dovahcrow / safer_ffi

Write safer FFI code in Rust without polluting it with unsafe code

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

Click to hide

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 downstream Rust dependents: `examples/`, `tests/` etc.
]

[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.

Generated C header (rust_points.h)

/*! \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 }

🚀🚀