Crate closure_ffi

Provides wrappers around closures which allows them to be called through context-free unsafe bare functions.

Context-free bare functions are not needed very often, as properly designed C APIs typically allow the user to specify an opaque pointer to a context object which will be provided to the function pointer. However, this is not always the case, and may be impossible in less common scenarios, e.g. function hooking for game modding/hacking.

Features

The crate comes with the following feature flags:

• no_std: Makes the crate compatible with #![no_std]. A dependency on alloc and spin is still required.
• bundled_jit_alloc: Provides a global JIT allocator through the jit-allocator crate. This is enabled by default.
• proc_macros: Provides the cc::hrtb proc macro which is necessary for creating bare functions with signatures that involve higher-kinded lifetimes (i.e. for<'a, ...> statements), as well as the bare_dyn proc macro for writing BareFn* types of boxed closures (i.e. Box<dyn Fn()>) more concisely.
• full: Enables all features except for no_std.

Examples

Passing a closure to a C API taking a contextless function pointer:

use closure_ffi::{cc, BareFnMut};
// Imagine we have an foreign C API for reigstering and unregistering some callback function.
// Notably, the API does not let the user provide a context object to the callback.
unsafe extern "C" fn ffi_register_callback(cb: unsafe extern "C" fn(u32)) {
    // ...
}
unsafe extern "C" fn ffi_unregister_callback(cb: unsafe extern "C" fn(u32)) {
    // ...
}

#[cfg(feature = "bundled_jit_alloc")]
{
    // We want to keep track of sum of callback arguments without using 
    // statics. This is where closure-ffi comes in:
    let mut sum = 0; // Non-'static closures work too!
    let wrapped = BareFnMut::new(cc::C, |x: u32| {
        sum += x;
    });

    // Safety: Here, we assert that the foreign API won't use the callback
    // in ways that break Rust's safety rules. Namely:
    // - The exclusivity of the FnMut's borrow is respected.
    // - If the calls are made from a different thread, the closure is Sync.
    // - We unregister the callback before the BareFnMut is dropped.
    unsafe {
        ffi_register_callback(wrapped.bare());
        // Do something that triggers the callback...
        ffi_unregister_callback(wrapped.bare());
    }

    drop(wrapped);
    println!("{sum}");
}

Modules

bare_closure

Provides the BareFnOnce, BareFnMut and BareFn wrapper types which allow closures to be called through context-free unsafe bare functions.

cc

Defines calling convention marker types for use with BareFn variants.

jit_alloc

Abstractions around allocators that provide dual-mapped memory with XOR protection rules (one RW view and one RX view) suitable for emitting code at runtime.

prelude

Common imports required to use closure-ffi.

Macros

bare_dynproc_macros

Shorthand for a BareFn* type taking a boxed closure.

Structs

BareFn

Wrapper around a Fn closure which exposes a bare function thunk that can invoke it without additional arguments.

BareFnMut

Wrapper around a FnMut closure which exposes a bare function thunk that can invoke it without additional arguments.

BareFnOnce

Wrapper around a FnOnce closure which exposes a bare function thunk that can invoke it without additional arguments.

JitAllocError

Anonymous error that may be returned by JitAlloc implementations when JitAlloc::alloc or JitAlloc::release fail.

Traits

JitAlloc

Generic allocator providing virtual memory suitable for emitting code at runtime.