X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/37f45a5b5b8f002b72000c48fb850faa0957f191..75e77d69640117d07fb70adcc1e2bdcb4f2739ac:/personal/_posts/2018-07-24-pointers-and-bytes.md?ds=sidebyside diff --git a/personal/_posts/2018-07-24-pointers-and-bytes.md b/personal/_posts/2018-07-24-pointers-and-bytes.md index 35a5c15..68a2cb0 100644 --- a/personal/_posts/2018-07-24-pointers-and-bytes.md +++ b/personal/_posts/2018-07-24-pointers-and-bytes.md @@ -173,7 +173,7 @@ We have to say what the value of `v` is, so we have to find some way to answer t (And this is an entirely separate issue from the problem with multiplication that came up in the last section. We just assume some abstract type `Pointer`.) We cannot represent a byte of a pointer as an element of `0..256`. -Essentially, if we use a naive model of memory, the extra "hidden" part of a pointer (the one that makes it more than just an integer) would be lost whne a pointer is stored to memory and loaded again. +Essentially, if we use a naive model of memory, the extra "hidden" part of a pointer (the one that makes it more than just an integer) would be lost when a pointer is stored to memory and loaded again. We have to fix this, so we have to extend our notion of a "byte" to accomodate that extra state. So, a byte is now *either* an element of `0..256` ("raw bits"), *or* the n-th byte of some abstract pointer. If we were to implement our memory model in Rust, this might look as follows: @@ -237,6 +237,8 @@ Finally, `Uninit` is also a better choice for interpreters like miri. Such interpreters have a hard time dealing with operations of the form "just choose any of these values" (i.e., non-deterministic operations), because if they want to fully explore all possible program executions, that means they have to try every possible value. Using `Uninit` instead of an arbitrary bit pattern means miri can, in a single execution, reliably tell you if your programs uses uninitialized values incorrectly. +**Update:** Since writing this section, I have written an entire [post dedicated to uninitialized memory and "real hardware"]({% post_url 2019-07-14-uninit %}) with more details, examples and references. **/Update** + ## Conclusion We have seen that in languages like C++ and Rust (unlike on real hardware), pointers can be different even when they point to the same address, and that a byte is more than just a number in `0..256`.