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Exposing a Rust library to C

Serhii Potapov August 10, 2017 #rust #C #FFI


Recently I've ported whatlang library to C (whatlang-ffi) and I'd like to share some experience.

DISCLAIMER: I am not a professional C/C++ developer, so it means:

Hello from Rust example

First let's make a minimal C program, that calls Rust.

cargo new whatlang-ffi
cd whatlang-ffi
mkdir examples

Add this to Cargo.toml:

name = "whatlang"
crate-type = ["staticlib", "cdylib"]

It tells cargo that we want to compile a static library and get .so object.

In src/ we implement a small function that prints a message to stdout:

pub extern fn print_hello_from_rust() {
    println!("Hello from Rust");

To explain #[no_mangle] and extern let me extract some quotes from: FFI with Haskell and Rust article:

{% blockquote %} The #[no_mangle] tells the Rust compiler not to do anything weird with the symbols of this function when compiled because we need to be able to call it from other languages. This is needed if you plan on doing any FFI. Not doing so means you won't be able to reference it in other languages {% endblockquote %}

{% blockquote %} extern means this is externally available outside our library and tells the compiler to follow the C calling convention when compiling {% endblockquote %}

Let's compile our lib:

cargo build --release

It creates target/release/ file.

Now using nm tool we can check that really contains print_hello_from_rust() function:

nm -D ./target/release/  | grep hello
0000000000003190 T print_hello_from_rust

Then we need src/whatlang.h header file with a function declaration:

void print_hello_from_rust();

And finally a C program itself (we put it into examples/hello.c):

#include "whatlang.h"

int main (void) {

Let's compile!

gcc -o ./examples/hello ./examples/hello.c -Isrc  -L. -l:target/release/

This produces examples/hello binary, which we can run:

Hello from Rust

During the development process we'll likely need to recompile and run the program frequently. To automate this let's create a Makefile with few commands:

GCC_BIN ?= $(shell which gcc)
CARGO_BIN ?= $(shell which cargo)

run: clean build
	$(CARGO_BIN) clean
	rm -f ./examples/hello
	$(CARGO_BIN) build --release
	$(GCC_BIN) -o ./examples/hello ./examples/hello.c -Isrc  -L. -l:target/release/

Now, we can run make run to recompile, hello.c and run hello binary.

In the rest for the article I'll go through common problems, design decisions and pitfalls I faced.

Naming conventions

Since C does not have namespaces (some people may disagree) I had to stick to some rules in order to avoid name collision with other libraries and confusion:

Similar logic rules apply to everything else. It may seem too verbose, but as I see it's a pretty common approach for many C libraries.

Representing plain enums

In whatlang I have enum Lang, which is probably is main entity in the library. First I had to add #[repr(C)] to make rust compiler represent the data in memory in the same way as C does:

pub enum Lang {
    // and so on

Lang represents 83 different languages, which can be encoded with 1 byte. That was my initial assumption an it seemed to be correct, until later I uncovered some bugs.

I decided to convert Lang enum into u8 with std::mem::transmute function, in order to figure out how it's encoded:

let lang_int: u8 = unsafe { std::mem::transmute(Lang::Eng) };
println!("lang_int = {:?}", lang_int);

and got the following error:

= note: source type: whatlang::Lang (32 bits)
= note: target type: u8 (8 bits)

Wow! So, actually the enum takes 4 bytes, instead of 1. Replacing u8 with u32 makes things work as expected:

lang_int = 14

Now it make sense, because English is on 15th position in the Lang declaration (remember, counting starts with 0).

So in C such enum can be mapped to uint32_t type from stdint.h. To define all the language I ended up with such list of constants:

#include <stdint.h>

static const uint32_t WHATLANG_LANG_AKA = 0;
static const uint32_t WHATLANG_LANG_AMH = 1;
static const uint32_t WHATLANG_LANG_ARB = 2;
// and so on

It's quite verbose, so one would rather use scripting a language to generate such boilerplate code.

UPDATE: later I figured out, that actually without #[repr(C)] Rust optimizes memory and uses 1 byte for Lang enum. So uint32_t can be replaced with uint8_t. It should work as far as number of enum variants does not exceed 256.

Returning a structure from a function


First I recommend you to read the docs for std::ffi::CStr and std::ffi::String from the standard library. Those types exist to represent C strings in Rust.

Passing a string to a function

From C side it's relative simple: just pass a pointer to a string, like it's done here with argument text:

uint8_t whatlang_detect(char* text, struct whatlang_info* info);

On Rust side you'll need to convert a pointer into &str or String so you can manipulate the data as a string.

use std::ffi::CStr;
use std::os::raw::c_char;

pub extern fn whatlang_detect(ptr: *const c_char, info: &mut Info) -> u8 {
    let cstr = unsafe { CStr::from_ptr(ptr) };

    match cstr.to_str() {
        Ok(s) => {
          // Here `s` is regular `&str` and we can work with it
        Err(_) => {
          // handle the error

In the example above the function accepts a raw pointer *const c_char (it can be also *mut c_char if you need to mutate data). Then we transform it into CStr calling unsafe method CStr::from_ptr(ptr). Finally we're calling CStr::to_str(&self) function, which converts C string into &str. This operation may fail, if the C string does not contain a valid UTF-8 sequence.

Returning a string from a function

Lang provides some methods that return static strings, like eng_name() to get language name in English. Example:

assert_eq!(Eng::Rus.eng_name(), "Russian")

My first thought was "I just can return a raw pointer to the string", so the initial solution was like:

C function declaration:

char* whatlang_lang_eng_name(uint32_t lang);

Rust implementation:

pub extern fn whatlang_lang_eng_name(lang: Lang) -> *const u8 {

But there is problem. C expects strings to be terminated with \0 character, while Rust actually organizes static strings in a different way. When I expected the output to be simple Russian, the output was the entire massive of static data:


So, I've decided that I actually need to copy string from Rust static memory and ensure that \0 is added.

So I came up with the following function:

void whatlang_lang_eng_name(uint32_t lang, char* buffer);

Now user needs to pass a pointer to a buffer, where result must be written.

Rust implementation:

extern crate libc;

pub extern fn whatlang_lang_code(lang: Lang, buffer_ptr: *mut c_char) {
    // Here unwrap is safe, because whatlang always returns a valid &str
    let s = CString::new(lang.code()).unwrap();
    unsafe {
        libc::strcpy(buffer_ptr, s.as_ptr());

First, we convert &str into CString. Then we use libc::strcpy from libc crate to copy the string.

NOTE: it's responsibility of a caller to ensure, that the buffer size is big enough (at least 30 bytes).

Dealing with structures

Representing a structure

I have the following flat Rust structure Info

pub struct Info {
    lang: Lang,
    script: Script,
    confidence: f64

(where Lang and Script are plain enums), and it easily maps to whatlang_info:

struct whatlang_info {
  uint32_t lang;
  uint32_t script;
  double confidence;

It could be slightly more complex with nested structures, but the idea stays the same.

Returning a structure

I guess it can be done at least in few different approaches. The way I do it: a function receives a pointer to a preallocated memory for a structure as one of the arguments.

You've already seen whatlang_detect function above:

uint8_t whatlang_detect(char* text, struct whatlang_info* info);

Here info is pointer, where result must be written in case of success (0 is returned).

Another way to do this is to return a pointer directly:

struct whatlang_info* whatlang_get_info();

In this case Rust function must return boxed structure:

pub extern fn whatlang_get_info() -> Box<Info> {
    let info = Info { lang: Lang::Ukr, script: Script::Cyrillic, confidence: 0.9 };

NOTE: In this approach the memory for the structure is allocated by Rust, but it's responsibility of C program to free it.

Complex enums?

There is also some thing, that I am actually not aware how do properly: how to represent complex Rust enum in C?

Therefore I also don't know how gracefully represent Result<T,E> and Option<T> types.

Maybe, it's not actually necessary. For know as a workaround my function

uint8_t whatlang_detect(char* text, struct whatlang_info* info);

returns 0 in case of Some and 1 in case of None. It writes a result to preallocated memory by a given pointer info.

But I would appreciate if you share some other insights about this.

There are also some things, that are not covered in this article like tuples and arrays. But you may get some ideas from this article.


It was shown how to create C bindings for a Rust library. It may not be something, that you would do often, but having such option is always nice. It means also, that Rust libraries may be ported to plenty other languages that has FFI support, and this sounds really cool!

Thanks for reading. Below you'll find some useful links that helped me during this investigation.


People on Reddit gave me a very good constructive feedback. Some things I did wrong here and I highly recommend you to read this comment in addition.

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