For FFI, can I depend on const &'static addresses?

help
1

Hello Rust forum!

I'm unsure as to when I can depend on a &'static reference to a const to be stable enough to send over an FFI boundary.

Originally I thought all consts were inlined but it seems more nuanced than that. Can I depend on const going through static memory promotion to send a pointer to it over an FFI boundary? My thinking is that if a const is inlined (therefore on the stack), then I can't depend on that address.

Here's an example of what I was trying to work with to get a better understanding:

struct TestInner {
    val: usize
}

struct ConstTest;

impl ConstTest {
    const TEST_INNER: TestInner = TestInner {
        val: 12
    };
    
    pub fn get() -> &'static TestInner {
        &Self::TEST_INNER
    }
}

fn main() {
    let test_1 = ConstTest::get();
    let test_2 = ConstTest::get();
    println!("{:p}", test_1 as *const TestInner);
    println!("{:p}", test_2 as *const TestInner);
}

Can I send test_1 as *const TestInner over to C code? The C code assumes the pointer will be valid past the current stack frame (and in my head this is valid since it lives in static memory).

Thank you!

2

If you have a &'static T to anything, then it's fine to send over FFI, no matter how you got it. It will definitely be valid forever.

3 Likes
3

If you get a static reference out of a const, you can be sure it's been promoted. However, due to the inlining-like quality you mention, you may get more than one distinct &'static address from a single const definition. If this is a problem, you'll need a static to reference.

2 Likes
4

Ooo, okay. So in that example I posted, I could theoretically get 2 different addresses in the println! statements but I can confident that both will be valid forever? A static item, though, guarantees me the same address.

5

Right. Even if you have a const SR: &'static i32 = &32, the entire expression is inlined, so the literal on the right may get promoted in different places. If so, though, they'll all be valid forever.

See: constant reference has unstable address acrosss compilation units. · Issue #72004 · rust-lang/rust · GitHub

1 Like
6

I really hate it when people say "well actually...", but there are obscure situations where the guarantees Rust makes around &'static T references are broken.

One that comes to mind is where the OS unloads part of your address space. For example, if you load a library at runtime with something like dlopen(), all &'static str string literals passed to the main application by the library will become invalid when the library is closed.

7

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