initialising structs and passing by reference

Question

I'm fairly new to C and I am having trouble working with structs. I have the following code:

typedef struct uint8array {
 uint8 len;
 uint8 data[];
} uint8array;

int compare_uint8array(uint8array* arr1, uint8array* arr2) {
  printf("%i %i\n data: %i, %i\n", arr1->len, arr2->len, arr1->data[0], arr2->data[0]);
  if (arr1->len != arr2->len) return 1;
  return 0;
  }

int compuint8ArrayTest() {
 printf("--compuint8ArrayTest--\n");
 uint8array arr1;
 arr1.len = 2;
 arr1.data[0] = 3;
 arr1.data[1] = 5;    

 uint8array arr2;
 arr2.len = 4;
 arr2.data[0] = 3;
 arr2.data[1] = 5;    
 arr2.data[2] = 7;    
 arr2.data[3] = 1;    

 assert(compare_uint8array(&arr1, &arr2) != 0);
}

Now the output of this program is:

 --compuint8ArrayTest--
 3 4
 data: 5, 3

Why are the values not what I initialized them to? What am I missing here?

Answer 1

In your case, uint8 data[]; is a flexible array member. You need to allocate memory to data before you can actually access it.

In your code, you're trying to access invalid memory location, causing undefined behavior.

Quoting C11, chapter §6.7.2.1 (emphasis mine)

As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. In most situations, the flexible array member is ignored. In particular, the size of the structure is as if the flexible array member were omitted except that it may have more trailing padding than the omission would imply. Howev er, when a . (or ->) operator has a left operand that is (a pointer to) a structure with a flexible array member and the right operand names that member, it behaves as if that member were replaced with the longest array (with the same element type) that would not make the structure larger than the object being accessed; the offset of the array shall remain that of the flexible array member, even if this would differ from that of the replacement array. If this array would have no elements, it behaves as if it had one element but the behavior is undefined if any attempt is made to access that element or to generate a pointer one past it.

A proper usage example can also be found in chapter §6.7.2.1

EXAMPLE 2 After the declaration:

struct s { int n; double d[]; };

the structure struct s has a flexible array member d. A typical way to use this is:

int m = /* some value */;
struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));

and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if p had been declared as:

struct { int n; double d[m]; } *p;