asterinas/kernel/src/syscall/read.rs

// SPDX-License-Identifier: MPL-2.0

use super::SyscallReturn;
use crate::{
    fs::file_table::{get_file_fast, FileDesc},
    prelude::*,
};

pub fn sys_read(
    fd: FileDesc,
    user_buf_addr: Vaddr,
    buf_len: usize,
    ctx: &Context,
) -> Result<SyscallReturn> {
    debug!(
        "fd = {}, user_buf_ptr = 0x{:x}, buf_len = 0x{:x}",
        fd, user_buf_addr, buf_len
    );

    let mut file_table = ctx.thread_local.borrow_file_table_mut();
    let file = get_file_fast!(&mut file_table, fd);

    // According to <https://man7.org/linux/man-pages/man2/read.2.html>, if
    // the user specified an empty buffer, we should detect errors by checking
    // the file descriptor. If no errors detected, return 0 successfully.
    let read_len = {
        if buf_len != 0 {
            let user_space = ctx.user_space();
            let mut writer = user_space.writer(user_buf_addr, buf_len)?;
            file.read(&mut writer)
        } else {
            file.read_bytes(&mut [])
        }
    }
    .map_err(|err| match err.error() {
        Errno::EINTR => Error::new(Errno::ERESTARTSYS),
        _ => err,
    })?;

    Ok(SyscallReturn::Return(read_len as _))
}