Unable to use recvfrom() on TCP socket

[thib-ack]

Hello,

I try to use the socket::recvfrom() function on a TCP socket and I get an error ENOTCONN.

I agree I could use the socket::recv() function but the docs says using recvfrom is allowed
https://linux.die.net/man/2/recvfrom

The recvfrom() [...] may be used to receive data on a socket whether or not it is connection-oriented.

I believe the issue comes from the function sockaddr_storage_to_addr() :
https://github.com/nix-rust/nix/blob/master/src/sys/socket/mod.rs#L1243

    if len < mem::size_of_val(&addr.ss_family) {
        return Err(Error::Sys(Errno::ENOTCONN));
    }

Here is an example of the problem. If you connect to this server with a tcp client (telnet for ex). You will get errors: Read err ENOTCONN: Transport endpoint is not connected

use nix::sys::socket;
use nix::sys::socket::{AddressFamily, SockType, SockFlag, InetAddr, SockAddr, IpAddr};

fn main() {
    let localhost = InetAddr::new(IpAddr::new_v4(127, 0, 0, 1), 6678);

    let tcp_listener = socket::socket(AddressFamily::Inet, SockType::Stream, SockFlag::empty(), None).unwrap();
    socket::bind(tcp_listener, &SockAddr::new_inet(localhost)).unwrap();
    socket::listen(tcp_listener, 1).unwrap();

    loop {
        let client_socket = socket::accept(tcp_listener).unwrap();

        loop {
            let mut buf: [u8; 256] = [0; 256];

            match socket::recvfrom(client_socket, &mut buf[..]) {
                Ok((len, from)) => {
                    buf[len] = 0;
                    let message = String::from_utf8_lossy(&buf);
                    println!("Message from {} of len {}: {}", from, len, message);
                },
                Err(e) => {
                    println!("Read err {}", e);
                }
            }
        }
    }
}

On the other hand, with this C code, it works as expected

#include <stdio.h>
#include <arpa/inet.h>
#include <string.h>

#define READ_SIZE 200

int main() {
    struct sockaddr_in localhost;
    memset(&localhost, 0, sizeof(struct sockaddr_in));
    localhost.sin_family = AF_INET;
    localhost.sin_port = htons(6677);
    inet_pton(AF_INET, "127.0.0.1", &(localhost.sin_addr));

    int tcp_listener = socket(AF_INET, SOCK_STREAM, 0);

    bind(tcp_listener, (struct sockaddr *) &localhost, sizeof(struct sockaddr_in));
    listen(tcp_listener, 1);

    while (1) {

        int client_socket = accept(tcp_listener, NULL, NULL);

        while(1) {
            char read_buffer[READ_SIZE + 1];
            int bytes_read = recvfrom(client_socket, read_buffer, READ_SIZE, 0, NULL, NULL);
            read_buffer[bytes_read] = '\0';
            printf("Message of len %d: %s\n", bytes_read, read_buffer);
        }
    }

    return 0;
}

Thanks,
Thibaut

[asomers]

You're correct, and it's somewhat shocking that nobody has noticed that bug for more than four years. Maybe most people don't use recvfrom that way because the standard library works well enough? In any case, I'll fix it.

[thib-ack]

Amazing, thank you !

[asomers]

Could you please check PR #1145 and tell me if that change works for you?

[thib-ack]

Hi,

It works like a charm with your fix 👍
The returned Option<SockAddr> behave as expected, with Some(SockAddr) in case of UDP socket and None in case of TCP socket.

Thanks,