Function pointers

These are the most simple callback objects. They consist of a single (function) pointer, that is, the address of the (beginning of the) code that is to be called.

  • In Rust, this is the family of fn(...) -> _ types.

    To avoid (very bad) bugs when mixing calling conventions (ABIs), Rust includes the ABI within the type of the function pointer, granting additional type-level safety. When dealing with C, the calling convention that matches C's is almost always extern "C" and is never extern "Rust".

    When unspecified, the calling convention defaults to extern "Rust", which is different from extern "C"!

    This is why all function pointers involved in FFI need to be extern-annotated. Forgetting it results in code that triggers Undefined Behavior (and traditional FFI fails to guard against it)⚠️

  • In C, these are written as ret_t (*name)(function_args), where name is the name of a variable or parameter that has the function pointer type, or the name of the type being type-aliased to the function pointer type.

Examples

RustC
cb: extern "C" fn()void (*cb)(void)
f: extern "C" fn(arg1_t, arg2_t) -> ret_tret_t (*f)(arg1_t, arg2_t)
transmute::<_, extern "C" fn(arg_t) -> ret_t>(f)(ret_t (*)(arg_t)) (f)
type cb_t = extern "C" fn(arg_t) -> ret_t;
let f: cb_t = ...;
transmute::<_, cb_t>(f)		typedef ret_t (*cb_t)(arg_t);
cb_t f = ...;
(cb_t) (f)

So, for instance,

#[ffi_export]
fn call (
    ctx: *mut c_void,
    cb: unsafe extern "C" fn(ctx: *mut c_void),
)

becomes

void call (
    void * ctx,
    void (*cb)(void * ctx)
);

Nullable function pointers

A Rust fn pointer cannot possibly be NULL!

This means that when NULL-able function pointers are involved, forgetting to Option-wrap them can lead to Undefined Behavior. Luckily, this is something that is easily caught by ::safer_ffi's sanity checks.