C Quiz: When Is f(n) Actually Called?

Does sizeof evaluate a function call? The answer depends on the type whose size is being determined.

Test your understanding with four expressions involving the same function declaration:

int f(int n);

The question

In which of the following cases is f(n) guaranteed to be called? Select all that apply.

• A: (void)f(n)

• B: sizeof(f(n))

• C: sizeof(int[f(n)])

• D: sizeof(int (*)[f(n)])

The examples use C with support for variable-length arrays (VLAs). Assume each expression is evaluated separately in a valid program, with positive, valid array bounds.

Take a moment to choose your answers before reading on.

Answer and explanation

The correct answers are A and C.

B does not call the function. In D, whether the array-bound expression is evaluated is unspecified, so the call is not guaranteed.

A: The return value is discarded

(void)f(n)

Casting the result to void discards the return value. The function call is still evaluated.

Result: f(n) is called.

B: Only the return type matters

sizeof(f(n))

The declared return type of f is int. To determine the size of this expression, the compiler needs the size of an int. It does not evaluate the function call.

Result: f(n) is not called.

C: The array size depends on the function result

sizeof(int[f(n)])

Here, int[f(n)] specifies a variable-length array type. Its number of elements depends on the value returned by f(n).

That value is needed to determine the array’s size.

Result: f(n) is called.

D: The pointer size does not depend on the array bound

sizeof(int (*)[f(n)])

This expression asks for the size of a pointer to a variable-length array.

Changing the array bound does not change the size of that pointer. For this case, C leaves it unspecified whether the bound expression is evaluated. Portable code must therefore allow for either outcome. See the C11 draft, sections 6.5.3.4 and 6.7.6.2.

Result: f(n) is not guaranteed to be called.

What to remember

The familiar rule that sizeof does not evaluate its operand has an important exception involving variable-length arrays.

When reviewing C code, pay attention to runtime-dependent array bounds and avoid relying on side effects inside sizeof. The SEI CERT C guidance on unevaluated operands explains the associated pitfalls.