Why is my integer value changed when passing a heap-allocated struct from Rust to C?

Question

I am passing data from Rust to C. While passing primitives seems to be easy, I am kind of lost with structs.

I have the following Rust code:

use ::std::os::raw::*;

static NAME: &[c_char] = &[65, 66, 67, 0];

#[repr(C)]
pub struct MyStruct {
    pub x: c_int,
    pub y: *const c_char,
}

#[no_mangle]
pub extern "C" fn get_my_struct() -> *const MyStruct {
    let my_struct = MyStruct {
        x: 11 as c_int,
        y: NAME.as_ptr(),
    };

    unsafe {
        ::std::mem::transmute(Box::new(my_struct))
    }
}

And the following C code:

typedef struct _my_struct my_struct;
extern const my_struct get_my_struct(void);

struct _my_struct {
    int x;
    const char *y;
};

int main(void) {
  my_struct my_complex_struct = get_my_struct();

  return 0;
}

The output from gdb says:

(gdb) p my_complex_struct
$1 = {x = 6295568, y = 0x7ffff7bce1e0 <ref> "ABC"}

The string looks fine but the int is definitely off. What am I missing here? Why is the value of x 6295568 and not 11?

Compiled by:

gcc (Debian 4.9.2-10) 4.9.2
rustc 1.20.0-nightly (8d22af87d 2017-07-22)
cargo 0.21.0-nightly (ffab51954 2017-07-18)

using:

cargo build
gcc --std=c11 -g -o main src/main.c /test/target/debug/libtest.so -L target/debug/

Answer 1

You have an issue because your ABI doesn't match. You are returning a pointer to an allocated structure but your C code claims the function returns a struct directly.

As demonstrated in The Rust FFI Omnibus chapter on objects, you should use Box::into_raw:

#[no_mangle]
pub extern "C" fn get_my_struct() -> *const MyStruct {
    let my_struct = MyStruct {
        x: 11 as c_int,
        y: NAME.as_ptr(),
    };

    Box::into_raw(Box::new(my_struct))
}

Your C function should be marked as returning a pointer:

extern const my_struct *get_my_struct(void);

// ...

int main(void) {
  const my_struct *my_complex_struct = get_my_struct();
  // ...
}

(lldb) p *my_complex_struct
(my_struct) $1 = (x = 11, y = "ABC")

The code also has a memory leak; you need to return the pointer back to Rust so it can be properly deallocated.

---

If you meant to return the struct directly, change your Rust code to not perform an allocation:

#[no_mangle]
pub extern "C" fn get_my_struct() -> MyStruct {
    MyStruct {
        x: 11 as c_int,
        y: NAME.as_ptr(),
    }
}

(lldb) p my_complex_struct
(my_struct) $0 = (x = 11, y = "ABC")

---

Disclaimer: I'm the primary author of the Omnibus.