External blocks

^Syntax
ExternBlock :
   unsafe^? extern Abi^? {
      InnerAttribute^*
      ExternalItem^*
   }

ExternalItem :
   OuterAttribute^* (
         MacroInvocationSemi
      | ( Visibility^? ( StaticItem | Function ) )
   )

External blocks provide declarations of items that are not defined in the current crate and are the basis of Rust's foreign function interface. These are akin to unchecked imports.

Two kinds of item declarations are allowed in external blocks: functions and statics. Calling functions or accessing statics that are declared in external blocks is only allowed in an unsafe context.

The unsafe keyword is syntactically allowed to appear before the extern keyword, but it is rejected at a semantic level. This allows macros to consume the syntax and make use of the unsafe keyword, before removing it from the token stream.

Functions

Functions within external blocks are declared in the same way as other Rust functions, with the exception that they must not have a body and are instead terminated by a semicolon. Patterns are not allowed in parameters, only IDENTIFIER or _ may be used. Function qualifiers (const, async, unsafe, and extern) are not allowed.

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.

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.

Statics

Statics within external blocks are declared in the same way as statics outside of external blocks, except that they do not have an expression initializing their value. It is unsafe to access a static item declared in an extern block, whether or not it's mutable, because there is nothing guaranteeing that the bit pattern at the static's memory is valid for the type it is declared with, since some arbitrary (e.g. C) code is in charge of initializing the static.

Extern statics can be either immutable or mutable just like statics outside of external blocks. An immutable static must be initialized before any Rust code is executed. It is not enough for the static to be initialized before Rust code reads from it.

ABI

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:


#![allow(unused)]
fn main() {
// 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))

Variadic functions

Functions within external blocks may be variadic by specifying ... as the last argument. There must be at least one parameter before the variadic parameter. The variadic parameter may optionally be specified with an identifier.


#![allow(unused)]
fn main() {
extern "C" {
    fn foo(x: i32, ...);
    fn with_name(format: *const u8, args: ...);
}
}

Attributes on extern blocks

The following attributes control the behavior of external blocks.

The `link` attribute

The link attribute specifies the name of a native library that the compiler should link with for the items within an extern block. It uses the MetaListNameValueStr syntax to specify its inputs. The name key is the name of the native library to link. The kind key is an optional value which specifies the kind of library with the following possible values:

dylib — Indicates a dynamic library. This is the default if kind is not specified.
static — Indicates a static library.
framework — Indicates a macOS framework. This is only valid for macOS targets.

The name key must be included if kind is specified.

The optional modifiers argument is a way to specify linking modifiers for the library to link. Modifiers are specified as a comma-delimited string with each modifier prefixed with either a + or - to indicate that the modifier is enabled or disabled, respectively. Specifying multiple modifiers arguments in a single link attribute, or multiple identical modifiers in the same modifiers argument is not currently supported.
Example: #[link(name = "mylib", kind = "static", modifiers = "+whole-archive").

The wasm_import_module key may be used to specify the WebAssembly module name for the items within an extern block when importing symbols from the host environment. The default module name is env if wasm_import_module is not specified.

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

#[link(name = "CoreFoundation", kind = "framework")]
extern {
    // …
}

#[link(wasm_import_module = "foo")]
extern {
    // …
}

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.

Linking modifiers: `bundle`

This modifier is only compatible with the static linking kind. Using any other kind will result in a compiler error.

When building a rlib or staticlib +bundle means that all object files from the native static library will be added to the rlib or staticlib archive, and then used from it during linking of the final binary.

When building a rlib -bundle means that the native static library is registered as a dependency of that rlib "by name", and object files from it are included only during linking of the final binary, the file search by that name is also performed during final linking.
When building a staticlib -bundle means that the native static library is simply not included into the archive and some higher level build system will need to add it later during linking of the final binary.

This modifier has no effect when building other targets like executables or dynamic libraries.

The default for this modifier is +bundle.

More implementation details about this modifier can be found in bundle documentation for rustc.

Linking modifiers: `whole-archive`

This modifier is only compatible with the static linking kind. Using any other kind will result in a compiler error.

+whole-archive means that the static library is linked as a whole archive without throwing any object files away.

The default for this modifier is -whole-archive.

More implementation details about this modifier can be found in whole-archive documentation for rustc.

The `link_name` attribute

The link_name attribute may be specified on declarations inside an extern block to indicate the symbol to import for the given function or static. It uses the MetaNameValueStr syntax to specify the name of the symbol.


#![allow(unused)]
fn main() {
extern {
    #[link_name = "actual_symbol_name"]
    fn name_in_rust();
}
}

Attributes on function parameters

Attributes on extern function parameters follow the same rules and restrictions as regular function parameters.

The Rust Reference