New upstream version 1.49.0~beta.4+dfsg1

[rustc.git] / src / doc / reference / src / items / external-blocks.md

# External blocks

> **<sup>Syntax</sup>**\
> _ExternBlock_ :\
> &nbsp;&nbsp; `unsafe`<sup>?</sup> `extern` [_Abi_]<sup>?</sup> `{`\
> &nbsp;&nbsp; &nbsp;&nbsp; [_InnerAttribute_]<sup>\*</sup>\
> &nbsp;&nbsp; &nbsp;&nbsp; _ExternalItem_<sup>\*</sup>\
> &nbsp;&nbsp; `}`
>
> _ExternalItem_ :\
> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> (\
> &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; [_MacroInvocationSemi_]\
> &nbsp;&nbsp; &nbsp;&nbsp; | ( [_Visibility_]<sup>?</sup> ( _ExternalStaticItem_ | _ExternalFunctionItem_ ) )\
> &nbsp;&nbsp; )
>
> _ExternalStaticItem_ :\
> &nbsp;&nbsp; `static` `mut`<sup>?</sup> [IDENTIFIER] `:` [_Type_] `;`
>
> _ExternalFunctionItem_ :\
> &nbsp;&nbsp; `fn` [IDENTIFIER]&nbsp;[_Generics_]<sup>?</sup>\
> &nbsp;&nbsp; `(` ( _NamedFunctionParameters_ | _NamedFunctionParametersWithVariadics_ )<sup>?</sup> `)`\
> &nbsp;&nbsp; [_FunctionReturnType_]<sup>?</sup> [_WhereClause_]<sup>?</sup> `;`
>
> _NamedFunctionParameters_ :\
> &nbsp;&nbsp; _NamedFunctionParam_ ( `,` _NamedFunctionParam_ )<sup>\*</sup> `,`<sup>?</sup>
>
> _NamedFunctionParam_ :\
> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> ( [IDENTIFIER] | `_` ) `:` [_Type_]
>
> _NamedFunctionParametersWithVariadics_ :\
> &nbsp;&nbsp; ( _NamedFunctionParam_ `,` )<sup>\*</sup> _NamedFunctionParam_ `,` [_OuterAttribute_]<sup>\*</sup> `...`

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 kind 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 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.

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:

```rust
// 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 `...` after one
or more named arguments in the argument list:

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

## 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 `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.

<!-- ignore: requires extern linking -->
```rust,ignore
#[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.

### 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.

```rust
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].

[IDENTIFIER]: ../identifiers.md
[WebAssembly module]: https://webassembly.github.io/spec/core/syntax/modules.html
[functions]: functions.md
[statics]: static-items.md
[_Abi_]: functions.md
[_FunctionReturnType_]: functions.md
[_Generics_]: generics.md
[_InnerAttribute_]: ../attributes.md
[_MacroInvocationSemi_]: ../macros.md#macro-invocation
[_MetaListNameValueStr_]: ../attributes.md#meta-item-attribute-syntax
[_MetaNameValueStr_]: ../attributes.md#meta-item-attribute-syntax
[_OuterAttribute_]: ../attributes.md
[_Type_]: ../types.md#type-expressions
[_Visibility_]: ../visibility-and-privacy.md
[_WhereClause_]: generics.md#where-clauses
[attributes]: ../attributes.md
[regular function parameters]: functions.md#attributes-on-function-parameters