.. Networking:

==========
Networking
==========

Emscripten compiled applications have a number of ways to connect with online
servers. Check the subtopics here to learn about the different strategies that
are available.

If you are familiar with networking concepts provided by different web APIs,
such as XmlHttpRequest, Fetch, WebSockets and WebRTC, you can quickly get
started by leveraging what you already know: by calling out from C/C++ code to
JavaScript (see the "Connecting C++ and JavaScript" section), you can establish
networked connections by writing regular JavaScript. For C/C++ developers,
Emscripten provides a few approaches, described here.

Emscripten WebSockets API
=========================

WebSockets API provides connection-oriented message-framed bidirectional
asynchronous networking communication to the browser. It is the closest to TCP
on the web that web sites can access, direct access to TCP sockets is not
possible from web browsers.

Emscripten provides a passthrough API for accessing the WebSockets API from
C/C++ code. This is useful for developers who would prefer not to write any
JavaScript code, or deal with the C/C++ and JavaScript language interop. See the
system include file ``<emscripten/websocket.h>`` for details. One benefit that
the Emscripten WebSockets API provides over manual WebSockets access in
JavaScript is the ability to share access to a WebSocket handle across multiple
threads, something that can be time consuming to develop from scratch.

To target Emscripten WebSockets API, you must link it in with a
``-lwebsocket.js`` linker directive.

Emulated POSIX TCP Sockets over WebSockets
==========================================

If you have existing TCP networking code written in C/C++ that utilizes the
Posix Sockets API, by default Emscripten attempts to emulate such connections to
take place over the WebSocket protocol instead. For this to work, you will need
to use something like WebSockify on the server side to enable the TCP server
stack to receive incoming WebSocket connections. This emulation is not very
complete at the moment, it is likely that you will run into problems out of the
box and need to adapt the code to work within the limitations that this
emulation provides.

This is the default build mode for Emscripten. Use the linker flag
``-sWEBSOCKET_URL`` or ``Module['websocket']['url']`` to specify the WebSocket URL
to connect to, and the linker flag ``-sWEBSOCKET_SUBPROTOCOL`` or
``Module['websocket']['subprotocol']`` to control the connection type
(``'binary'`` or ``'text'``).

Full POSIX Sockets over WebSocket Proxy Server
==============================================

Emscripten provides a native POSIX Sockets proxy server program, located in
directory ``tools/websocket_to_posix_proxy/``, that allows full POSIX Sockets
API access from a web browser. This support works by proxying all POSIX Sockets
API calls from the browser to the Emscripten POSIX Sockets proxy server (via
transparent use of the WebSockets API), and the proxy server then performs the
native TCP/UDP calls on behalf of the page. This allows a web browser page to
run full TCP & UDP connections, act as a server to accept incoming connections,
and perform host name lookups and reverse lookups. Because all API calls are
individually proxied, this support can be slow. This support is mostly useful
for developing testing infrastructure and debugging.

The following POSIX sockets functions are proxied in this manner:
 - ``socket()``, ``socketpair()``, ``shutdown()``, ``bind()``, ``connect()``, ``listen()``, ``accept()``, ``getsockname()``, ``getpeername()``, ``send()``, ``recv()``, ``sendto()``, ``recvfrom()``, ``sendmsg()``, ``recvmsg()``, ``getsockopt()``, ``setsockopt()``, ``getaddrinfo()``, ``getnameinfo()``.

The following POSIX sockets functions are currently not proxied (and will not work):
 - ``poll()``, ``close()`` (use ``shutdown()`` instead), ``select()``

To use POSIX sockets proxying, link the application with flags ``-lwebsocket.js
-sPROXY_POSIX_SOCKETS -pthread -sPROXY_TO_PTHREAD``. That is,
POSIX sockets proxying builds on top of the Emscripten WebSockets library, and
requires multithreading and proxying the application ``main()`` to a pthread.

For an example of how the POSIX Sockets proxy server works in an Emscripten
client program, see the file ``test/websocket/tcp_echo_client.cpp``.

XmlHttpRequests and Fetch API
=============================

For HTTP transfers, one can use the browser built-in XmlHttpRequest (XHR) API
and the newer Fetch API. These can be accessed directly from JavaScript.
Emscripten also provides passthrough APIs to perform HTTP requests. For more
information, see the ``emscripten_async_wget*()`` C API and the Emscripten Fetch
API.

WebRTC and UDP
==============

Direct UDP communication is not available in browsers, but as a close
alternative, the WebRTC specification provides a mechanism to perform UDP-like
communication with WebRTC Data Channels. Currently Emscripten does not provide a
C/C++ API for interacting with WebRTC.
