Method get

src/weak.rs:67–100

(&self)

    }

    pub(crate) fn get(&self) -> Option<F> {
        assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
        unsafe {
            // Relaxed is fine here because we fence before reading through the
            // pointer (see the comment below).
            match self.addr.load(Ordering::Relaxed) {
                INVALID => self.initialize(),
                NULL => None,
                addr => {
                    let func = mem::transmute_copy::<*mut c_void, F>(&addr);
                    // The caller is presumably going to read through this
                    // value (by calling the function we've dlsymed). This
                    // means we'd need to have loaded it with at least C11's
                    // consume ordering in order to be guaranteed that the data
                    // we read from the pointer isn't from before the pointer
                    // was stored. Rust has no equivalent to
                    // memory_order_consume, so we use an acquire fence (sorry,
                    // ARM).
                    //
                    // Now, in practice this likely isn't needed even on CPUs
                    // where relaxed and consume mean different things. The
                    // symbols we're loading are probably present (or not) at
                    // init, and even if they aren't the runtime dynamic loader
                    // is extremely likely have sufficient barriers internally
                    // (possibly implicitly, for example the ones provided by
                    // invoking `mprotect`).
                    //
                    // That said, none of that's *guaranteed*, and so we fence.
                    atomic::fence(Ordering::Acquire);
                    Some(func)
                }
            }
        }
    }

    // Cold because it should only happen during first-time initialization.
    #[cold]