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Read all data from TCP stream in Rust
I'd like to write a TCP client in Rust that can receive the entire message sent by a server, but I have no information about the length of a message.
I'm aware that TCP doesn't preserve message boundaries.
Still, what's the best I can do to read the entire message from the TcpStream
?
In the scenario the client should work, there's no protocol for the messages sent from server to client: The client doesn't know beforehand how many bytes are in a message and the message doesn't have a header (as in HTTP) that would contain the message length. There's no special delimiter that marks the end of a message.
Also, the server might keep the TCP connection open after sending a message, meaning I can not rely on every message being completed with a server FIN.
I'd like to do this using Rust's standard library only, no other dependencies.
1 answer
The following users marked this post as Works for me:
User | Comment | Date |
---|---|---|
Matthias Braun | (no comment) | May 4, 2023 at 06:41 |
One way to do this, is:
- Set a read timeout on the
TcpStream
usingset_read_timeout
. This avoids a hanging client in case the server has stopped sending but left the connection open. - Read from the stream in a loop, using a
BufReader
, and aggregate the read bytes. - Break out of the loop in case the read timed out or the server closed the connection.
Here's some example code:
use std::{
io::{self, BufReader, LineWriter, Read, Write},
net::TcpStream,
time::Duration,
};
fn main() -> io::Result<()> {
println!("Start of program");
// This is just to get data via TCP, we could of course parse the HTTP header for
// the message length
let server_host = "www.neverssl.com";
let server_port = "80";
let stream = TcpStream::connect(server_host.to_string() + ":" + server_port)?;
let mut reader = BufReader::new(&stream);
let mut writer = LineWriter::new(&stream);
// Don't block indefinitely on `reader.read` when there's no data to read
let max_read_time = Duration::from_millis(200);
stream.set_read_timeout(Some(max_read_time))?;
writer.write_all(format!("GET / HTTP/1.1\r\nhost: {server_host}\r\n\r\n").as_bytes())?;
let mut total_bytes_read = vec![];
let bytes_to_read_per_attempt = 1024;
let mut read_attempt_nr = 0;
loop {
read_attempt_nr += 1;
println!("Read attempt nr {read_attempt_nr}");
let mut cur_buffer = vec![0; bytes_to_read_per_attempt];
// If the reader has no data but the server hasn't closed the connection,
// by default `reader.read()` would block until the server closes the connection.
// Hence, we need `stream.set_read_timeout`.
let nr_of_bytes_read = match reader.read(&mut cur_buffer) {
Ok(nr_of_bytes_read) => nr_of_bytes_read,
Err(err) => {
if err.kind() == io::ErrorKind::WouldBlock || err.kind() == io::ErrorKind::TimedOut
{
println!("Read attempt timed out");
break;
} else {
return Err(err);
}
}
};
// Reading zero bytes indicates that the server closed the connection
if nr_of_bytes_read == 0 {
println!("Read zero bytes → Connection seems closed");
break;
}
// Remove the excess null bytes at the end of cur_buffer
cur_buffer.truncate(nr_of_bytes_read);
total_bytes_read.append(&mut cur_buffer);
println!("Read {nr_of_bytes_read} bytes in attempt nr {read_attempt_nr}");
}
let response = String::from_utf8_lossy(&total_bytes_read).to_string();
let nr_of_bytes_received = total_bytes_read.len();
println!("Server response : {response}");
println!("{nr_of_bytes_received} bytes received");
Ok(println!("End of program"))
}
This client assumes that if it hasn't received data from the server for 200 milliseconds (while the connection is still open), that the server's message is complete.
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