Error in examples/readme.rs

[jiangyinzuo]

I download this repository and run

 cargo run --example readme

The program panics and outputs

thread 'main' panicked at 'read error: -22', examples/readme.rs:28:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

My OS envirenment is

Linux version 5.4.0-67-generic (buildd@lcy01-amd64-025) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)) #75-Ubuntu SMP Fri Feb 19 18:03:38 UTC 2021

My rust version is 1.51 and io-uring version is 0.5.1.

[quininer]

Read opcode is available in the 5.6 kernel, and you use 5.4 kernel. You can use Readv opcode instead.

[jiangyinzuo]

I use Readv and modify the code as follows, and it runs successfully. Thanks a lot.

use io_uring::{opcode, types, IoUring};
use std::os::unix::io::AsRawFd;
use std::{fs, io};
use libc::c_void;

fn main() -> io::Result<()> {
    let mut ring = IoUring::new(8)?;

    let fd = fs::File::open("README.md")?;
    let mut buf = vec![0u8; 4096];

    let iovec = libc::iovec {
        iov_base: buf.as_mut_ptr() as *mut c_void,
        iov_len: 4096,
    };
    let read_e = opcode::Readv::new(types::Fd(fd.as_raw_fd()), &iovec as *const _, 1)
        .build()
        .user_data(0x42);

    // Note that the developer needs to ensure
    // that the entry pushed into submission queue is valid (e.g. fd, buffer).
    unsafe {
        ring.submission()
            .push(&read_e)
            .expect("submission queue is full");
    }

    ring.submit_and_wait(1)?;

    let cqe = ring.completion().next().expect("completion queue is empty");

    assert_eq!(cqe.user_data(), 0x42);
    assert!(cqe.result() >= 0, "read error: {}", cqe.result());

    println!("{}", cqe.result());
    println!("{}", std::str::from_utf8(buf.as_slice()).unwrap());
    Ok(())
}