Reference · UDP

udp.reply

udp.reply(map, any) -> null | error(str)

What it does

Sends a packet back to where one came from: given the packet map that udp.listen handed to its function, it sends to packet.from from the port the packet came to, packet.port, as most protocols expect the answer. The data is as udp.send takes it: text, a list of byte values, or any other value as JSON, up to 65,507 bytes. It gives null, or an error value when the system refuses the packet. It needs the net effect.

Examples

udp.listen(9000, (p) => {    let cmd = json.parse(p.text)    match cmd.op {        "ping" => udp.reply(p, "pong"),        "get" => udp.reply(p, { value: state[cmd.key] }),        _ => udp.reply(p, { error: "unknown op" }),    }})

Edge cases

• The answer leaves from the port the packet came to, not from a port of the system's choosing, so a sender that waits on the port it sent to hears it.
• A packet's map can be kept and answered later, from another request or a schedule, as long as the program still listens on its port.
• The sender may be gone: the packet is sent all the same and the result is null, since UDP has no way to know.
• The map must have port and from: { host, port }; a map made by hand with those fields is accepted, as long as the program listens on port.

udp.listen(9000, (p) => {    channel.broadcast("readings", p.text)    udp.reply(p, [6])                          // one byte: ACK})

Errors

A first argument that is not a packet map (no port, no from), null data, data over 65,507 bytes, and a packet whose port the program does not listen on are runtime errors; the last is what a packet map made by hand in a program without udp.listen meets. A refused send is an error value.

print(udp.reply({}, "x"))

line 1: `udp.reply` takes a packet that `udp.listen` gave, then what to send

let packet = { text: "hi", bytes: [104, 105], from: { host: "127.0.0.1", port: 5000 }, port: 9000, at: now() }print(udp.reply(packet, "ok"))

line 2: `udp.reply`: the program does not listen on UDP port 9000

Chaining

The packet comes first, so inside the handler p | udp.reply("ok") reads as an answer; the result is null or an error, so ?? ignores a failure and is_err sees it.

udp.listen(9000, (p) => {    let r = p | udp.reply("ok")    if is_err(r) { print("could not answer", p.from.host, r.error) }})

See also

udp.listen, udp.send, json.parse, is_err

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