unspecified dimension of array initialisation with {}

Question

I am wondering if:

int a[] = {1, 2};

allocates sizeof(int) * number of constants inside brackets

int a[5] = {1, 2};

assigns constants to array fields from 0 to 1 and then fills with 0

int a[5] = {};

fills with 0

What happens when I do:

int a[] = {};

Thanks.

Answer 1

int a[5] = {};

and

 int a[] = {};

are not valid C definitions.

In GNU C (C with gcc extensions), you can use empty {} and it is considered the same as {0}.

Note that int [] is a incomplete type. When initializing an array of an incomplete type with explicit initializers, the type is completed and the number of elements of the array is then the number of elements in the brace enclosed initializer list.

So int a[] = {0}; defines an array of 1 element in C and in GNU C int a[] = {}; does the same.