如何从 Rust 写入特定的原始文件描述符？

Question

我需要写入文件描述符 3。我一直在寻找它，但文档很差。我发现的唯一一件事是 libc 库和 fdopen 方法的使用，但我还没有找到任何关于如何使用它或在其上编写的示例。

谁能给我一个用 Rust 写入文件描述符的例子吗？

Answer 1

libc crate 是 "just" C 和 Rust 之间接口的包装库，因此要了解如何使用函数，应该阅读 C 函数的手册，有很多来源那，这里是 fdopen():

The fdopen() function associates a stream with the existing file descriptor, fd. The mode of the stream (one of the values "r", "r+", "w", "w+", "a", "a+") must be compatible with the mode of the file descriptor. The file position indicator of the new stream is set to that belonging to fd, and the error and end-of-file indicators are cleared. Modes "w" or "w+" do not cause truncation of the file. The file descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed. The result of applying fdopen() to a shared memory object is undefined.

基本使用是这样的：

use libc::fdopen;
use std::ffi::CString;

fn main() {
    let mode = CString::new("w").unwrap();
    unsafe {
        let _ = fdopen(3, mode.as_ptr());
    }
}

要使用它，您可以使用 fwrite():

The function fwrite() writes nmemb elements of data, each size bytes long, to the stream pointed to by stream, obtaining them from the location given by ptr.

所以，完整的例子：

use libc::{c_void, fdopen, fwrite};
use std::ffi::CString;

fn main() {
    let mode = CString::new("w").unwrap();
    let file = unsafe {
        let file = fdopen(3, mode.as_ptr());
        if file.is_null() {
            panic!("can't open file");
        }
        file
    };

    let welcome = "Hello world!";

    let result = unsafe { fwrite(welcome.as_ptr() as *const c_void, 1, welcome.len(), file) };
    if result != welcome.len() {
        panic!("write not successful");
    }
}

Answer 2

您可以使用来自特定文件描述符的 FromRawFd to create a File，但仅限于类 UNIX 操作系统：

use std::{
    fs::File,
    io::{self, Write},
    os::unix::io::FromRawFd,
};

fn main() -> io::Result<()> {
    let mut f = unsafe { File::from_raw_fd(3) };
    write!(&mut f, "Hello, world!")?;
    Ok(())
}

$ target/debug/example 3> /tmp/output
$ cat /tmp/output
Hello, world!

from_raw_fd 是不安全的，因为不能保证文件描述符是有效的或者谁实际负责该文件描述符。

创建的 File 将取得文件描述符的所有权：当 File 超出范围时，文件描述符将被关闭。您可以使用 IntoRawFd or mem::forget.

来避免这种情况

另请参阅：

How can I read from a specific raw file descriptor in Rust?

如何从 Rust 写入特定的原始文件描述符？

How do I write to a specific raw file descriptor from Rust?

unix

io

file-descriptor

rust