Allocating an object for C / FFI library calls - c

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();
}