sarasacw-omrf/assets/c-friendly-ffi-design/ffi-function-signatures.md

FFI Function Signatures

Under this section we discuss the signature requirements of the exported FFI functions.

C-style Name Export

FFI functions must be exported under C names, that is, they must not go through the C++ name mangling mechanism.

In Rust, you need to decorate the function with the #[unsafe(no_mangle)] attribute together with extern "C", for example:

#[unsafe(no_mangle)]
pub extern "C" fn WFStartup() -> CError {
    // Function body omitted...
}

// More FFI functions omitted...

Export Name Style

In this subsection, we discuss the naming style of exported functions.

In this design, there are exactly two styles to choose from. We call them the Windows style and the POSIX style. Either style can be chosen freely, but within a single library the two styles must not be mixed.

We recommend the following selection rule: if your library is Windows-only, choose the Windows style; otherwise, choose the POSIX style.

Windows Style

The Windows style format is <PREFIX><FUNC> for ordinary functions, and <PREFIX><STRUCT><FUNC> for opaque struct functions.

In the format, <STRUCT> is the name of the opaque struct and <FUNC> is the name of the function. Both of them must use PascalCase naming.

For <PREFIX>, it is a prefix identifier that is specific to this library. As we all know, C has no namespace concept, and all functions are exported into the same space. Therefore, avoiding name collisions between functions is an important task. By prepending <PREFIX> to the function name, name duplication can be avoided as much as possible. For this reason, you need to choose <PREFIX> carefully and try to pick a prefix that is unlikely to collide.

In the Windows style, there are two spellings for <PREFIX>. One is the all-uppercase style and the other is the style in which only the first letter is uppercase and all remaining letters are lowercase. Choose according to your own needs. In addition, the length of <PREFIX> must not exceed 5 characters, and it must not contain anything other than letters and digits (that is, underscores are not allowed).

The following examples show some legal and illegal Windows style function names:

Name	Verdict	Reason
WFStartup	Legal	An ordinary (non opaque struct) function.
WFIconGetHandle	Legal	An opaque struct function whose <STRUCT> is Icon.
WFWordHandleGetHandle	Legal	An opaque struct function whose <STRUCT> is WordHandle.
FlGetMessage	Legal	Uses a <PREFIX>, which is Fl, that capitalizes only its first letter.
MyRustGetMessage	Illegal	<PREFIX>, which is MyRust, does not keep all remaining letters lowercase.
My_RustGetMessage	Illegal	<PREFIX> contains an underscore.
IconGetHandle	Illegal	Missing <PREFIX>.
QwertyuiopGetData	Illegal	<PREFIX>, which is Qwertyuiop, is too long.
WFget_message	Illegal	<FUNC>, which is get_message, is not PascalCase.
WFicon_handleGetMessage	Illegal	<STRUCT>, which is icon_handle, is not PascalCase.

POSIX Style

The POSIX style format is <PREFIX>_<FUNC> for ordinary functions, and <PREFIX>_<STRUCT>_<FUNC> for opaque struct functions.

The <PREFIX>, <STRUCT> and <FUNC> components in the format have the same meaning as in the Windows style, but the format is different. <STRUCT> and <FUNC> must use snake_case naming. <PREFIX> must be all lowercase, and it must not contain anything other than letters and digits (that is, underscores are not allowed). Its length and selection requirements are the same as in the Windows style.

The following examples show some legal and illegal POSIX style function names:

Name	Verdict	Reason
wf_startup	Legal	An ordinary (non opaque struct) function.
wf_icon_get_handle	Legal	An opaque struct function whose <STRUCT> is icon.
wf_word_handle_get_handle	Legal	An opaque struct function whose <STRUCT> is word_handle.
Fl_get_message	Illegal	<PREFIX>, which is Fl, is not all lowercase.
my_rust_get_message	Illegal	<PREFIX>, which is my_rust, contains an underscore.
icon_get_handle	Illegal	Missing <PREFIX>.
qwertyuiop_get_data	Illegal	<PREFIX>, which is qwertyuiop, is too long.
wf_GetMessage	Illegal	<FUNC>, which is GetMessage, is not snake_case.
wf_Icon_get_message	Illegal	<STRUCT>, which is Icon, is not snake_case.