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The pitfalls of uninitialized memory #Programming #C #Rust

Via ralfj.de, when you allocate memory and you don’t initialize it, what are its contents? This is called this “uninitialized memory”, but what exactly does this mean and what happens when it gets read?

This post is about uninitialized memory, but also about the semantics of highly optimized “low-level” languages in general. I will try to convince you that reasoning by “what the hardware does” is inherently flawed when talking about languages such as Rust, C or C++. These are not low-level languages. I have made this point before in the context of pointers; this time it is going to be about uninitialized memory.

The trigger for this post is the deprecation of mem::uninitialized() with Rust 1.36, but the post is just as relevant for C/C++ as it is for Rust.

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When you allocate some memory (whether it be on the stack or heap), and you do not initialize it, what are its contents? We call this “uninitialized memory”, but what exactly does this mean and what happens when it gets read? For many languages, this question is inconsequential: in Java, Haskell, OCaml and generally in all safe languages, uninitialized memory cannot be read, this is prevented by the type system. The same is true in safe Rust, actually. However, in unsafe Rust as well as in inherently unsafe languages such as C and C++, not having to initialize memory can be an important optimization, so this is a very relevant question.

The C and C++ specifications (without going into all the detail here) say that uninitialized memory is “indeterminate”, but the details of what exactly that means are unclear. Many people will tell you that “uninitialized memory contains a random bit pattern”. This is wrong. They might also talk about the system allocator or how the OS kernel allocates pages for the program to use. That is just irrelevant information.

See the blog post for a detailed explanation of uninitialized memory and what to do with it.


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