Force non blocking read with TcpStream

Question

I've got a thread, that maintains a list of sockets, and I'd like to traverse the list, see if there is anything to read, if so - act upon it, if not - move onto the next. The problem is, as soon as I come across the first node, all execution is halted until something comes through on the read.

I'm using std::io::Read::read(&mut self, buf: &mut [u8]) -> Result<usize>

From the doc

This function does not provide any guarantees about whether it blocks waiting for data, but if an object needs to block for a read but cannot it will typically signal this via an Err return value.

Digging into the source, the TcpStream Read implementation is

impl Read for TcpStream {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { self.0.read(buf) }
}

Which invokes

pub fn read(&mut self, buf: &mut [u8]) -> IoResult<uint> {
    let fd = self.fd();
    let dolock = || self.lock_nonblocking();
    let doread = |nb| unsafe {
        let flags = if nb {c::MSG_DONTWAIT} else {0};
        libc::recv(fd,
                   buf.as_mut_ptr() as *mut libc::c_void,
                   buf.len() as wrlen,
                   flags) as libc::c_int
    };
    read(fd, self.read_deadline, dolock, doread)
}

And finally, calls read<T, L, R>(fd: sock_t, deadline: u64, mut lock: L, mut read: R)

Where I can see loops over non blocking reads until data has been retrieved or an error has occurred.

Is there a way to force a non-blocking read with TcpStream?

Answer 1

Updated Answer

It should be noted, that as of Rust 1.9.0, std::net::TcpStream has added functionality:

fn set_nonblocking(&self, nonblocking: bool) -> Result<()>

Original Answer

Couldn't exactly get it with TcpStream, and didn't want to pull in a separate lib for IO operations, so I decided to set the file descriptor as Non-blocking before using it, and executing a system call to read/write. Definitely not the safest solution, but less work than implementing a new IO lib, even though MIO looks great.

extern "system" {
    fn read(fd: c_int, buffer: *mut c_void, count: size_t) -> ssize_t;
}

pub fn new(user: User, stream: TcpStream) -> Socket {

    // First we need to setup the socket as Non-blocking on POSIX
    let fd = stream.as_raw_fd();
    unsafe {
        let ret_value = libc::fcntl(fd,
            libc::consts::os::posix01::F_SETFL,
            libc::consts::os::extra::O_NONBLOCK);

        // Ensure we didnt get an error code
        if ret_value < 0 {
            panic!("Unable to set fd as non-blocking")
        }
    }

    Socket {
        user: user,
        stream: stream
    }
}

pub fn read(&mut self) {
    let count = 512 as size_t;
    let mut buffer = [0u8; 512];
    let fd = self.stream.as_raw_fd();

    let mut num_read = 0 as ssize_t;
    unsafe {
        let buf_ptr = buffer.as_mut_ptr();
        let void_buf_ptr: *mut c_void = mem::transmute(buf_ptr);
        num_read = read(fd, void_buf_ptr, count);
        if num_read > 0 {
            println!("Read: {}", num_read);
        }

        println!("test");
    }
}