Module asyncnet

This module implements a high-level asynchronous sockets API based on the asynchronous dispatcher defined in the asyncdispatch module.

Asynchronous IO in Nim

Async IO in Nim consists of multiple layers (from highest to lowest):

  • asyncnet module
  • Async await
  • asyncdispatch module (event loop)
  • selectors module

Each builds on top of the layers below it. The selectors module is an abstraction for the various system select() mechanisms such as epoll or kqueue. If you wish you can use it directly, and some people have done so successfully. But you must be aware that on Windows it only supports select().

The async dispatcher implements the proactor pattern and also has an implementation of IOCP. It implements the proactor pattern for other OS' via the selectors module. Futures are also implemented here, and indeed all the procedures return a future.

The final layer is the async await transformation. This allows you to write asynchronous code in a synchronous style and works similar to C#'s await. The transformation works by converting any async procedures into an iterator.

This is all single threaded, fully non-blocking and does give you a lot of control. In theory you should be able to work with any of these layers interchangeably (as long as you only care about non-Windows platforms).

For most applications using asyncnet is the way to go as it builds over all the layers, providing some extra features such as buffering.

SSL

SSL can be enabled by compiling with the -d:ssl flag.

You must create a new SSL context with the newContext function defined in the net module. You may then call wrapSocket on your socket using the newly created SSL context to get an SSL socket.

Examples

Chat server

The following example demonstrates a simple chat server.

import asyncnet, asyncdispatch

var clients {.threadvar.}: seq[AsyncSocket]

proc processClient(client: AsyncSocket) {.async.} =
  while true:
    let line = await client.recvLine()
    if line.len == 0: break
    for c in clients:
      await c.send(line & "\c\L")

proc serve() {.async.} =
  clients = @[]
  var server = newAsyncSocket()
  server.setSockOpt(OptReuseAddr, true)
  server.bindAddr(Port(12345))
  server.listen()
  
  while true:
    let client = await server.accept()
    clients.add client
    
    asyncCheck processClient(client)

asyncCheck serve()
runForever()

Types

AsyncSocket = ref AsyncSocketDesc
  Source Edit

Procs

proc newAsyncSocket(fd: AsyncFD; domain: Domain = AF_INET;
                   sockType: SockType = SOCK_STREAM;
                   protocol: Protocol = IPPROTO_TCP; buffered = true): AsyncSocket {.
raises: [], tags: []
.}
Creates a new AsyncSocket based on the supplied params.   Source Edit
proc newAsyncSocket(domain: Domain = AF_INET; sockType: SockType = SOCK_STREAM;
                   protocol: Protocol = IPPROTO_TCP; buffered = true): AsyncSocket {.
raises: [OSError], tags: []
.}

Creates a new asynchronous socket.

This procedure will also create a brand new file descriptor for this socket.

  Source Edit
proc newAsyncSocket(domain, sockType, protocol: cint; buffered = true): AsyncSocket {.
raises: [OSError], tags: []
.}

Creates a new asynchronous socket.

This procedure will also create a brand new file descriptor for this socket.

  Source Edit
proc connect(socket: AsyncSocket; address: string; port: Port): Future[void] {.
raises: [FutureError], tags: [RootEffect]
.}

Connects socket to server at address:port.

Returns a Future which will complete when the connection succeeds or an error occurs.

  Source Edit
proc recvInto(socket: AsyncSocket; buf: pointer; size: int; flags = {SafeDisconn}): Future[
    int] {.
raises: [FutureError], tags: [RootEffect]
.}

Reads up to size bytes from socket into buf.

For buffered sockets this function will attempt to read all the requested data. It will read this data in BufferSize chunks.

For unbuffered sockets this function makes no effort to read all the data requested. It will return as much data as the operating system gives it.

If socket is disconnected during the recv operation then the future may complete with only a part of the requested data.

If socket is disconnected and no data is available to be read then the future will complete with a value of 0.

  Source Edit
proc recv(socket: AsyncSocket; size: int; flags = {SafeDisconn}): Future[string] {.
raises: [FutureError], tags: [RootEffect]
.}

Reads up to size bytes from socket.

For buffered sockets this function will attempt to read all the requested data. It will read this data in BufferSize chunks.

For unbuffered sockets this function makes no effort to read all the data requested. It will return as much data as the operating system gives it.

If socket is disconnected during the recv operation then the future may complete with only a part of the requested data.

If socket is disconnected and no data is available to be read then the future will complete with a value of "".

  Source Edit
proc send(socket: AsyncSocket; buf: pointer; size: int; flags = {SafeDisconn}): Future[
    void] {.
raises: [FutureError], tags: [RootEffect]
.}
Sends size bytes from buf to socket. The returned future will complete once all data has been sent.   Source Edit
proc send(socket: AsyncSocket; data: string; flags = {SafeDisconn}): Future[void] {.
raises: [FutureError], tags: [RootEffect]
.}
Sends data to socket. The returned future will complete once all data has been sent.   Source Edit
proc acceptAddr(socket: AsyncSocket; flags = {SafeDisconn}): Future[
    tuple[address: string, client: AsyncSocket]] {.
raises: [ValueError, OSError, Exception, FutureError], tags: [RootEffect]
.}
Accepts a new connection. Returns a future containing the client socket corresponding to that connection and the remote address of the client. The future will complete when the connection is successfully accepted.   Source Edit
proc accept(socket: AsyncSocket; flags = {SafeDisconn}): Future[AsyncSocket] {.
raises: [ValueError, OSError, Exception, FutureError], tags: [RootEffect]
.}
Accepts a new connection. Returns a future containing the client socket corresponding to that connection. The future will complete when the connection is successfully accepted.   Source Edit
proc recvLineInto(socket: AsyncSocket; resString: FutureVar[string];
                 flags = {SafeDisconn}; maxLength = MaxLineLength): Future[void] {.
raises: [FutureError], tags: [RootEffect]
.}

Reads a line of data from socket into resString.

If a full line is read \r\L is not added to line, however if solely \r\L is read then line will be set to it.

If the socket is disconnected, line will be set to "".

If the socket is disconnected in the middle of a line (before \r\L is read) then line will be set to "". The partial line will be lost.

The maxLength parameter determines the maximum amount of characters that can be read before a ValueError is raised. This prevents Denial of Service (DOS) attacks.

Warning: The Peek flag is not yet implemented.

Warning: recvLineInto on unbuffered sockets assumes that the protocol uses \r\L to delimit a new line.

  Source Edit
proc recvLine(socket: AsyncSocket; flags = {SafeDisconn}; maxLength = MaxLineLength): Future[
    string] {.
raises: [FutureError], tags: [RootEffect]
.}

Reads a line of data from socket. Returned future will complete once a full line is read or an error occurs.

If a full line is read \r\L is not added to line, however if solely \r\L is read then line will be set to it.

If the socket is disconnected, line will be set to "".

If the socket is disconnected in the middle of a line (before \r\L is read) then line will be set to "". The partial line will be lost.

The maxLength parameter determines the maximum amount of characters that can be read before a ValueError is raised. This prevents Denial of Service (DOS) attacks.

Warning: The Peek flag is not yet implemented.

Warning: recvLine on unbuffered sockets assumes that the protocol uses \r\L to delimit a new line.

  Source Edit
proc listen(socket: AsyncSocket; backlog = SOMAXCONN) {.
tags: [ReadIOEffect], raises: [OSError]
.}

Marks socket as accepting connections. Backlog specifies the maximum length of the queue of pending connections.

Raises an EOS error upon failure.

  Source Edit
proc bindAddr(socket: AsyncSocket; port = Port(0); address = "") {.
tags: [ReadIOEffect], raises: [ValueError, OSError]
.}

Binds address:port to the socket.

If address is "" then ADDR_ANY will be bound.

  Source Edit
proc close(socket: AsyncSocket) {.
raises: [SslError, OSError], tags: []
.}
Closes the socket.   Source Edit
proc wrapSocket(ctx: SslContext; socket: AsyncSocket) {.
raises: [SslError, OSError], tags: []
.}

Wraps a socket in an SSL context. This function effectively turns socket into an SSL socket.

Disclaimer: This code is not well tested, may be very unsafe and prone to security vulnerabilities.

  Source Edit
proc wrapConnectedSocket(ctx: SslContext; socket: AsyncSocket;
                        handshake: SslHandshakeType) {.
raises: [SslError, OSError], tags: []
.}

Wraps a connected socket in an SSL context. This function effectively turns socket into an SSL socket.

This should be called on a connected socket, and will perform an SSL handshake immediately.

Disclaimer: This code is not well tested, may be very unsafe and prone to security vulnerabilities.

  Source Edit
proc getSockOpt(socket: AsyncSocket; opt: SOBool; level = SOL_SOCKET): bool {.
tags: [ReadIOEffect], raises: [OSError]
.}
Retrieves option opt as a boolean value.   Source Edit
proc setSockOpt(socket: AsyncSocket; opt: SOBool; value: bool; level = SOL_SOCKET) {.
tags: [WriteIOEffect], raises: [OSError]
.}
Sets option opt to a boolean value specified by value.   Source Edit
proc isSsl(socket: AsyncSocket): bool {.
raises: [], tags: []
.}
Determines whether socket is a SSL socket.   Source Edit
proc getFd(socket: AsyncSocket): SocketHandle {.
raises: [], tags: []
.}
Returns the socket's file descriptor.   Source Edit
proc isClosed(socket: AsyncSocket): bool {.
raises: [], tags: []
.}
Determines whether the socket has been closed.   Source Edit