Behavior of sizeof on variable length arrays (C only)

Question

My question is how exactly sizeof() behaves when passed argument is a dynamic array variable length array.

Let\'s consider an example:<

Answer 1

My question is how exactly sizeof() behaves when passed argument is a dynamic array.

1. Well, you rather meant a "variable-length array" (VLA).

2. It behaves almost exactly the same: it returns the size of the array in bytes.

sizeof() 'behaves like' a function and not as an operator.

Nah, it never was a function. The only thing that changes is that if used on VLAs, this operator doesn't yield a compile-time constant, otherwise it does.

Answer 2

It still behaves as an operator. Cast is also operator and also evaluates it's argument and so does * or & . Being an operator is a syntactic category. That does not change.

The important distinction is that it behaves as expression while in other cases it behaves as constant.

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Update: I commented below that I don't see why the evaluation makes difference, but now I realized there are two ways you can write sizeof with variable length array. Either you can pass variable declared as variable lenght array:

int a[x];
sizeof(a)

in which case evaluating a indeed makes no difference. But you can also use a type as the argument, which would be

sizeof(int[x])

and in this case the result is x * sizeof(int) and x must be evaluated. Which I suppose is why the specification mentions it.