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 |_
):
TypeNamedFunctionParametersWithVariadics :
( 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.
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.
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:
// 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 normalfn foo()
in any Rust code.extern "C"
-- This is the same asextern fn foo()
; whatever the default your C compiler supports.extern "system"
-- Usually the same asextern "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"
-- Thefastcall
ABI -- corresponds to MSVC's__fastcall
and GCC and clang's__attribute__((fastcall))
extern "vectorcall"
-- Thevectorcall
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.
Variadic functions
Functions within external blocks may be variadic by specifying ...
after one
or more named arguments in the argument list:
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 ifkind
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.
#[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.
extern {
#[link_name = "actual_symbol_name"]
fn name_in_rust();
}