIM],
        b;
    a[1][1][1][1][1][1][1][1][1][1][1] = 2;
    b=a[1][1][1][1][1][1][1][1][1][1][1];
    printf("%d \n",b);
    return 0;
}

If DIM is 8 it generated no run-time error, but just if DIM is greater than 8, it causes run-time Error "Segmentation Fault". Why ???

int3 · Accepted Answer · 2009-11-20 16:25:49Z

7

Almost certainly a stack overflow. You are allocating, what, 3 ^ 10 * 9 * sizeof(int) bytes! Use int *a = (int*)malloc(N * sizeof(int)) instead, where N is the number of ints you want. Then you can simulate an N-dimensional array on it.

I'll explain how to simulate a 2D array on a 1D array.. Say it has rows of width 10. Then you access the 5th value on row three by taking a[10 * 2 + 5]. In general, you do a[width * (row - 1) + column].

Second method. You can allocate an array of pointers to pointers of ints:

int **a = (int**)malloc(rows * sizeof(int*))
for (int i=0; i<row; ++i)
    a[i] = (int*)malloc(columns * sizeof(int))

... extending this to more dimensions left as an exercise for the reader.

edited Nov 20, 2009 at 16:25

answered Nov 20, 2009 at 16:07

int3

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3 Comments

psihodelia Over a year ago

but I need 11-dimensional array and C has no new[] operand

Ant Over a year ago

You mean use malloc() - this is a C question, not C++.

int3 Over a year ago

@Ant: sorry, fixed it. @Norma: why would you need that in the first place?! You can simulate it (I'll add that in), or use an int*******... but seriously speaking you probably have a design flaw.

pmg · Accepted Answer · 2009-11-20 16:12:23Z

1

3*3*3*3*3*3*3*3*3*3*8 = 472392; 472392*4 /* sizeof (int) */ = 1889568
3*3*3*3*3*3*3*3*3*3*9 = 531441; 531441*4 /* sizeof (int) */ = 2125764

I guess your stack is limited to 2Mbytes

answered Nov 20, 2009 at 16:12

pmg

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Comments

shf301 · Accepted Answer · 2009-11-20 16:11:36Z

1

The size of your array is 3^10 * 8 * sizeof(int). Assuming a 32 bit int, sizeof(int) is four bytes and the size of your array is:

3^10 * 8 * 4 = 1,889,568 bytes

So you're stack isn't that big and you're overflowing the stack.

answered Nov 20, 2009 at 16:11

shf301

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