Crate safer_ffi

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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 📚


Minimum Supported Rust Version: 1.66.1


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Small self-contained demo

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

git clone && 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:

name = "crate_name"
version = "0.1.0"
edition = "2021"

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/`

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

# 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.

# 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/

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(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]`.
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.
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<()> {
Step 3: src/bin/
fn main() -> ::std::io::Result<()> {

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
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" {

#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]`.
mid_point (
    Point_t const * a,
    Point_t const * b);

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

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

#endif /* __RUST_CRATE_NAME__ */

Testing it from C

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


#include <stdlib.h>

#include "rust_points.h"

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);
    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
  • 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 }




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



  • boxedalloc
    #[repr(C)] Boxed types.
  • char *-compatible strings (slim pointers), for easier use from within C. They thus do not support inner nulls, nor string appending.
  • Closures with a #[repr(C)] layout (inlined vtable), up to 9 function arguments.
  • dyn_traitsdyn-traits
  • headersheaders
    C headers generation.
  • Trait abstractions describing the semantics of “being #[repr(C)]
  • On certain platforms, ::libc has no definitions for pervasive types such as size_t.
  • Wrappers around NonNull to better express the semantics of such pointer.
  • Logic common to all fat pointers.
  • Rust string types with a defined #[repr(C)] layout, albeit not char * compatible (fat pointers).
  • Tuple types with a guaranteed #[repr(C)] layout.
  • vecalloc


  • Safely implement CType for a #[repr(C)] struct when all its fields are CType.
  • Transitioning helper macro: still uses the old ReprC! syntax, but just to forward to the new #[derive_ReprC2($(js)?)] one.
  • Creates a compile-time checked char_p::Ref<'static> out of a string literal.
  • c_vecalloc
  • ffi_export_future_helpersfutures and dyn-traits


Attribute Macros

  • Identity macro when feature = "headers" is enabled, otherwise this macro outputs nothing.
  • Safely implement ReprC for a #[repr(C)] struct when all its fields are ReprC.
  • Export a function to be callable by C.