Allocating an object for C / FFI library calls

Question

I have a C library, which has gpio implementation. There's gpio_type which is target specific, each MCU has different definition for gpio_type. One of the functions in the library:

void gpio_init(gpio_type *object, int32_t pin);

I want to write abstraction of Gpio object in Rust, using C library functions. Therefore need something like opaque pointer type (in C++ I would just create a member variable with type: gpio_type). I figured I would create an empty enum (or struct), allocate a space needed for the object and transmute it to match the type in C layer.

pub enum gpio_type {}

#[link(name = "gpio_lib", kind = "static")]
extern {
    pub fn gpio_init(obj: *mut gpio_type, value: i32);
}

pub struct Gpio {
    gpio : *mut gpio_type,
}

impl Gpio {
    pub fn new(pin: u32) -> Gpio {
        unsafe {
            let mut gpio_ptr : &'static [u8; 4] = init(); // size of gpio in C is 4 bytes for one target, will be changed later to obtain it dynamically
            let gpio_out = Gpio { gpio: transmute(gpio_ptr)};
            gpio_init(gpio_out.gpio, pin);
            gpio_out
        }
    }
}

This targets embedded devices, therefore no std, no libc. I don't want to redefine gpio_type for each target in rust (copy the C declaration for each target), looking for something to just allocate memory for the object which C will handle.

The following snippet below produces pointer to address 0 according to disassembly. Disassembly for Gpio new method:

 45c:   b580        push    {r7, lr}
 45e:   466f        mov r7, sp
 460:   4601        mov r1, r0
 462:   2000        movs    r0, #0
 464:   f000 fae6   bl  a34 <gpio_init>
 468:   2000        movs    r0, #0
 46a:   bd80        pop {r7, pc}

Any ideas why 462 is 0 ?

Answer 1

looking for something to just allocate memory for the object which C will handle

What about something like this? Give the struct an actual size (in this case by giving it a fixed-size array of byte-sized items), allocate that space on the heap, then treat that as a raw pointer.

use std::mem;

#[allow(missing_copy_implementations)]
pub struct Gpio([u8; 4]);

impl Gpio {
    fn new() -> Gpio { Gpio([0,0,0,0]) }
}

fn main() {
    // Allocate some bytes and get a raw pointer
    let a: *mut u8 = unsafe { mem::transmute(Box::new(Gpio::new())) };

    // Use it here!

    // When done... back to a box
    let b: Box<Gpio> = unsafe { mem::transmute(a) };

    // Now it will be dropped automatically (and free the allocated memory)

    // Or you can be explicit
    drop(b);
}

However, I'd suggest doing something like this; it's a lot more obvious and doesn't need a heap allocation:

#[allow(missing_copy_implementations)]
pub struct Gpio([u8; 4]);

impl Gpio {
    fn new() -> Gpio { Gpio([0,0,0,0]) }

    fn as_mut_ptr(&mut self) -> *mut u8 {
        self.0.as_mut_ptr()
    }
}

fn main() {
    let mut g = Gpio::new();
    let b = g.as_mut_ptr();
}

As a bonus, you get a nice place to hang some methods on. Potentially as_mut_ptr wouldn't need to be public, and could be hidden behind public methods on the Gpio struct.

(might also be able to use uninitialized instead of [0,0,0,0])

An expanded example of the second suggestion

// This depends on your library, check the FFI guide for details
extern {
    fn gpio_init(gpio: *mut u8, pin: u8);
    fn gpio_pin_on(gpio: *mut u8);
    fn gpio_pin_off(gpio: *mut u8);
}

#[allow(missing_copy_implementations)]
pub struct Gpio([u8; 4]);

impl Gpio {
    fn new(pin: u8) -> Gpio {
        let mut g = Gpio([0,0,0,0]);
        g.init(pin);
        g
    }

    fn as_mut_ptr(&mut self) -> *mut u8 {
        self.0.as_mut_ptr()
    }

    fn init(&mut self, pin: u8) { unsafe { gpio_init(self.as_mut_ptr(), pin) } }
    pub fn on(&mut self) { unsafe { gpio_pin_on(self.as_mut_ptr()) } }
    pub fn off(&mut self) { unsafe { gpio_pin_off(self.as_mut_ptr()) } }
}

static BLUE_LED_PIN: u8 = 0x4;

fn main() {
    let mut g = Gpio::new(BLUE_LED_PIN);
    g.on();
    g.off();
}