Programming Rust, 3rd Edition

Chapter 23. Unsafe Code

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Let no one think of me that I am humble or weak or passive; Let them understand I am of a different kind: dangerous to my enemies, loyal to my friends. To such a life glory belongs.

Euripides, Medea

The secret joy of systems programming is that, underneath every single safe language and carefully designed abstraction is a swirling maelstrom of wildly unsafe machine language and bit fiddling. You can write that in Rust, too.

The language we’ve presented up to this point in the book ensures your programs are free of memory errors and data races entirely automatically, through types, lifetimes, bounds checks, and so on. But this sort of automated reasoning has its limits; there are many valuable techniques that Rust cannot recognize as safe.

Unsafe code lets you tell Rust, “I am opting to use features whose safety you cannot guarantee.” By marking off a block of code as unsafe, you acquire the ability to call unsafe functions in the standard library, dereference ...