How to use a C typedef struct and functions with that struct from Rust?

Question

I have these C files and I want to use pair_add, add and PAIR from Rust.

adder.c

#include <stdlib.h>
#include "adder.h"

int pair_add(PAIR * ppair) {
    return ppair->x + ppair->y;
}

int add(int x, int y) {
    return x + y;
}

adder.h

typedef struct {
    int x;
    int y;
} PAIR;

int pair_add(PAIR * ppair);
int add(int, int);

I compile them using:

gcc -c adder.c
ar rc libadder.a adder.o  # Static link

The documentation does not detail how to integrate C typedef structs and the example is for functions which return and accept i32. Other online resources were also limited.

I attempted the following but could not add the PAIR typedef:

extern crate libc;

use libc::c_int;

#[link(name = "adder")]
extern "C" {
    // Define PAIR

    // int pair_add(PAIR * ppair);
    fn pair_add(input: Pair) -> c_int;

    // int add(int, int);
    fn add(input1: c_int) -> c_int;
}

fn main() {}

@Stargateur Updated the post and posted a new question on that topic stackoverflow.com/questions/50619070 — Greg
– Greg, Commented May 31, 2018 at 7:38

Shepmaster · Accepted Answer · 2018-05-31 13:50:36Z

5

First:

typedef struct {
    int x;
    int y;
} PAIR;

This declares an anonymous struct and this is not currently supported by Rust. There is a RFC that proposes to add anonymous types.

Secondly, a typedef is just an alias and the name of the struct isn't important to be compatible. That means that you can simply do:

extern crate libc;
use libc::c_int;

#[repr(C)]
struct PAIR {
    x: c_int,
    y: c_int,
}

// optional "equivalent" of typedef
type Pair = PAIR;

extern "C" {
    fn pair_add(input: *mut Pair) -> c_int;
    // could be
    // fn pair_add(input: *mut PAIR) -> c_int;
}

You could easily ignore the typedef and just use PAIR as a name for this struct. You can even just write struct PAIR; to make it opaque.

edited May 31, 2018 at 13:50

Shepmaster

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answered May 31, 2018 at 8:02

Stargateur

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2 Comments

Shepmaster Over a year ago

You don't need to match the struct's name to start with, you can just do #[repr(C)] struct Pair { x: c_int, y: c_int }. You can give it any name you want, because as you mention, the name doesn't matter to the compiler.

Shepmaster Over a year ago

to make it opaque — I would not use this to define an opaque type. Another Q&A has suggestions, but struct PAIR; would allow anyone to create an instance of a PAIR, which should not be allowed if it's opaque.

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