What is safer_ffi?

safer_ffi is a framework that helps you write foreign function interfaces (FFI) without polluting your Rust code with unsafe { ... } code blocks while making functions far easier to read and maintain.

📚 Read The User Guide 📚

Prerequisites

Minimum Supported Rust Version: 1.66.1

Quickstart

Small self-contained demo

You may try working with the examples/point example embedded in the repo:

git clone https://github.com/getditto/safer_ffi && cd safer_ffi
(cd examples/point && make)

Otherwise, to start using ::safer_ffi, follow the following steps:

Crate layout

Step 1: Cargo.toml

Edit your Cargo.toml like so:

[package]
name = "crate_name"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = [
    "staticlib",  # Ensure it gets compiled as a (static) C library
  # "cdylib",     # If you want a shared/dynamic C library (advanced)
    "lib",        # For `generate-headers` and other downstream rust dependents
                  # such as integration `tests/`, doctests, and `examples/`
]

[[bin]]
name = "generate-headers"
required-features = ["headers"]  # Do not build unless generating headers.

[dependencies]
# Use `cargo add` or `cargo search` to find the latest values of x.y.z.
# For instance:
#   cargo add safer-ffi
safer-ffi.version = "x.y.z"
safer-ffi.features = [] # you may add some later on.

[features]
# If you want to generate the headers, use a feature-gate
# to opt into doing so:
headers = ["safer-ffi/headers"]

• Where "x.y.z" ought to be replaced by the last released version, which you can find by running cargo search safer-ffi.

• See the dedicated chapter on Cargo.toml for more info.

Step 2: src/lib.rs

Then, to export a Rust function to FFI, add the #[derive_ReprC] and #[ffi_export] attributes like so:

use ::safer_ffi::prelude::*;

/// A `struct` usable from both Rust and C
#[derive_ReprC]
#[repr(C)]
#[derive(Debug, Clone, Copy)]
pub struct Point {
    x: f64,
    y: f64,
}

/* Export a Rust function to the C world. */
/// Returns the middle point of `[a, b]`.
#[ffi_export]
fn mid_point(a: &Point, b: &Point) -> Point {
    Point {
        x: (a.x + b.x) / 2.,
        y: (a.y + b.y) / 2.,
    }
}

/// Pretty-prints a point using Rust's formatting logic.
#[ffi_export]
fn print_point(point: &Point) {
    println!("{:?}", point);
}

// The following function is only necessary for the header generation.
#[cfg(feature = "headers")] // c.f. the `Cargo.toml` section
pub fn generate_headers() -> ::std::io::Result<()> {
    ::safer_ffi::headers::builder()
        .to_file("rust_points.h")?
        .generate()
}

• See the dedicated chapter on src/lib.rs for more info.

Step 3: src/bin/generate-headers.rs

fn main() -> ::std::io::Result<()> {
    ::crate_name::generate_headers()
}

Compilation & header generation

# Compile the C library (in `target/{debug,release}/libcrate_name.ext`)
cargo build # --release

# Generate the C header
cargo run --features headers --bin generate-headers

• See the dedicated chapter on header generation for more info.

/*! \file */
/*******************************************
 *                                         *
 *  File auto-generated by `::safer_ffi`.  *
 *                                         *
 *  Do not manually edit this file.        *
 *                                         *
 *******************************************/

#ifndef __RUST_CRATE_NAME__
#define __RUST_CRATE_NAME__
#ifdef __cplusplus
extern "C" {
#endif


#include <stddef.h>
#include <stdint.h>

/** \brief
 *  A `struct` usable from both Rust and C
 */
typedef struct Point {
    /** <No documentation available> */
    double x;

    /** <No documentation available> */
    double y;
} Point_t;

/** \brief
 *  Returns the middle point of `[a, b]`.
 */
Point_t
mid_point (
    Point_t const * a,
    Point_t const * b);

/** \brief
 *  Pretty-prints a point using Rust's formatting logic.
 */
void
print_point (
    Point_t const * point);


#ifdef __cplusplus
} /* extern \"C\" */
#endif

#endif /* __RUST_CRATE_NAME__ */

Testing it from C

Here is a basic example to showcase FFI calling into our exported Rust functions:

main.c

#include <stdlib.h>

#include "rust_points.h"

int
main (int argc, char const * const argv[])
{
    Point_t a = { .x = 84, .y = 45 };
    Point_t b = { .x = 0, .y = 39 };
    Point_t m = mid_point(&a, &b);
    print_point(&m);
    return EXIT_SUCCESS;
}

Compilation command

cc -o main{,.c} -L target/debug -l crate_name -l{pthread,dl,m}

# Now feel free to run the compiled binary
./main

• Note regarding the extra -l… flags.

  Those vary based on the version of the Rust standard library being used, and the system being used to compile it. In order to reliably know which ones to use, rustc itself ought to be queried for it.

  Simple command:

  rustc --crate-type=staticlib --print=native-static-libs -</dev/null

  this yields, to the stderr, output along the lines of:

  note: Link against the following native artifacts when linking against this static library. The order and any duplication can be significant on some platforms.
  
  note: native-static-libs: -lSystem -lresolv -lc -lm -liconv

  Using something like sed -nE 's/^note: native-static-libs: (.*)/\1/p' is thus a convenient way to extract these flags:

  rustc --crate-type=staticlib --print=native-static-libs -</dev/null \
      2>&1 | sed -nE 's/^note: native-static-libs: (.*)/\1/p'

  Ideally, you would not query for this information in a vacuum (e.g., /dev/null file being used as input Rust code just above), and rather, would apply it for your actual code being compiled:

  cargo rustc -q -- --print=native-static-libs \
      2>&1 | sed -nE 's/^note: native-static-libs: (.*)/\1/p'

  And if you really wanted to polish things further, you could use the JSON-formatted compiler output (this, for instance, avoids having to redirect stderr). But then you'd have to use a JSON parser, such as jq:

  RUST_STDLIB_DEPS=$(set -eo pipefail && \
      cargo rustc \
          --message-format=json \
          -- --print=native-static-libs \
      | jq -r '
          select (.reason == "compiler-message")
          | .message.message
      ' | sed -nE 's/^native-static-libs: (.*)/\1/p' \
  )

  and then use:

  cc -o main{,.c} -L target/debug -l crate_name ${RUST_STDLIB_DEPS}

which does output:

Point { x: 42.0, y: 42.0 }

🚀🚀

Development

To test the code with a certain amount of FFI integration baked into the tests (since safer-ffi, alone, only exports APIs to the FFI, so doesn't come with FFI callsites on its own), the ffi_tests/ project directory is used to test against C, C#, and Lua callsites.

You can run these tests directly by doing:

make -C ffi_tests

or by adding --features ffi-tests to the cargo test command.

Formatting

Code is formatted using the "{MSRV}-nightly toolchain". That is, formatting uses some unstable features of rustfmt, so we use a Versioned Nightly™ approach. For the sake of version consistency, we stick to that of our MSRV.

But since it needs to be a genuine nightly toolchain, we are forced to pick an actual nightly toolchain, only one whose date matches the birth of the corresponding MSRV-stable toolchain.

• See ./scripts/formatting/rust-toolchain.toml.
• See also, w.r.t. versions and dates: https://releases.rs

To format the code, you have three options:

• ./scripts/format.sh

• cargo fmt-nightly (which is defined as an alias of the previous bullet)

• cargo fmt but only after having run ./scripts/formatting/setup_cargo_fmt_override.sh at least once.

  This does mutate a bit your rustup setup, but in an unobservable way (but for allowing the matching stable toolchain to use unstable features when running cargo fmt, the very point of this maneuver).

  This is probably the preferred approach for those with IDEs or whatnot which automagically runs cargo fmt/rustfmt on save.

Dependencies

• Lua dependencies

  For running the C# FFI integration tests please install dotnet (v8.0) dependency:

  See https://aka.ms/dotnet-download for guidance about this.

• Lua dependencies

  For running Lua FFI integration tests please install luajit dependency:

  MacOS:

  brew install luajit

  Ubuntu/Debian:

  sudo apt-get install -y luajit

Various integration test suites

safer-ffi includes three different tests suites that can be run.

# In the project root:
cargo test

# FFI tests

make -C ffi_tests

# JavaScript tests

make -C js_tests

# Running the JS tests also gives you instructions for running browser tests.
# Run this command in the `js_tests` directory, open a browser and navigate to
# http://localhost:13337/
wasm-pack build --target web && python3 -m http.server 13337