src/library_sockfs.js

// Copyright 2013 The Emscripten Authors.  All rights reserved.
// Emscripten is available under two separate licenses, the MIT license and the
// University of Illinois/NCSA Open Source License.  Both these licenses can be
// found in the LICENSE file.

mergeInto(LibraryManager.library, {
  $SOCKFS__postset: function() {
    addAtInit('SOCKFS.root = FS.mount(SOCKFS, {}, null);');
  },
  $SOCKFS__deps: ['$FS'],
  $SOCKFS: {
    mount: function(mount) {
      // If Module['websocket'] has already been defined (e.g. for configuring
      // the subprotocol/url) use that, if not initialise it to a new object.
      Module['websocket'] = (Module['websocket'] && 
                             ('object' === typeof Module['websocket'])) ? Module['websocket'] : {};

      // Add the Event registration mechanism to the exported websocket configuration
      // object so we can register network callbacks from native JavaScript too.
      // For more documentation see system/include/emscripten/emscripten.h
      Module['websocket']._callbacks = {};
      Module['websocket']['on'] = function(event, callback) {
	    if ('function' === typeof callback) {
		  this._callbacks[event] = callback;
        }
	    return this;
      };

      Module['websocket'].emit = function(event, param) {
	    if ('function' === typeof this._callbacks[event]) {
		  this._callbacks[event].call(this, param);
        }
      };

      // If debug is enabled register simple default logging callbacks for each Event.
#if SOCKET_DEBUG
      Module['websocket']['on']('error', function(error) {err('Socket error ' + error);});
      Module['websocket']['on']('open', function(fd) {out('Socket open fd = ' + fd);});
      Module['websocket']['on']('listen', function(fd) {out('Socket listen fd = ' + fd);});
      Module['websocket']['on']('connection', function(fd) {out('Socket connection fd = ' + fd);});
      Module['websocket']['on']('message', function(fd) {out('Socket message fd = ' + fd);});
      Module['websocket']['on']('close', function(fd) {out('Socket close fd = ' + fd);});
#endif

      return FS.createNode(null, '/', {{{ cDefine('S_IFDIR') }}} | 511 /* 0777 */, 0);
    },
    createSocket: function(family, type, protocol) {
      var streaming = type == {{{ cDefine('SOCK_STREAM') }}};
      if (protocol) {
        assert(streaming == (protocol == {{{ cDefine('IPPROTO_TCP') }}})); // if SOCK_STREAM, must be tcp
      }

      // create our internal socket structure
      var sock = {
        family: family,
        type: type,
        protocol: protocol,
        server: null,
        error: null, // Used in getsockopt for SOL_SOCKET/SO_ERROR test
        peers: {},
        pending: [],
        recv_queue: [],
#if SOCKET_WEBRTC
#else
        sock_ops: SOCKFS.websocket_sock_ops
#endif
      };

      // create the filesystem node to store the socket structure
      var name = SOCKFS.nextname();
      var node = FS.createNode(SOCKFS.root, name, {{{ cDefine('S_IFSOCK') }}}, 0);
      node.sock = sock;

      // and the wrapping stream that enables library functions such
      // as read and write to indirectly interact with the socket
      var stream = FS.createStream({
        path: name,
        node: node,
        flags: FS.modeStringToFlags('r+'),
        seekable: false,
        stream_ops: SOCKFS.stream_ops
      });

      // map the new stream to the socket structure (sockets have a 1:1
      // relationship with a stream)
      sock.stream = stream;

      return sock;
    },
    getSocket: function(fd) {
      var stream = FS.getStream(fd);
      if (!stream || !FS.isSocket(stream.node.mode)) {
        return null;
      }
      return stream.node.sock;
    },
    // node and stream ops are backend agnostic
    stream_ops: {
      poll: function(stream) {
        var sock = stream.node.sock;
        return sock.sock_ops.poll(sock);
      },
      ioctl: function(stream, request, varargs) {
        var sock = stream.node.sock;
        return sock.sock_ops.ioctl(sock, request, varargs);
      },
      read: function(stream, buffer, offset, length, position /* ignored */) {
        var sock = stream.node.sock;
        var msg = sock.sock_ops.recvmsg(sock, length);
        if (!msg) {
          // socket is closed
          return 0;
        }
        buffer.set(msg.buffer, offset);
        return msg.buffer.length;
      },
      write: function(stream, buffer, offset, length, position /* ignored */) {
        var sock = stream.node.sock;
        return sock.sock_ops.sendmsg(sock, buffer, offset, length);
      },
      close: function(stream) {
        var sock = stream.node.sock;
        sock.sock_ops.close(sock);
      }
    },
    nextname: function() {
      if (!SOCKFS.nextname.current) {
        SOCKFS.nextname.current = 0;
      }
      return 'socket[' + (SOCKFS.nextname.current++) + ']';
    },
    // backend-specific stream ops
    websocket_sock_ops: {
      //
      // peers are a small wrapper around a WebSocket to help in
      // emulating dgram sockets
      //
      // these functions aren't actually sock_ops members, but we're
      // abusing the namespace to organize them
      //
      createPeer: function(sock, addr, port) {
        var ws;

        if (typeof addr === 'object') {
          ws = addr;
          addr = null;
          port = null;
        }

        if (ws) {
          // for sockets that've already connected (e.g. we're the server)
          // we can inspect the _socket property for the address
          if (ws._socket) {
            addr = ws._socket.remoteAddress;
            port = ws._socket.remotePort;
          }
          // if we're just now initializing a connection to the remote,
          // inspect the url property
          else {
            var result = /ws[s]?:\/\/([^:]+):(\d+)/.exec(ws.url);
            if (!result) {
              throw new Error('WebSocket URL must be in the format ws(s)://address:port');
            }
            addr = result[1];
            port = parseInt(result[2], 10);
          }
        } else {
          // create the actual websocket object and connect
          try {
            // runtimeConfig gets set to true if WebSocket runtime configuration is available.
            var runtimeConfig = (Module['websocket'] && ('object' === typeof Module['websocket']));

            // The default value is 'ws://' the replace is needed because the compiler replaces '//' comments with '#'
            // comments without checking context, so we'd end up with ws:#, the replace swaps the '#' for '//' again.
            var url = '{{{ WEBSOCKET_URL }}}'.replace('#', '//');

            if (runtimeConfig) {
              if ('string' === typeof Module['websocket']['url']) {
                url = Module['websocket']['url']; // Fetch runtime WebSocket URL config.
              }
            }

            if (url === 'ws://' || url === 'wss://') { // Is the supplied URL config just a prefix, if so complete it.
              var parts = addr.split('/');
              url = url + parts[0] + ":" + port + "/" + parts.slice(1).join('/');
            }

            // Make the WebSocket subprotocol (Sec-WebSocket-Protocol) default to binary if no configuration is set.
            var subProtocols = '{{{ WEBSOCKET_SUBPROTOCOL }}}'; // The default value is 'binary'

            if (runtimeConfig) {
              if ('string' === typeof Module['websocket']['subprotocol']) {
                subProtocols = Module['websocket']['subprotocol']; // Fetch runtime WebSocket subprotocol config.
              }
            }

            // The regex trims the string (removes spaces at the beginning and end, then splits the string by
            // <any space>,<any space> into an Array. Whitespace removal is important for Websockify and ws.
            subProtocols = subProtocols.replace(/^ +| +$/g,"").split(/ *, */);

            // The node ws library API for specifying optional subprotocol is slightly different than the browser's.
            var opts = ENVIRONMENT_IS_NODE ? {'protocol': subProtocols.toString()} : subProtocols;

            // some webservers (azure) does not support subprotocol header
            if (runtimeConfig && null === Module['websocket']['subprotocol']) {
              subProtocols = 'null';
              opts = undefined;
            }

#if SOCKET_DEBUG
            out('connect: ' + url + ', ' + subProtocols.toString());
#endif
            // If node we use the ws library.
            var WebSocketConstructor;
#if ENVIRONMENT_MAY_BE_NODE
            if (ENVIRONMENT_IS_NODE) {
              WebSocketConstructor = require('ws');
            } else
#endif // ENVIRONMENT_MAY_BE_NODE
#if ENVIRONMENT_MAY_BE_WEB
            if (ENVIRONMENT_IS_WEB) {
              WebSocketConstructor = window['WebSocket'];
            } else
#endif // ENVIRONMENT_MAY_BE_WEB
            {
              WebSocketConstructor = WebSocket;
            }
            ws = new WebSocketConstructor(url, opts);
            ws.binaryType = 'arraybuffer';
          } catch (e) {
            throw new FS.ErrnoError(ERRNO_CODES.EHOSTUNREACH);
          }
        }

#if SOCKET_DEBUG
        out('websocket adding peer: ' + addr + ':' + port);
#endif

        var peer = {
          addr: addr,
          port: port,
          socket: ws,
          dgram_send_queue: []
        };

        SOCKFS.websocket_sock_ops.addPeer(sock, peer);
        SOCKFS.websocket_sock_ops.handlePeerEvents(sock, peer);

        // if this is a bound dgram socket, send the port number first to allow
        // us to override the ephemeral port reported to us by remotePort on the
        // remote end.
        if (sock.type === {{{ cDefine('SOCK_DGRAM') }}} && typeof sock.sport !== 'undefined') {
#if SOCKET_DEBUG
          out('websocket queuing port message (port ' + sock.sport + ')');
#endif
          peer.dgram_send_queue.push(new Uint8Array([
              255, 255, 255, 255,
              'p'.charCodeAt(0), 'o'.charCodeAt(0), 'r'.charCodeAt(0), 't'.charCodeAt(0),
              ((sock.sport & 0xff00) >> 8) , (sock.sport & 0xff)
          ]));
        }

        return peer;
      },
      getPeer: function(sock, addr, port) {
        return sock.peers[addr + ':' + port];
      },
      addPeer: function(sock, peer) {
        sock.peers[peer.addr + ':' + peer.port] = peer;
      },
      removePeer: function(sock, peer) {
        delete sock.peers[peer.addr + ':' + peer.port];
      },
      handlePeerEvents: function(sock, peer) {
        var first = true;

        var handleOpen = function () {
#if SOCKET_DEBUG
          out('websocket handle open');
#endif

          Module['websocket'].emit('open', sock.stream.fd);

          try {
            var queued = peer.dgram_send_queue.shift();
            while (queued) {
#if SOCKET_DEBUG
              out('websocket sending queued data (' + queued.byteLength + ' bytes): ' + [Array.prototype.slice.call(new Uint8Array(queued))]);
#endif
              peer.socket.send(queued);
              queued = peer.dgram_send_queue.shift();
            }
          } catch (e) {
            // not much we can do here in the way of proper error handling as we've already
            // lied and said this data was sent. shut it down.
            peer.socket.close();
          }
        };

        function handleMessage(data) {
          assert(typeof data !== 'string' && data.byteLength !== undefined);  // must receive an ArrayBuffer

          // An empty ArrayBuffer will emit a pseudo disconnect event
          // as recv/recvmsg will return zero which indicates that a socket
          // has performed a shutdown although the connection has not been disconnected yet.
          if (data.byteLength == 0) {
            return;
          }
          data = new Uint8Array(data);  // make a typed array view on the array buffer

#if SOCKET_DEBUG
          out('websocket handle message (' + data.byteLength + ' bytes): ' + [Array.prototype.slice.call(data)]);
#endif

          // if this is the port message, override the peer's port with it
          var wasfirst = first;
          first = false;
          if (wasfirst &&
              data.length === 10 &&
              data[0] === 255 && data[1] === 255 && data[2] === 255 && data[3] === 255 &&
              data[4] === 'p'.charCodeAt(0) && data[5] === 'o'.charCodeAt(0) && data[6] === 'r'.charCodeAt(0) && data[7] === 't'.charCodeAt(0)) {
            // update the peer's port and it's key in the peer map
            var newport = ((data[8] << 8) | data[9]);
            SOCKFS.websocket_sock_ops.removePeer(sock, peer);
            peer.port = newport;
            SOCKFS.websocket_sock_ops.addPeer(sock, peer);
            return;
          }

          sock.recv_queue.push({ addr: peer.addr, port: peer.port, data: data });
          Module['websocket'].emit('message', sock.stream.fd);
        };

        if (ENVIRONMENT_IS_NODE) {
          peer.socket.on('open', handleOpen);
          peer.socket.on('message', function(data, flags) {
            if (!flags.binary) {
              return;
            }
            handleMessage((new Uint8Array(data)).buffer);  // copy from node Buffer -> ArrayBuffer
          });
          peer.socket.on('close', function() {
            Module['websocket'].emit('close', sock.stream.fd);
          });
          peer.socket.on('error', function(error) {
            // Although the ws library may pass errors that may be more descriptive than
            // ECONNREFUSED they are not necessarily the expected error code e.g. 
            // ENOTFOUND on getaddrinfo seems to be node.js specific, so using ECONNREFUSED
            // is still probably the most useful thing to do.
            sock.error = ERRNO_CODES.ECONNREFUSED; // Used in getsockopt for SOL_SOCKET/SO_ERROR test.
            Module['websocket'].emit('error', [sock.stream.fd, sock.error, 'ECONNREFUSED: Connection refused']);
            // don't throw
          });
        } else {
          peer.socket.onopen = handleOpen;
          peer.socket.onclose = function() {
            Module['websocket'].emit('close', sock.stream.fd);
          };
          peer.socket.onmessage = function peer_socket_onmessage(event) {
            handleMessage(event.data);
          };
          peer.socket.onerror = function(error) {
            // The WebSocket spec only allows a 'simple event' to be thrown on error,
            // so we only really know as much as ECONNREFUSED.
            sock.error = ERRNO_CODES.ECONNREFUSED; // Used in getsockopt for SOL_SOCKET/SO_ERROR test.
            Module['websocket'].emit('error', [sock.stream.fd, sock.error, 'ECONNREFUSED: Connection refused']);
          };
        }
      },

      //
      // actual sock ops
      //
      poll: function(sock) {
        if (sock.type === {{{ cDefine('SOCK_STREAM') }}} && sock.server) {
          // listen sockets should only say they're available for reading
          // if there are pending clients.
          return sock.pending.length ? ({{{ cDefine('POLLRDNORM') }}} | {{{ cDefine('POLLIN') }}}) : 0;
        }

        var mask = 0;
        var dest = sock.type === {{{ cDefine('SOCK_STREAM') }}} ?  // we only care about the socket state for connection-based sockets
          SOCKFS.websocket_sock_ops.getPeer(sock, sock.daddr, sock.dport) :
          null;

        if (sock.recv_queue.length ||
            !dest ||  // connection-less sockets are always ready to read
            (dest && dest.socket.readyState === dest.socket.CLOSING) ||
            (dest && dest.socket.readyState === dest.socket.CLOSED)) {  // let recv return 0 once closed
          mask |= ({{{ cDefine('POLLRDNORM') }}} | {{{ cDefine('POLLIN') }}});
        }

        if (!dest ||  // connection-less sockets are always ready to write
            (dest && dest.socket.readyState === dest.socket.OPEN)) {
          mask |= {{{ cDefine('POLLOUT') }}};
        }

        if ((dest && dest.socket.readyState === dest.socket.CLOSING) ||
            (dest && dest.socket.readyState === dest.socket.CLOSED)) {
          mask |= {{{ cDefine('POLLHUP') }}};
        }

        return mask;
      },
      ioctl: function(sock, request, arg) {
        switch (request) {
          case {{{ cDefine('FIONREAD') }}}:
            var bytes = 0;
            if (sock.recv_queue.length) {
              bytes = sock.recv_queue[0].data.length;
            }
            {{{ makeSetValue('arg', '0', 'bytes', 'i32') }}};
            return 0;
          default:
            return ERRNO_CODES.EINVAL;
        }
      },
      close: function(sock) {
        // if we've spawned a listen server, close it
        if (sock.server) {
          try {
            sock.server.close();
          } catch (e) {
          }
          sock.server = null;
        }
        // close any peer connections
        var peers = Object.keys(sock.peers);
        for (var i = 0; i < peers.length; i++) {
          var peer = sock.peers[peers[i]];
          try {
            peer.socket.close();
          } catch (e) {
          }
          SOCKFS.websocket_sock_ops.removePeer(sock, peer);
        }
        return 0;
      },
      bind: function(sock, addr, port) {
        if (typeof sock.saddr !== 'undefined' || typeof sock.sport !== 'undefined') {
          throw new FS.ErrnoError(ERRNO_CODES.EINVAL);  // already bound
        }
        sock.saddr = addr;
        sock.sport = port;
        // in order to emulate dgram sockets, we need to launch a listen server when
        // binding on a connection-less socket
        // note: this is only required on the server side
        if (sock.type === {{{ cDefine('SOCK_DGRAM') }}}) {
          // close the existing server if it exists
          if (sock.server) {
            sock.server.close();
            sock.server = null;
          }
          // swallow error operation not supported error that occurs when binding in the
          // browser where this isn't supported
          try {
            sock.sock_ops.listen(sock, 0);
          } catch (e) {
            if (!(e instanceof FS.ErrnoError)) throw e;
            if (e.errno !== ERRNO_CODES.EOPNOTSUPP) throw e;
          }
        }
      },
      connect: function(sock, addr, port) {
        if (sock.server) {
          throw new FS.ErrnoError(ERRNO_CODES.EOPNOTSUPP);
        }

        // TODO autobind
        // if (!sock.addr && sock.type == {{{ cDefine('SOCK_DGRAM') }}}) {
        // }

        // early out if we're already connected / in the middle of connecting
        if (typeof sock.daddr !== 'undefined' && typeof sock.dport !== 'undefined') {
          var dest = SOCKFS.websocket_sock_ops.getPeer(sock, sock.daddr, sock.dport);
          if (dest) {
            if (dest.socket.readyState === dest.socket.CONNECTING) {
              throw new FS.ErrnoError(ERRNO_CODES.EALREADY);
            } else {
              throw new FS.ErrnoError(ERRNO_CODES.EISCONN);
            }
          }
        }

        // add the socket to our peer list and set our
        // destination address / port to match
        var peer = SOCKFS.websocket_sock_ops.createPeer(sock, addr, port);
        sock.daddr = peer.addr;
        sock.dport = peer.port;

        // always "fail" in non-blocking mode
        throw new FS.ErrnoError(ERRNO_CODES.EINPROGRESS);
      },
      listen: function(sock, backlog) {
        if (!ENVIRONMENT_IS_NODE) {
          throw new FS.ErrnoError(ERRNO_CODES.EOPNOTSUPP);
        }
#if ENVIRONMENT_MAY_BE_NODE
        if (sock.server) {
           throw new FS.ErrnoError(ERRNO_CODES.EINVAL);  // already listening
        }
        var WebSocketServer = require('ws').Server;
        var host = sock.saddr;
#if SOCKET_DEBUG
        console.log('listen: ' + host + ':' + sock.sport);
#endif
        sock.server = new WebSocketServer({
          host: host,
          port: sock.sport
          // TODO support backlog
        });
        Module['websocket'].emit('listen', sock.stream.fd); // Send Event with listen fd.

        sock.server.on('connection', function(ws) {
#if SOCKET_DEBUG
          console.log('received connection from: ' + ws._socket.remoteAddress + ':' + ws._socket.remotePort);
#endif
          if (sock.type === {{{ cDefine('SOCK_STREAM') }}}) {
            var newsock = SOCKFS.createSocket(sock.family, sock.type, sock.protocol);

            // create a peer on the new socket
            var peer = SOCKFS.websocket_sock_ops.createPeer(newsock, ws);
            newsock.daddr = peer.addr;
            newsock.dport = peer.port;

            // push to queue for accept to pick up
            sock.pending.push(newsock);
            Module['websocket'].emit('connection', newsock.stream.fd);
          } else {
            // create a peer on the listen socket so calling sendto
            // with the listen socket and an address will resolve
            // to the correct client
            SOCKFS.websocket_sock_ops.createPeer(sock, ws);
            Module['websocket'].emit('connection', sock.stream.fd);
          }
        });
        sock.server.on('closed', function() {
          Module['websocket'].emit('close', sock.stream.fd);
          sock.server = null;
        });
        sock.server.on('error', function(error) {
          // Although the ws library may pass errors that may be more descriptive than
          // ECONNREFUSED they are not necessarily the expected error code e.g. 
          // ENOTFOUND on getaddrinfo seems to be node.js specific, so using EHOSTUNREACH
          // is still probably the most useful thing to do. This error shouldn't
          // occur in a well written app as errors should get trapped in the compiled
          // app's own getaddrinfo call.
          sock.error = ERRNO_CODES.EHOSTUNREACH; // Used in getsockopt for SOL_SOCKET/SO_ERROR test.
          Module['websocket'].emit('error', [sock.stream.fd, sock.error, 'EHOSTUNREACH: Host is unreachable']);
          // don't throw
        });
#endif // ENVIRONMENT_MAY_BE_NODE
      },
      accept: function(listensock) {
        if (!listensock.server) {
          throw new FS.ErrnoError(ERRNO_CODES.EINVAL);
        }
        var newsock = listensock.pending.shift();
        newsock.stream.flags = listensock.stream.flags;
        return newsock;
      },
      getname: function(sock, peer) {
        var addr, port;
        if (peer) {
          if (sock.daddr === undefined || sock.dport === undefined) {
            throw new FS.ErrnoError(ERRNO_CODES.ENOTCONN);
          }
          addr = sock.daddr;
          port = sock.dport;
        } else {
          // TODO saddr and sport will be set for bind()'d UDP sockets, but what
          // should we be returning for TCP sockets that've been connect()'d?
          addr = sock.saddr || 0;
          port = sock.sport || 0;
        }
        return { addr: addr, port: port };
      },
      sendmsg: function(sock, buffer, offset, length, addr, port) {
        if (sock.type === {{{ cDefine('SOCK_DGRAM') }}}) {
          // connection-less sockets will honor the message address,
          // and otherwise fall back to the bound destination address
          if (addr === undefined || port === undefined) {
            addr = sock.daddr;
            port = sock.dport;
          }
          // if there was no address to fall back to, error out
          if (addr === undefined || port === undefined) {
            throw new FS.ErrnoError(ERRNO_CODES.EDESTADDRREQ);
          }
        } else {
          // connection-based sockets will only use the bound
          addr = sock.daddr;
          port = sock.dport;
        }

        // find the peer for the destination address
        var dest = SOCKFS.websocket_sock_ops.getPeer(sock, addr, port);

        // early out if not connected with a connection-based socket
        if (sock.type === {{{ cDefine('SOCK_STREAM') }}}) {
          if (!dest || dest.socket.readyState === dest.socket.CLOSING || dest.socket.readyState === dest.socket.CLOSED) {
            throw new FS.ErrnoError(ERRNO_CODES.ENOTCONN);
          } else if (dest.socket.readyState === dest.socket.CONNECTING) {
            throw new FS.ErrnoError(ERRNO_CODES.EAGAIN);
          }
        }

        // create a copy of the incoming data to send, as the WebSocket API
        // doesn't work entirely with an ArrayBufferView, it'll just send
        // the entire underlying buffer
        if (ArrayBuffer.isView(buffer)) {
          offset += buffer.byteOffset;
          buffer = buffer.buffer;
        }

        var data;
#if USE_PTHREADS
        // WebSockets .send() does not allow passing a SharedArrayBuffer, so clone the portion of the SharedArrayBuffer as a regular
        // ArrayBuffer that we want to send.
        if (buffer instanceof SharedArrayBuffer) {
          data = new Uint8Array(new Uint8Array(buffer.slice(offset, offset + length))).buffer;
        } else {
#endif
          data = buffer.slice(offset, offset + length);
#if USE_PTHREADS
        }
#endif

        // if we're emulating a connection-less dgram socket and don't have
        // a cached connection, queue the buffer to send upon connect and
        // lie, saying the data was sent now.
        if (sock.type === {{{ cDefine('SOCK_DGRAM') }}}) {
          if (!dest || dest.socket.readyState !== dest.socket.OPEN) {
            // if we're not connected, open a new connection
            if (!dest || dest.socket.readyState === dest.socket.CLOSING || dest.socket.readyState === dest.socket.CLOSED) {
              dest = SOCKFS.websocket_sock_ops.createPeer(sock, addr, port);
            }
#if SOCKET_DEBUG
            out('websocket queuing (' + length + ' bytes): ' + [Array.prototype.slice.call(new Uint8Array(data))]);
#endif
            dest.dgram_send_queue.push(data);
            return length;
          }
        }

        try {
#if SOCKET_DEBUG
          out('websocket send (' + length + ' bytes): ' + [Array.prototype.slice.call(new Uint8Array(data))]);
#endif
          // send the actual data
          dest.socket.send(data);
          return length;
        } catch (e) {
          throw new FS.ErrnoError(ERRNO_CODES.EINVAL);
        }
      },
      recvmsg: function(sock, length) {
        // http://pubs.opengroup.org/onlinepubs/7908799/xns/recvmsg.html
        if (sock.type === {{{ cDefine('SOCK_STREAM') }}} && sock.server) {
          // tcp servers should not be recv()'ing on the listen socket
          throw new FS.ErrnoError(ERRNO_CODES.ENOTCONN);
        }

        var queued = sock.recv_queue.shift();
        if (!queued) {
          if (sock.type === {{{ cDefine('SOCK_STREAM') }}}) {
            var dest = SOCKFS.websocket_sock_ops.getPeer(sock, sock.daddr, sock.dport);

            if (!dest) {
              // if we have a destination address but are not connected, error out
              throw new FS.ErrnoError(ERRNO_CODES.ENOTCONN);
            }
            else if (dest.socket.readyState === dest.socket.CLOSING || dest.socket.readyState === dest.socket.CLOSED) {
              // return null if the socket has closed
              return null;
            }
            else {
              // else, our socket is in a valid state but truly has nothing available
              throw new FS.ErrnoError(ERRNO_CODES.EAGAIN);
            }
          } else {
            throw new FS.ErrnoError(ERRNO_CODES.EAGAIN);
          }
        }

        // queued.data will be an ArrayBuffer if it's unadulterated, but if it's
        // requeued TCP data it'll be an ArrayBufferView
        var queuedLength = queued.data.byteLength || queued.data.length;
        var queuedOffset = queued.data.byteOffset || 0;
        var queuedBuffer = queued.data.buffer || queued.data;
        var bytesRead = Math.min(length, queuedLength);
        var res = {
          buffer: new Uint8Array(queuedBuffer, queuedOffset, bytesRead),
          addr: queued.addr,
          port: queued.port
        };

#if SOCKET_DEBUG
        out('websocket read (' + bytesRead + ' bytes): ' + [Array.prototype.slice.call(res.buffer)]);
#endif

        // push back any unread data for TCP connections
        if (sock.type === {{{ cDefine('SOCK_STREAM') }}} && bytesRead < queuedLength) {
          var bytesRemaining = queuedLength - bytesRead;
#if SOCKET_DEBUG
          out('websocket read: put back ' + bytesRemaining + ' bytes');
#endif
          queued.data = new Uint8Array(queuedBuffer, queuedOffset + bytesRead, bytesRemaining);
          sock.recv_queue.unshift(queued);
        }

        return res;
      }
    }
  },

  /*
   * Mechanism to register handlers for the various Socket Events from C code.
   * The registration functions are mostly variations on a theme, so we use this
   * generic handler. Most of the callback functions take a file descriptor as a
   * parameter, which will get passed to them by the emitting call. The error
   * callback also takes an int representing the errno and a char* representing the
   * error message, which we extract from the data passed to _callback and convert
   * to a char* string before calling the registered C callback.
   * Passing a NULL callback function to a emscripten_set_socket_*_callback call
   * will deregister the callback registered for that Event.
   */
  __set_network_callback: function(event, userData, callback) {
    function _callback(data) {
      try {
        if (event === 'error') {
          var sp = stackSave();
          var msg = allocate(intArrayFromString(data[2]), 'i8', ALLOC_STACK);
          {{{ makeDynCall('viiii') }}}(callback, data[0], data[1], msg, userData);
          stackRestore(sp);
        } else {
          {{{ makeDynCall('vii') }}}(callback, data, userData);
        }
      } catch (e) {
        if (e instanceof ExitStatus) {
          return;
        } else {
          if (e && typeof e === 'object' && e.stack) err('exception thrown: ' + [e, e.stack]);
          throw e;
        }
      }
    };

    Module['noExitRuntime'] = true;
    Module['websocket']['on'](event, callback ? _callback : null);
  },
  emscripten_set_socket_error_callback__deps: ['__set_network_callback'],
  emscripten_set_socket_error_callback: function(userData, callback) {
    ___set_network_callback('error', userData, callback);
  },
  emscripten_set_socket_open_callback__deps: ['__set_network_callback'],
  emscripten_set_socket_open_callback: function(userData, callback) {
    ___set_network_callback('open', userData, callback);
  },
  emscripten_set_socket_listen_callback__deps: ['__set_network_callback'],
  emscripten_set_socket_listen_callback: function(userData, callback) {
    ___set_network_callback('listen', userData, callback);
  },
  emscripten_set_socket_connection_callback__deps: ['__set_network_callback'],
  emscripten_set_socket_connection_callback: function(userData, callback) {
    ___set_network_callback('connection', userData, callback);
  },
  emscripten_set_socket_message_callback__deps: ['__set_network_callback'],
  emscripten_set_socket_message_callback: function(userData, callback) {
    ___set_network_callback('message', userData, callback);
  },
  emscripten_set_socket_close_callback__deps: ['__set_network_callback'],
  emscripten_set_socket_close_callback: function(userData, callback) {
    ___set_network_callback('close', userData, callback);
  }
});