X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/c4b1c46b46d8b78098e4cac1cc9eb41dee2a4d3d..64c2c59f4ed7644f6ce457e190c316cf8c189608:/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 c6ee054..5aa60df 100644 --- a/personal/_posts/2019-07-14-uninit.md +++ b/personal/_posts/2019-07-14-uninit.md @@ -54,7 +54,7 @@ However, if you [run the example](https://play.rust-lang.org/?version=stable&mod ## What *is* uninitialized memory? How is this possible? -The answer is that every byte in memory cannot just have a value in `0..256` (this is Rust/Ruby syntax for a left-inclusive right-exclusive range), it can also be "uninitialized". +The answer is that, in the "abstract machine" that is used to specify the behavior of our program, every byte in memory cannot just have a value in `0..256` (this is Rust/Ruby syntax for a left-inclusive right-exclusive range), it can also be "uninitialized". Memory *remembers* if you initialized it. The `x` that is passed to `always_return_true` is *not* the 8-bit representation of some number, it is an uninitialized byte. Performing operations such as comparison on uninitialized bytes is undefined behavior. @@ -108,7 +108,7 @@ It runs on the Rust abstract machine, and that machine (which only exists in our The real, physical hardware that we end up running the compiled program on is a very efficient *but imprecise* implementation of this abstract machine, and all the rules that Rust has for undefined behavior work together to make sure that this imprecision is not visible for *well-behaved* (UB-free) programs. But for programs that do have UB, this "illusion" breaks down, and [anything is possible](https://raphlinus.github.io/programming/rust/2018/08/17/undefined-behavior.html). -UB-free programs can be made sense of by looking at their assembly, but *whether* a program has UB is impossible to tell on that level. +*Only* UB-free programs can be made sense of by looking at their assembly, but *whether* a program has UB is impossible to tell on that level. For that, you need to think in terms of the abstract machine.[^sanitizer] [^sanitizer]: This does imply that tools like valgrind, that work on the final assembly, can never reliably detect *all* UB.