# net FAQ

## What does net.listen return?

 returns different handles according to type:

| type | return type | public field | public method |
|------|------|------|------|
| web | WebServer | addr、type | close() |
| tcp | TcpServer | addr、type | close() |
| udp | UdpSocket | addr、type | send()、close() |
| ws | WsServer | addr、type | close() |

`addr` is the actual listening address, `type` is the service type string, `close()` Used to close a service or socket.

## What does net.connect return?

 returns different connection handles according to type:

| type | return type | field | common methods |
|------|------|------|------|
| tcp | TcpClient | addr、type | write()、send()、read()、close() |
| udp | UdpSocket | addr、type | send()、close() |
| ws | WsSocket | addr、type | send()、write()、close()、on_message()、on_close()、on_error() |

TCP and UDP use `host`, `port` to configure the connection target; WebSocket uses `url` to configure the complete connection address.

## Callback field names

 event callbacks uniformly use snake_case: on_connect, on_message, on_close, on_error.

## Port occupation

bind will throw an error when the port is occupied. Development tests can use 127.0.0.1:0 to let the operating system allocate free ports.

## What happens when the queue is full

TCP, UDP, and WebSocket background event queues have a fixed upper limit, the default is 4096. When the queue is full, it will not occupy unlimited memory: TCP and WebSocket will close the corresponding connection, and UDP will discard the current message. The number of occurrences can be viewed via `BT.stats().net.event_queue_rejected`.

## Error message

 User-visible errors of TCP, UDP, and WebSocket will try to use a unified Chinese format. Common types include port occupied, insufficient permissions, address unavailable, connection refused, connection reset by peer, handshake failure, write queue full, and message size exceeding upper limit.

## Stress test script

 `net-stress-tcp-server.bt` / `net-stress-tcp-client.bt`, `net-stress-udp-server.bt` / `net-stress-udp-client.bt`, `net-stress-ws-server.bt` / `net-stress-ws-client.bt` are provided under `examples/`. Start the corresponding server first, and then start the client. When you need to push down resource boundaries, set `BT_NET_EVENT_QUEUE`, `BT_NET_WRITE_QUEUE`, or `BT_NET_MESSAGE_LIMIT` before starting the process.

## How to receive binary messages

 After setting `binary: true` in the TCP, UDP, and WebSocket listening configuration, the message parameters of the `on_message` callback will return Bytes. TCP client synchronous reading can use `read_bytes()`.