clarification

[web.git] / personal / _posts / 2019-07-14-uninit.md

diff --git a/personal/_posts/2019-07-14-uninit.md b/personal/_posts/2019-07-14-uninit.md
index 7d056c7edc4e4d1fd2ef4602b57133333dd917c2..bbb1468535f65b94a406fcf6b2d2205485484842 100644 (file)
--- a/personal/_posts/2019-07-14-uninit.md
+++ b/personal/_posts/2019-07-14-uninit.md
@@ -1,6 +1,6 @@
 ---
 title: '"What The Hardware Does" is not What Your Program Does: Uninitialized Memory'
 ---
 title: '"What The Hardware Does" is not What Your Program Does: Uninitialized Memory'

-categories: rust research
+categories: rust research programming

 forum: https://internals.rust-lang.org/t/what-the-hardware-does-is-not-what-your-program-does-uninitialized-memory/10561
 ---
 
 forum: https://internals.rust-lang.org/t/what-the-hardware-does-is-not-what-your-program-does-uninitialized-memory/10561
 ---
 


@@ -36,20 +36,22 @@ Here is an example to demonstrate why "random bit pattern" cannot describe unini
 use std::mem;
 
 fn always_returns_true(x: u8) -> bool {
 use std::mem;
 
 fn always_returns_true(x: u8) -> bool {

-    x < 150 || x > 120
+    x < 120 || x == 120 || x > 120

 }
 
 fn main() {
 }
 
 fn main() {

-    let x: u8 = unsafe { mem::uninitialized() };
+    let x: u8 = unsafe { mem::MaybeUninit::uninit().assume_init() };

     assert!(always_returns_true(x));
 }
 {% endhighlight %}
     assert!(always_returns_true(x));
 }
 {% endhighlight %}

+**Update (2022-11-17):** Switched to `MaybeUninit` to keep the example working in newer versions of Rust.
+

 `always_returns_true` is a function that, clearly, will return `true` for any possible 8-bit unsigned integer.
 `always_returns_true` is a function that, clearly, will return `true` for any possible 8-bit unsigned integer.

-After all, *every* possible value for `x` will be less than 150 or bigger than 120.
-A quick loop [confirms this](https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=58168009e601a2f01b08981f907a473c).
-However, if you [run the example](https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=da278adb50142d14909df74ea1e43069), you can see the assertion fail.[^godbolt]
+After all, *every* possible value for `x` will be either less than 120, equal to 120, or bigger than 120.
+A quick loop [confirms this](https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=65b690fa3c1691e11d4d45955358cdbe).
+However, if you [run the example](https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=c17d299cacd626c572def0c4262aed69), you can see the assertion fail.[^godbolt]

 
 

-[^godbolt]: In case this changes with future Rust versions, [here is the same example on godbolt](https://godbolt.org/z/JX4B4N); the `xor eax, eax` indicates that the function returns 0, aka `false`. And [here is a version for C++](https://godbolt.org/z/PvZGQB); imagine calling `make_true(true)` which *should* always return `true` but as the assembly shows will return `false`.
+[^godbolt]: In case this changes with future Rust versions, [here is the same example on godbolt](https://godbolt.org/z/9G67hP); the `xor eax, eax` indicates that the function returns 0, aka `false`. And [here is a version for C++](https://godbolt.org/z/TWrvcq).

 
 ## What *is* uninitialized memory?
 
 
 ## What *is* uninitialized memory?