pub type ShutdownCallbackFn = unsafe extern "C" fn(*mut c_void);Expand description
A shutdown hook: extern "C" so the same slot serves Rust, C and C++
without a second registry, and so no allocator is involved.
Deliberately NOT a closure. rclcpp takes std::function<void()>, which is
a heap allocation per registration; the callback shape the rest of this
tree’s FFI surface already uses is a bare function pointer plus an opaque
context the callee casts back to its own state.