External blocks - The Rust Reference

# External blocks

> Syntax
> ExternBlock :
> `extern` Abi? `{`
> InnerAttribute*
> ExternalItem*
> `}`

> ExternalItem :
> OuterAttribute*
> Visibility?
> ( ExternalStaticItem | ExternalFunctionItem )

> ExternalStaticItem :
> `static` `mut`? IDENTIFIER `:` Type `;`

> ExternalFunctionItem :
> `fn` IDENTIFIER Generics?
> `(` NamedFunctionParameters? | NamedFunctionParametersWithVariadics ) `)`
> FunctionReturnType? WhereClause? `;`

> NamedFunctionParameters :
> NamedFunctionParam ( `,` NamedFunctionParam )* `,`?

> NamedFunctionParam :
> ( IDENTIFIER | `_` ) `:` Type

> NamedFunctionParametersWithVariadics :
> ( NamedFunctionParam `,` )* NamedFunctionParam `,` `...`

External blocks form the basis for Rust's foreign function interface. Declarations in an external block describe symbols in external, non-Rust libraries.

Functions within external blocks are declared in the same way as other Rust functions, with the exception that they may not have a body and are instead terminated by a semicolon. Patterns are not allowed in parameters, only IDENTIFIER or `_` may be used.

Functions within external blocks may be called by Rust code, just like functions defined in Rust. The Rust compiler automatically translates between the Rust ABI and the foreign ABI.

Functions within external blocks may be variadic by specifying `...` after one or more named arguments in the argument list:

    extern {
        fn foo(x: i32, ...);
    }

A number of attributes control the behavior of external blocks.

By default external blocks assume that the library they are calling uses the standard C ABI on the specific platform. Other ABIs may be specified using an `abi` string, as shown here:

    // Interface to the Windows API
    extern "stdcall" { }

There are three ABI strings which are cross-platform, and which all compilers are guaranteed to support:

- `extern "Rust"` -- The default ABI when you write a normal `fn foo()` in any Rust code.
- `extern "C"` -- This is the same as `extern fn foo()`; whatever the default your C compiler supports.
- `extern "system"` -- Usually the same as `extern "C"`, except on Win32, in which case it's `"stdcall"`, or what you should use to link to the Windows API itself

There are also some platform-specific ABI strings:

- `extern "cdecl"` -- The default for x86_32 C code.
- `extern "stdcall"` -- The default for the Win32 API on x86_32.
- `extern "win64"` -- The default for C code on x86_64 Windows.
- `extern "sysv64"` -- The default for C code on non-Windows x86_64.
- `extern "aapcs"` -- The default for ARM.
- `extern "fastcall"` -- The `fastcall` ABI -- corresponds to MSVC's `__fastcall` and GCC and clang's `__attribute__((fastcall))`
- `extern "vectorcall"` -- The `vectorcall` ABI -- corresponds to MSVC's `__vectorcall` and clang's `__attribute__((vectorcall))`

Finally, there are some rustc-specific ABI strings:

- `extern "rust-intrinsic"` -- The ABI of rustc intrinsics.
- `extern "rust-call"` -- The ABI of the Fn::call trait functions.
- `extern "platform-intrinsic"` -- Specific platform intrinsics -- like, for example, `sqrt` -- have this ABI. You should never have to deal with it.

The `link` attribute allows the name of the library to be specified. When specified the compiler will attempt to link against the native library of the specified name.

    #[link(name = "crypto")]
    extern { }

A function declared in an extern block is implicitly `unsafe`. When coerced to a function pointer, a function declared in an extern block has type `unsafe extern "abi" for<'l1, ..., 'lm> fn(A1, ..., An) -> R`, where `'l1`, ... `'lm` are its lifetime parameters, `A1`, ..., `An` are the declared types of its parameters and `R` is the declared return type.

It is `unsafe` to access a static item declared in an extern block, whether or not it's mutable.

It is valid to add the `link` attribute on an empty extern block. You can use this to satisfy the linking requirements of extern blocks elsewhere in your code (including upstream crates) instead of adding the attribute to each extern block.