21. FFI¶

Legality Rules

21:1 Foreign Function Interface or FFI employs ABI, attributes, external blocks, external functions, linkage, and type layout to interface a Rust program with foreign code.

21:2 The following attributes affect FFI:

21.1. ABI¶

Syntax

AbiSpecification ::=
    extern AbiKind?

AbiKind ::=
    RawStringLiteral
  | StringLiteral

Legality Rules

21.1:1 Application Binary Interface or ABI is a set of conventions that dictate how data and computation cross language boundaries.

21.1:2 The ABI kind indicates the ABI of a construct.

21.1:3 The following ABIs are supported:

21.1:4 extern "C" - The default ABI of C code, referred to as extern C ABI.
21.1:5 extern "C-unwind" - The same as extern "C" with the addition that unwinding across FFI is permitted.
21.1:6 extern "Rust" - The default ABI of a Rust program, referred to as Rust ABI.
21.1:7 extern "system" - The operating system-dependent ABI, referred to as external system ABI.

21.1:8 A function without an explicit ABI has implicit Rust ABI, unless it appears within an external block.

21.1:9 A function with an ABI but without a specified ABI kind has implicit extern C ABI.

Implementation Permissions

21.1:10 A tool is allowed to specify additional ABIs. These ABIs may include, but may not be limited to, the following:

21.1:11 extern "aapcs" - The ARM ABI.
21.1:12 extern "cdecl" - The x86_32 ABI of C code.
21.1:13 extern "fastcall" - The fastcall ABI that corresponds to MSVC’s __fastcall and GCC and clang’s __attribute__((fastcall)).
21.1:14 extern "stdcall" - The x86_32 ABI of the Win32 API.
21.1:15 extern "sysv64" - The x86_64 non-Windows ABI of C code.
21.1:16 extern "vectorcall" - The vectorcall ABI that corresponds to MSVC’s __vectorcall and clang’s __attribute__((vectorcall)).
21.1:17 extern "win64" - The x86_64 Windows ABI of C code.

Undefined Behavior

21.1:18 It is undefined behavior when a foreign exception crosses a foreign function interface boundary with an ABI other than extern "C-unwind".

Examples

extern
extern "C"

21.2. External Blocks¶

Syntax

ExternalBlock ::=
    unsafe? extern AbiSpecification? {
      InnerAttributeOrDoc*
      ExternItem*
    }

ExternItem ::=
    OuterAttributeOrDoc* (ExternalItemWithVisibility | TerminatedMacroInvocation)

ExternalItemWithVisibility ::=
    VisibilityModifier? (
        FunctionDeclaration
      | StaticDeclaration
    )

Legality Rules

21.2:1 An external block is a construct that provides the declarations of foreign functions as unchecked imports.

21.2:2 The unsafe keyword of an external block is rejected, but may still be consumed by macros.

Examples

extern "C" {
    static MAX_LENGTH: size_t;

    fn compress
        (input: *const u8,
         input_length: size_t,
         compressed: *mut u8,
         compressed_length: *mut size_t) -> c_int;
    fn log(msg: *const c_char, ...);
}