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% Interfacing with other Languages (FFI)
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*Obviously* we'd all love to live in a **glorious** world where everything is
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written in Rust, Rust, and More Rust. Tragically, programs have been written
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in Not Rust for over 50 years. Crufty enterprises are doomed to
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support ancient code bases, and greybeard programmers stuck in their ways
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*insist* on writing programs in other languages, even to this day!
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In all seriousness, there's a myriad of reasons for your codebase to be a
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hybrid of different languages, and Rust is well-designed to interface with
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all of them as painlessly as possible. It does this through the tried and
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true strategy of all languages: pretend to be C, and understand C.
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Thanks to Rust's minimal runtime and C-like semantics, this is about as
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painless as FFI with C++. Obviously, most of Rust's features are completely
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incompatible with other languages: tagged unions, zero-sized-types, dynamically-
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sized types, destructors, methods, traits, references, and lifetimes are all
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concepts that you won't be able to expose or accept in your foreign function
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interface.
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All mapping through C will give you is functions, structs, globals, raw pointers,
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and C-like enums. That's it. Rust's default data layouts are also incompatible
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with the C layout. See [the section on data layout][data.html] for details.
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Long story short: mark FFI structs and enums with `#[repr(C)]`, mark FFI
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functions as `extern`.
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## Runtime
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Rust's runtime is sufficiently minimal that it requires *no* special handling.
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You don't need to set anything up. You don't need to tear anything down.
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Awesome.
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The only runtime detail you *really* need to worry about is unwinding. Rust's
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unwinding model is not defined to be incompatible with any particular language.
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That means that if you call Rust from another language and it unwinds into the
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calling language, this will cause Undefined Behaviour. Similarly, if another
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language unwinds into Rust, it will also cause Undefined Behaviour.
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Rust can't really do anything about other languages unwinding into it (FFI is unsafe
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for a reason!), but you can be a good FFI citizen by catching panics in any
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FFI functions you export. Rust provides `thread::catch_panic` for exactly this.
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Unfortunately, this API is still unstable.
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## libc
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