CODEWITHSUNDEEP
cc23
Matthew Schell
Matthew Schell

Reputation: 679

What's the point of the __unused attribute in C?

I've got a question. Let's say you define a function or variable in C. If you don't use it, the compiler generates a warning that this variable/function wasn't used. To suppress the warning, you use the __unused attribute before the declaration. If you have to use that, what's the point of declaring a function/variable in the first place? What's good about using __unused?

Upvotes: 1

Views: 2383

Answers (1)

Acorn
Acorn

Reputation: 26136

__unused is usually a macro that expands to a C language extension. However, C23 will give us [[maybe_unused]], and the standard comes with an example:

EXAMPLE

[[maybe_unused]] void f([[maybe_unused]] int i) {
    [[maybe_unused]] int j = i + 100;
    assert(j);
}

Implementations are encouraged not to diagnose that j is unused, whether or not NDEBUG is defined.

That is, the assert() macro will expand to nothing in NDEBUG, which means the compiler will think j is not used.

Of course, an alternative to this would be to put the entire j definition (and optionally its usage) inside an #ifdef "block", but that would be cumbersome in this case. Furthermore, if you do that, then i would be the one unused, etc.

There are other use cases, like unused function parameters if you want to keep their names and avoid the (void)x; trick, dealing with a confused optimizer/compiler in some cases, external uses of symbols that cannot be seen by the compiler/linker, etc.

Upvotes: 1

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