gwsocket is a standalone, simple, yet powerful rfc6455 compliant WebSocket Server.

.LP

.B gwsocket -p [--addr][--origin][gwsocket options ...]

.B gwsocket

is a free (MIT Licensed), standalone, WebSocket Server. It sits

between your application and the client's browser, giving the ability for

bidirectional communication between these two with ease and flexibility.

.P

You can run gwsocket without passing any options to the command line. By

default, it will listen on port 7890 on all the interfaces, e.g., 127.0.0.1,

::1, etc. For instance:

.LP

.B # gwsocket --access-log=/tmp/access.log

.I Note:

See a basic client-side example at the bottom of the man page.

.TP

Specifies the port to bind.

.TP

Command line help.

.TP

Display version information and exit.

.TP

Specifies the path/file for the access log.

.TP

Specifies the address to bind.

.TP

Set the server to echo all received messages.

.TP

Maximum size of a websocket frame. This includes received frames from the

client and messages through the named pipe.

.TP

Ensure clients send the specified origin header upon the WebSocket handshake.

.TP

Creates a named pipe (FIFO) that reads from on the given path/file.

.TP

Creates a named pipe (FIFO) that writes to the given path/file.

.TP

Enable --stdin and --stdout. By default named pipes are used for I/O. This adds the option to use stdin / stdout as well. For example, this allows:

./gwsocket --std > log.txt

tail -F /var/log/syslog | ./gwsocket --std

--stdin and --stdout are added for fine grained control.

.TP

Send stdin to the websocket.

.TP

Send received websocket data to stdout.

.TP

Parse messages using strict mode. See below for more details.

.TP

Path to TLS/SSL certificate.

.TP

Path to TLS/SSL private key.

.TP

Specify UNIX-domain socket address to bind server to.

.TP

Compile with debugging symbols and turn off compiler optimizations.

.TP

Compile gwsocket with OpenSSL support for its WebSocket server.

.P

In order to establish a channel between your application and the client's

browser, gwsocket provides two methods that allow the user to send data in and

out. The first one is through the use of the standard input

.I (stdin)

,and the standard output

.I (stdout)

.The second method is through a fixed-size header

followed by the payload. See options below for more details.

.I

1. stdin/stdout

.P

The standard input/output is the simplest way of sending/receiving data to/from

a client. However, it's limited to broadcasting messages to all clients. To

send messages to or receive from a specific client, use the strict mode in

section 2.

.I

1.1 Sending Data to all Clients — stdout

.P

If you need to broadcast data from your application to all clients connected to

gwsocket, then, the simplest way of doing it is by piping your application

output into a named pipe (also known as FIFO) that gwsocket makes use of. Once

gwsocket receives the payload, then it will automatically broadcast the message

to all connected clients.

.BR

1.1. Examples

#include <fcntl.h>

#include <sys/stat.h>

#include <sys/types.h>

#include <unistd.h>

int main() {

int fd;

char *myfifo = "/tmp/wspipein.fifo";

const char *msg = "Message to broadcast";

fd = open(myfifo, O_WRONLY);

write(fd, msg, strlen(msg));

close(fd);

return 0;

}

.P

.I Note:

You can send as many bytes

.I PIPE_BUF

can hold. If a message is greater than PIPE_BUF, it would send the rest on a

second message or third, and so on. See strict mode below for more control over

messages.

.I

1.2 Receiving Data from Clients — stdin

.P

When a client sends a message to the server, it is possible to capture that

message in your application. To do this, your application simply needs to read

from a named pipe. By default, gwsocket creates a FIFO under

.I /tmp/wspipeout.fifo.

.BR

1.2. Examples

.P

Receiving data can be as simple as doing a

.I # cat /tmp/wspipeout.fifo

or you can do it in the language of your choice. See examples below.

#include <stdio.h>

#include <stdlib.h>

#include <fcntl.h>

#include <limits.h>

static void read_message (int fd, fd_set set) {

int bytes = 0;

char buf[PIPE_BUF] = { 0 };

FD_ZERO (&set);

FD_SET (fd, &set);

if ((select (fd + 1, &set, NULL, NULL, NULL)) < 1)

exit (1);

if (!FD_ISSET (fd, &set))

return;

if (read (fd, buf, PIPE_BUF) > 0)

printf ("%s\n", buf);

}

int main (void) {

fd_set set;

char *fifo = "/tmp/wspipeout.fifo";

int fd = 0;

if ((fd = open (fifo, O_RDWR | O_NONBLOCK)) < 0)

exit (1);

while (1)

read_message(fd, set);

return 0;

}

.I

Make sure the reader in your application is set as non-blocking to get a

constant feed.

.P

.I Tip

If you need to know which client sent the message, for example, in a chat

application, please see the strict mode below.

.I

2. Strict Mode

.P

gwsocket implements its own tiny protocol for sending/receiving data. In

contrast to the stdin/stdout mode, the strict mode allows you to send/receive

data to/from specific connected clients as well as to keep track of who

opened/closed a WebSocket connection. It also gives you the ability to pack and

send as much data as you would like on a single message.

.P

2. Data Format

.P

The message header is a fixed-size header. The first 12 bytes (uint32_t) are

packed in network byte order and contain the "meta-data" of the message we are

sending/receiving. The rest of it is the actual message.

.P

0 1 2 3

+---------------------------------------------+

| Client Socket Id (listener) |

+---------------------------------------------+

| Message Type (binary: 0x2 / text: 0x1) |

+---------------------------------------------+

| Payload length |

+---------------------------------------------+

| Payload Data |

+---------------------------------------------+

.I

2.1 Sending Data — Strict Mode

.P

If you need to send a message to a specific client, then you can do so by

specifying the client id in the message header. If set to 0, the message will

be broadcasted to all clients. The first 4 bytes are reserved for the client id

or listener. The following 4 bytes are reserved for the message type. 0x01 for

a text message, and 0x02 for a binary message. And the last 4 bytes are

reserved for the payload's length.

.P

Once the header has been written to the pipe, you may now write the message.

.BR

2.1. Examples

.P

First, start the server in strict-mode.

.LP

.B # gwsocket --strict-mode

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <fcntl.h>

size_t pack_uint32(void* buf, uint32_t val) {

uint32_t v32 = htonl(val);

memcpy(buf, &v32, sizeof(uint32_t));

return sizeof(uint32_t);

}

int main() {

char *p = calloc (sizeof(uint32_t) * 3, sizeof(char)), *ptr;

const char *msg = "Message to broadcast";

const char *fifo = "/tmp/wspipein.fifo";

int fd;

ptr = p;

ptr += pack_uint32(ptr, 0);

ptr += pack_uint32(ptr, 0x01);

ptr += pack_uint32(ptr, strlen(msg));

fd = open(fifo, O_WRONLY);

write(fd, p, sizeof(uint32_t) * 3);

write(fd, msg, strlen(msg));

close(fd);

free (p);

return 0;

}

.I

2.2 Receiving Data from Clients — Strict Mode

.P

Now, to get a message from a specific client and route it to another client,

you just need to do the opposite of sending data. First you unpack the header

from network byte order to host byte order and then read the payload.

.BR

2.2. Examples

.P

First, start the server in strict-mode.

.LP

.B # gwsocket --strict-mode

#include <stdio.h>

#include <stdlib.h>

#include <fcntl.h>

#include <limits.h>

#include <int.h>

static size_t unpack_uint32 (const void *b, uint32_t * val) {

uint32_t v32 = 0;

memcpy (&v32, b, sizeof (uint32_t));

*val = ntohl (v32);

return sizeof (uint32_t);

}

static void read_message (int fd, fd_set set) {

int bytes = 0;

uint32_t size = 0, listener = 0, type = 0;

char hdr[PIPE_BUF] = { 0 }, buf[PIPE_BUF] = {0};

char *ptr = NULL;

FD_ZERO (&set);

FD_SET (fd, &set);

if ((select (fd + 1, &set, NULL, NULL, NULL)) < 1)

exit (1);

if (!FD_ISSET (fd, &set))

return;

if (hdr[0] == '\0') {

if (read (fd, hdr, sizeof (uint32_t) * 3) < 1)

return;

}

ptr = hdr;

ptr += unpack_uint32(ptr, &listener);

ptr += unpack_uint32(ptr, &type);

ptr += unpack_uint32(ptr, &size);

if (read (fd, buf, size) < 1)

return;

printf ("client: %d, msg: %s\n", listener, buf);

}

int main (void) {

fd_set set;

char *fifo = "/tmp/wspipeout.fifo";

int fd = 0;

if ((fd = open (fifo, O_RDWR | O_NONBLOCK)) < 0)

exit (1);

while (1)

read_message(fd, set);

return 0;

}

.P

.I Note:

If you read/write to a stream, be aware that they do not necessarily read/write

the full amount of data you have requested. Your application will need to

handle the case where only a single byte is read or written. Examples above do

not handle this.

.P

Here's the basic example, client and server side. First start the server and

set it in echo mode.

.LP

.B # gwsocket --echo-mode

.P

Now, let's create the client side.

<!DOCTYPE html>

<html lang="en">

<style>

pre {

background: #EEE;

border: 1px solid #CCC;

padding: 10px;

}

#page-wrapper {

border-top: 5px solid #69c773;

margin: 1em auto;

width: 950px;

}

</style>

<script>

window.onload = function() {

function $(selector) {

return document.querySelector(selector);

}

var socket = new WebSocket('ws://localhost:7890');

socket.onopen = function(event) {

$('#messages').innerHTML = 'Connected<br>';

};

socket.onmessage = function(event) {

$('#messages').innerHTML += 'Received:<br>' + event.data + '<br>';

};

socket.onclose = function(event) {

$('#messages').innerHTML = 'Disconnected ' + event.reason;

};

$('#submit').onclick = function(e) {

socket.send($('input').value);

$('#messages').innerHTML += 'Sent:<br>' + $('input').value + '<br>';

$('input').value = '';

};

};

</script>

<div id="page-wrapper">

<pre id="messages">Connecting...</pre>

<input id="message" required>

<button id="submit">Send Message</button>

</div>

.P

If you think you have found a bug, please send me an email to hello [@at]

goaccess.io.

.P

Gerardo Orellana. For more details about it, or new releases, please visit

http://gwsocket.io