How to use ioctl + nix macros to get a variable size buffer

This is related to How to use nix's ioctl? but it is not the same question.

I want to retrieve a variable size buffer. There is another ioctl that tells me that I need to read X bytes. The C header tells me the following too:

#define HID_MAX_DESCRIPTOR_SIZE     4096
#define HIDIOCGRDESC        _IOR('H', 0x02, struct hidraw_report_descriptor)

struct hidraw_report_descriptor {
    __u32 size;
    __u8 value[HID_MAX_DESCRIPTOR_SIZE];
};

I define the macro in the following way:

ioctl_read_buf!(hid_read_descr, b'H', 0x02, u8);

And later call:

let mut desc_raw = [0u8; 4 + 4096];
let err = unsafe { hid_read_descr(file.as_raw_fd(), &mut desc_raw); };

When doing this, desc_raw is full of zeros. I would have expected the first 4 bytes to contain size based on the struct definition.

The alternative, does not seem to work either

ioctl_read!(hid_read_descr2, b'H', 0x02, [u8; 4+4096]);
// ...
let mut desc_raw = [0xFFu8; 4 + 4096];
let err = unsafe { hid_read_descr2(file.as_raw_fd(), &mut desc_raw); };

In both cases, I have tried initializing desc_raw with 0xFF and after the call, it seems untouched.

Am I using the ioctl_read_buf macro incorrectly?

Solution

Now that Digikata has thoughtfully provided enough code to drive the program...

Am I using the ioctl_read_buf macro incorrectly?

I'd say that using it at all is incorrect here. You don't want to read an array of data, you want to read a single instance of a specific type. That's what ioctl_read! is for.

We define a repr(C) struct that mimics the C definition. This ensures that important details like alignment, padding, field ordering, etc., all match one-to-one with the code we are calling.

We can then construct an uninitialized instance of this struct and pass it to the newly-defined function.

use libc; // 0.2.66
use nix::ioctl_read; // 0.16.1
use std::{
    fs::OpenOptions,
    mem::MaybeUninit,
    os::unix::{fs::OpenOptionsExt, io::AsRawFd},
};

const HID_MAX_DESCRIPTOR_SIZE: usize = 4096;

#[repr(C)]
pub struct hidraw_report_descriptor {
    size: u32,
    value: [u8; HID_MAX_DESCRIPTOR_SIZE],
}

ioctl_read!(hid_read_sz, b'H', 0x01, libc::c_int);
ioctl_read!(hid_read_descr, b'H', 0x02, hidraw_report_descriptor);

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let file = OpenOptions::new()
        .read(true)
        .write(true)
        .custom_flags(libc::O_NONBLOCK)
        .open("/dev/hidraw0")?;

    unsafe {
        let fd = file.as_raw_fd();

        let mut size = 0;
        hid_read_sz(fd, &mut size)?;
        println!("{}", size);

        let mut desc_raw = MaybeUninit::<hidraw_report_descriptor>::uninit();
        (*desc_raw.as_mut_ptr()).size = size as u32;
        hid_read_descr(file.as_raw_fd(), desc_raw.as_mut_ptr())?;
        let desc_raw = desc_raw.assume_init();
        let data = &desc_raw.value[..desc_raw.size as usize];
        println!("{:02x?}", data);
    }

    Ok(())
}