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Simplify the unix Weak functionality #58442

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merged 2 commits into from
Feb 24, 2019
Merged

Simplify the unix Weak functionality #58442

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70c5af8
Avoid allocation in std::sys::unix::weak
cuviper Feb 13, 2019
33d80bf
Return without a reference in unix Weak::get()
cuviper Feb 13, 2019
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15 changes: 7 additions & 8 deletions src/libstd/sys/unix/weak.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,15 +18,15 @@

use libc;

use ffi::CString;
use ffi::CStr;
use marker;
use mem;
use sync::atomic::{AtomicUsize, Ordering};

macro_rules! weak {
(fn $name:ident($($t:ty),*) -> $ret:ty) => (
static $name: ::sys::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
::sys::weak::Weak::new(stringify!($name));
::sys::weak::Weak::new(concat!(stringify!($name), '\0'));
)
}
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@RalfJung RalfJung Feb 23, 2019 •
edited
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Shouldn't something about this abstraction be unsafe? Like, either the constructor or get or so?

Seems we are using it only with unsafe fn, but the Weak type itself can't know that.

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Sure, I think it would make sense for the constructor to be unsafe, which the macro can hide knowing this is unsafe fn. All uses do go through this macro already though.


Expand All @@ -45,23 +45,22 @@ impl<F> Weak<F> {
}
}

pub fn get(&self) -> Option<&F> {
pub fn get(&self) -> Option<F> {
assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
unsafe {
if self.addr.load(Ordering::SeqCst) == 1 {
self.addr.store(fetch(self.name), Ordering::SeqCst);
}
if self.addr.load(Ordering::SeqCst) == 0 {
None
} else {
mem::transmute::<&AtomicUsize, Option<&F>>(&self.addr)
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match self.addr.load(Ordering::SeqCst) {
0 => None,
addr => Some(mem::transmute_copy::<usize, F>(&addr)),
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Please add a comment saying that the sizes are equal because we asserted this earlier. (Took me a bit to see that. As I said, I don't know this code and feel somewhat uncomfortable reviewing it.)

}
}
}
}

unsafe fn fetch(name: &str) -> usize {
let name = match CString::new(name) {
let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
Ok(cstr) => cstr,
Err(..) => return 0,
};
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