Crate safer_ffi

Expand description

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

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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.

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 }

🚀🚀

Development

Tests

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

Re-exports

pub use dyn_traits::futures;
futures
pub use tuple::*;

Modules

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

Macros

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

Structs

Stringalloc
Same as String, but with guaranteed #[repr(C)] layout
Vecalloc
Same as Vec<T>, but with guaranteed #[repr(C)] layout
c_char
A ReprC standalone type with the same layout and ABI as ::libc::c_char.
c_int
A ReprC standalone type with the same layout and ABI as ::libc::c_int.

Attribute Macros

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