Creating a 2d Array having issues (C)

Question

The code below is me trying to instantiate a 2d array, and it instantiating incorrectly: THE CODE:

    FILE* kernalFile = fopen(argv[1], "r");
    int rKernalSize;
    fscanf(kernalFile, "%d", &rKernalSize);
    unsigned int rKernal[rKernalSize][rKernalSize];

Data froma break point right after that code is ran: rKernalSize VALUES:

Name : rKernalSize
    Details:3
    Default:3
    Decimal:3
    Hex:0x3
    Binary:11
    Octal:03

rKernal VALUES:

Name : rKernal
    Details:0x7ffffffe0cd0
    Default:[0]
    Decimal:[0]
    Hex:[0]
    Binary:[0]
    Octal:[0]

or

rKernal[][0]

It should be rKernal[3][3] and here is the file so you can look at it. If wanted:

3 -1 1 0 1 0 -1 0 -1 1 3 -1 1 0 1 0 -1 0 -1 1 3 -1 1 0 1 0 -1 0 -1 1

TLDR: rKernalSize is correct (3) but when I create the 2d Array with rKernal[rKernalSize][rKernalSize] it does not instantiate correctly! It instatiates as rKernal[][0] maybe thats default but should be rKernal[3][3]

Answer 1

The basic problem here is that rKernalSize isn't known at compile time when that array is being allocated. At compile time, the value of int rKernalSize is compiler-dependent (unless the new C standard explicitly makes it 0; 0's the value I'd bet on anyway.) So when the code is loaded, there's a symbol rKernal that represents the address of a secrtion of memory containing no bytes.

But then you run the program, and read '3' with your scanf; when you dumpt the results, you see '3'.

This code, by the way, wouldn't work in straight C -- you must allocate before the first executable statement. This will compile in C++.

Now, if you want, using straight C, to do something like this, here's what you need:

• Read your size as you have using scanf.

• Allocate the memory for your array using malloc, which will look something like

x

int ** ary;
int rkSize;  // what you scanf'd into
if((ary = malloc(rkSize*rkSize*sizeof(unsigned int)))==NULL){
     // for some reason your malloc failed.  You can't do much
     fprintf(stderr,"Oops!\n");
     exit(1);
 }
 // If you got here, then you have your array

• Now, because of the pointer-array duality in C, you can treat this as your array x

  ary[1][1] = 42; // The answer

Answer 2

Forget what the debugger is telling you. In your code, immediately following:

unsigned int rKernal[rKernalSize][rKernalSize];

put the statement:

printf ("%d\n", sizeof(rKernal) / sizeof(unsigned int));

and see what it prints out (hopefully 9).

It's possible the debugging information, created at compile time, is not enough to properly determine the sizes of variable length arrays.

---

By way of example, even though gcc supports variable length arrays, gdb itself couldn't properly handle them as recently as late 2009 and there's still no mention of them in the documentation dated October 2010.

So I suspect that is the problem, especially since the test code I provided above output 9 as expected.