What does—or did—“volatile void function( … )” do?

Question

I\'ve seen How many usage does "volatile" keyword have in C++ function, from grammar perspective? about use of the volatile keyword on functions, but there was no clea

Answer 1

The C99 standard says this in §6.7.3/3:

The properties associated with qualified types are meaningful only for expressions that are lvalues.¹¹⁴⁾

114) The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used

§6.2.5/19 says:

The void type comprises an empty set of values; it is an incomplete type that cannot be completed.

And §6.3.2.1/1 says:

An lvalue is an expression with an object type or an incomplete type other than void;⁵³⁾ [...]

Hence, void is not an lvalue, so the type qualifiers (const, volatile, and restrict) are not meaningful for expressions of type void. So, in any C99-compliant compiler, const void and volatile void are meaningless (although pointers to const void and const volatile are meaningful).

Furthermore, the constraints of §6.9.1/3 disallow a function to return a qualified type of void:

The return type of a function shall be void or an object type other than array type.

Since this is a constraint, a conforming compiler must issue a diagnostic (§5.1.1.3/1). So a function returning volatile void is not allowed in C99.

As for what volatile void may have used to do, I have no idea and can't really speculate. The AES code you're looking at probably just has old cruft that never got cleaned up, I'd guess.