libwebsockets-api-doc.html

<h2>libwebsockets_hangup_on_client - Server calls to terminate client connection</h2>
<i>void</i>
<b>libwebsockets_hangup_on_client</b>
(<i>struct libwebsocket_context *</i> <b>context</b>,
<i>int</i> <b>fd</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>libwebsockets context
<dt><b>fd</b>
<dd>Connection socket descriptor
</dl>
<hr>
<h2>libwebsockets_get_peer_addresses - Get client address information</h2>
<i>void</i>
<b>libwebsockets_get_peer_addresses</b>
(<i>int</i> <b>fd</b>,
<i>char *</i> <b>name</b>,
<i>int</i> <b>name_len</b>,
<i>char *</i> <b>rip</b>,
<i>int</i> <b>rip_len</b>)
<h3>Arguments</h3>
<dl>
<dt><b>fd</b>
<dd>Connection socket descriptor
<dt><b>name</b>
<dd>Buffer to take client address name
<dt><b>name_len</b>
<dd>Length of client address name buffer
<dt><b>rip</b>
<dd>Buffer to take client address IP qotted quad
<dt><b>rip_len</b>
<dd>Length of client address IP buffer
</dl>
<h3>Description</h3>
<blockquote>
This function fills in <tt><b>name</b></tt> and <tt><b>rip</b></tt> with the name and IP of
the client connected with socket descriptor <tt><b>fd</b></tt>.  Names may be
truncated if there is not enough room.  If either cannot be
determined, they will be returned as valid zero-length strings.
</blockquote>
<hr>
<h2>libwebsocket_service_fd - Service polled socket with something waiting</h2>
<i>int</i>
<b>libwebsocket_service_fd</b>
(<i>struct libwebsocket_context *</i> <b>context</b>,
<i>struct pollfd *</i> <b>pollfd</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>Websocket context
<dt><b>pollfd</b>
<dd>The pollfd entry describing the socket fd and which events
happened.
</dl>
<h3>Description</h3>
<blockquote>
This function closes any active connections and then frees the
context.  After calling this, any further use of the context is
undefined.
</blockquote>
<hr>
<h2>libwebsocket_context_destroy - Destroy the websocket context</h2>
<i>void</i>
<b>libwebsocket_context_destroy</b>
(<i>struct libwebsocket_context *</i> <b>context</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>Websocket context
</dl>
<h3>Description</h3>
<blockquote>
This function closes any active connections and then frees the
context.  After calling this, any further use of the context is
undefined.
</blockquote>
<hr>
<h2>libwebsocket_service - Service any pending websocket activity</h2>
<i>int</i>
<b>libwebsocket_service</b>
(<i>struct libwebsocket_context *</i> <b>context</b>,
<i>int</i> <b>timeout_ms</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>Websocket context
<dt><b>timeout_ms</b>
<dd>Timeout for poll; 0 means return immediately if nothing needed
service otherwise block and service immediately, returning
after the timeout if nothing needed service.
</dl>
<h3>Description</h3>
<blockquote>
This function deals with any pending websocket traffic, for three
kinds of event.  It handles these events on both server and client
types of connection the same.
<p>
1) Accept new connections to our context's server
<p>
2) Perform pending broadcast writes initiated from other forked
processes (effectively serializing asynchronous broadcasts)
<p>
3) Call the receive callback for incoming frame data received by
server or client connections.
<p>
You need to call this service function periodically to all the above
functions to happen; if your application is single-threaded you can
just call it in your main event loop.
<p>
Alternatively you can fork a new process that asynchronously handles
calling this service in a loop.  In that case you are happy if this
call blocks your thread until it needs to take care of something and
would call it with a large nonzero timeout.  Your loop then takes no
CPU while there is nothing happening.
<p>
If you are calling it in a single-threaded app, you don't want it to
wait around blocking other things in your loop from happening, so you
would call it with a timeout_ms of 0, so it returns immediately if
nothing is pending, or as soon as it services whatever was pending.
</blockquote>
<hr>
<h2>libwebsocket_callback_on_writable - Request a callback when this socket becomes able to be written to without blocking</h2>
<i>int</i>
<b>libwebsocket_callback_on_writable</b>
(<i>struct libwebsocket_context *</i> <b>context</b>,
<i>struct libwebsocket *</i> <b>wsi</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>libwebsockets context
<dt><b>wsi</b>
<dd>Websocket connection instance to get callback for
</dl>
<hr>
<h2>libwebsocket_callback_on_writable_all_protocol - Request a callback for all connections using the given protocol when it becomes possible to write to each socket without blocking in turn.</h2>
<i>int</i>
<b>libwebsocket_callback_on_writable_all_protocol</b>
(<i>const struct libwebsocket_protocols *</i> <b>protocol</b>)
<h3>Arguments</h3>
<dl>
<dt><b>protocol</b>
<dd>Protocol whose connections will get callbacks
</dl>
<hr>
<h2>libwebsocket_set_timeout - marks the wsi as subject to a timeout</h2>
<i>void</i>
<b>libwebsocket_set_timeout</b>
(<i>struct libwebsocket *</i> <b>wsi</b>,
<i>enum pending_timeout</i> <b>reason</b>,
<i>int</i> <b>secs</b>)
<h3>Arguments</h3>
<dl>
<dt><b>wsi</b>
<dd>Websocket connection instance
<dt><b>reason</b>
<dd>timeout reason
<dt><b>secs</b>
<dd>how many seconds
</dl>
<h3>Description</h3>
<blockquote>
<p>
You will not need this unless you are doing something special
</blockquote>
<hr>
<h2>libwebsocket_get_socket_fd - returns the socket file descriptor</h2>
<i>int</i>
<b>libwebsocket_get_socket_fd</b>
(<i>struct libwebsocket *</i> <b>wsi</b>)
<h3>Arguments</h3>
<dl>
<dt><b>wsi</b>
<dd>Websocket connection instance
</dl>
<h3>Description</h3>
<blockquote>
<p>
You will not need this unless you are doing something special
</blockquote>
<hr>
<h2>libwebsocket_rx_flow_control - Enable and disable socket servicing for receieved packets.</h2>
<i>int</i>
<b>libwebsocket_rx_flow_control</b>
(<i>struct libwebsocket *</i> <b>wsi</b>,
<i>int</i> <b>enable</b>)
<h3>Arguments</h3>
<dl>
<dt><b>wsi</b>
<dd>Websocket connection instance to get callback for
<dt><b>enable</b>
<dd>0 = disable read servicing for this connection, 1 = enable
</dl>
<h3>Description</h3>
<blockquote>
<p>
If the output side of a server process becomes choked, this allows flow
control for the input side.
</blockquote>
<hr>
<h2>libwebsocket_canonical_hostname - returns this host's hostname</h2>
<i>const char *</i>
<b>libwebsocket_canonical_hostname</b>
(<i>struct libwebsocket_context *</i> <b>context</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>Websocket context
</dl>
<h3>Description</h3>
<blockquote>
<p>
This is typically used by client code to fill in the host parameter
when making a client connection.  You can only call it after the context
has been created.
</blockquote>
<hr>
<h2>libwebsocket_create_context - Create the websocket handler</h2>
<i>struct libwebsocket_context *</i>
<b>libwebsocket_create_context</b>
(<i>int</i> <b>port</b>,
<i>const char *</i> <b>interf</b>,
<i>struct libwebsocket_protocols *</i> <b>protocols</b>,
<i>struct libwebsocket_extension *</i> <b>extensions</b>,
<i>const char *</i> <b>ssl_cert_filepath</b>,
<i>const char *</i> <b>ssl_private_key_filepath</b>,
<i>int</i> <b>gid</b>,
<i>int</i> <b>uid</b>,
<i>unsigned int</i> <b>options</b>)
<h3>Arguments</h3>
<dl>
<dt><b>port</b>
<dd>Port to listen on... you can use 0 to suppress listening on
any port, that's what you want if you are not running a
websocket server at all but just using it as a client
<dt><b>interf</b>
<dd>NULL to bind the listen socket to all interfaces, or the
interface name, eg, "eth2"
<dt><b>protocols</b>
<dd>Array of structures listing supported protocols and a protocol-
specific callback for each one.  The list is ended with an
entry that has a NULL callback pointer.
It's not const because we write the owning_server member
<dt><b>extensions</b>
<dd>NULL or array of libwebsocket_extension structs listing the
extensions this context supports
<dt><b>ssl_cert_filepath</b>
<dd>If libwebsockets was compiled to use ssl, and you want
to listen using SSL, set to the filepath to fetch the
server cert from, otherwise NULL for unencrypted
<dt><b>ssl_private_key_filepath</b>
<dd>filepath to private key if wanting SSL mode,
else ignored
<dt><b>gid</b>
<dd>group id to change to after setting listen socket, or -1.
<dt><b>uid</b>
<dd>user id to change to after setting listen socket, or -1.
<dt><b>options</b>
<dd>0, or LWS_SERVER_OPTION_DEFEAT_CLIENT_MASK
</dl>
<h3>Description</h3>
<blockquote>
This function creates the listening socket and takes care
of all initialization in one step.
<p>
After initialization, it returns a struct libwebsocket_context * that
represents this server.  After calling, user code needs to take care
of calling <b>libwebsocket_service</b> with the context pointer to get the
server's sockets serviced.  This can be done in the same process context
or a forked process, or another thread,
<p>
The protocol callback functions are called for a handful of events
including http requests coming in, websocket connections becoming
established, and data arriving; it's also called periodically to allow
async transmission.
<p>
HTTP requests are sent always to the FIRST protocol in <tt><b>protocol</b></tt>, since
at that time websocket protocol has not been negotiated.  Other
protocols after the first one never see any HTTP callack activity.
<p>
The server created is a simple http server by default; part of the
websocket standard is upgrading this http connection to a websocket one.
<p>
This allows the same server to provide files like scripts and favicon /
images or whatever over http and dynamic data over websockets all in
one place; they're all handled in the user callback.
</blockquote>
<hr>
<h2>libwebsockets_fork_service_loop - Optional helper function forks off a process for the websocket server loop. You don't have to use this but if not, you have to make sure you are calling libwebsocket_service periodically to service the websocket traffic</h2>
<i>int</i>
<b>libwebsockets_fork_service_loop</b>
(<i>struct libwebsocket_context *</i> <b>context</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>server context returned by creation function
</dl>
<hr>
<h2>libwebsockets_get_protocol - Returns a protocol pointer from a websocket connection.</h2>
<i>const struct libwebsocket_protocols *</i>
<b>libwebsockets_get_protocol</b>
(<i>struct libwebsocket *</i> <b>wsi</b>)
<h3>Arguments</h3>
<dl>
<dt><b>wsi</b>
<dd>pointer to struct websocket you want to know the protocol of
</dl>
<h3>Description</h3>
<blockquote>
<p>
This is useful to get the protocol to broadcast back to from inside
the callback.
</blockquote>
<hr>
<h2>libwebsockets_broadcast - Sends a buffer to the callback for all active connections of the given protocol.</h2>
<i>int</i>
<b>libwebsockets_broadcast</b>
(<i>const struct libwebsocket_protocols *</i> <b>protocol</b>,
<i>unsigned char *</i> <b>buf</b>,
<i>size_t</i> <b>len</b>)
<h3>Arguments</h3>
<dl>
<dt><b>protocol</b>
<dd>pointer to the protocol you will broadcast to all members of
<dt><b>buf</b>
<dd>buffer containing the data to be broadcase.  NOTE: this has to be
allocated with LWS_SEND_BUFFER_PRE_PADDING valid bytes before
the pointer and LWS_SEND_BUFFER_POST_PADDING afterwards in the
case you are calling this function from callback context.
<dt><b>len</b>
<dd>length of payload data in buf, starting from buf.
</dl>
<h3>Description</h3>
<blockquote>
This function allows bulk sending of a packet to every connection using
the given protocol.  It does not send the data directly; instead it calls
the callback with a reason type of LWS_CALLBACK_BROADCAST.  If the callback
wants to actually send the data for that connection, the callback itself
should call <b>libwebsocket_write</b>.
<p>
<b>libwebsockets_broadcast</b> can be called from another fork context without
having to take any care about data visibility between the processes, it'll
"just work".
</blockquote>
<hr>
<h2>libwebsocket_write - Apply protocol then write data to client</h2>
<i>int</i>
<b>libwebsocket_write</b>
(<i>struct libwebsocket *</i> <b>wsi</b>,
<i>unsigned char *</i> <b>buf</b>,
<i>size_t</i> <b>len</b>,
<i>enum libwebsocket_write_protocol</i> <b>protocol</b>)
<h3>Arguments</h3>
<dl>
<dt><b>wsi</b>
<dd>Websocket instance (available from user callback)
<dt><b>buf</b>
<dd>The data to send.  For data being sent on a websocket
connection (ie, not default http), this buffer MUST have
LWS_SEND_BUFFER_PRE_PADDING bytes valid BEFORE the pointer
and an additional LWS_SEND_BUFFER_POST_PADDING bytes valid
in the buffer after (buf + len).  This is so the protocol
header and trailer data can be added in-situ.
<dt><b>len</b>
<dd>Count of the data bytes in the payload starting from buf
<dt><b>protocol</b>
<dd>Use LWS_WRITE_HTTP to reply to an http connection, and one
of LWS_WRITE_BINARY or LWS_WRITE_TEXT to send appropriate
data on a websockets connection.  Remember to allow the extra
bytes before and after buf if LWS_WRITE_BINARY or LWS_WRITE_TEXT
are used.
</dl>
<h3>Description</h3>
<blockquote>
This function provides the way to issue data back to the client
for both http and websocket protocols.
<p>
In the case of sending using websocket protocol, be sure to allocate
valid storage before and after buf as explained above.  This scheme
allows maximum efficiency of sending data and protocol in a single
packet while not burdening the user code with any protocol knowledge.
</blockquote>
<hr>
<h2>libwebsockets_serve_http_file - Send a file back to the client using http</h2>
<i>int</i>
<b>libwebsockets_serve_http_file</b>
(<i>struct libwebsocket *</i> <b>wsi</b>,
<i>const char *</i> <b>file</b>,
<i>const char *</i> <b>content_type</b>)
<h3>Arguments</h3>
<dl>
<dt><b>wsi</b>
<dd>Websocket instance (available from user callback)
<dt><b>file</b>
<dd>The file to issue over http
<dt><b>content_type</b>
<dd>The http content type, eg, text/html
</dl>
<h3>Description</h3>
<blockquote>
This function is intended to be called from the callback in response
to http requests from the client.  It allows the callback to issue
local files down the http link in a single step.
</blockquote>
<hr>
<h2>libwebsockets_remaining_packet_payload - Bytes to come before "overall" rx packet is complete</h2>
<i>size_t</i>
<b>libwebsockets_remaining_packet_payload</b>
(<i>struct libwebsocket *</i> <b>wsi</b>)
<h3>Arguments</h3>
<dl>
<dt><b>wsi</b>
<dd>Websocket instance (available from user callback)
</dl>
<h3>Description</h3>
<blockquote>
This function is intended to be called from the callback if the
user code is interested in "complete packets" from the client.
libwebsockets just passes through payload as it comes and issues a buffer
additionally when it hits a built-in limit.  The LWS_CALLBACK_RECEIVE
callback handler can use this API to find out if the buffer it has just
been given is the last piece of a "complete packet" from the client --
when that is the case <b>libwebsockets_remaining_packet_payload</b> will return
0.
<p>
Many protocols won't care becuse their packets are always small.
</blockquote>
<hr>
<h2>libwebsocket_client_connect - Connect to another websocket server</h2>
<i>struct libwebsocket *</i>
<b>libwebsocket_client_connect</b>
(<i>struct libwebsocket_context *</i> <b>context</b>,
<i>const char *</i> <b>address</b>,
<i>int</i> <b>port</b>,
<i>int</i> <b>ssl_connection</b>,
<i>const char *</i> <b>path</b>,
<i>const char *</i> <b>host</b>,
<i>const char *</i> <b>origin</b>,
<i>const char *</i> <b>protocol</b>,
<i>int</i> <b>ietf_version_or_minus_one</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>Websocket context
<dt><b>address</b>
<dd>Remote server address, eg, "myserver.com"
<dt><b>port</b>
<dd>Port to connect to on the remote server, eg, 80
<dt><b>ssl_connection</b>
<dd>0 = ws://, 1 = wss:// encrypted, 2 = wss:// allow self
signed certs
<dt><b>path</b>
<dd>Websocket path on server
<dt><b>host</b>
<dd>Hostname on server
<dt><b>origin</b>
<dd>Socket origin name
<dt><b>protocol</b>
<dd>Comma-separated list of protocols being asked for from
the server, or just one.  The server will pick the one it
likes best.
<dt><b>ietf_version_or_minus_one</b>
<dd>-1 to ask to connect using the default, latest
protocol supported, or the specific protocol ordinal
</dl>
<h3>Description</h3>
<blockquote>
This function creates a connection to a remote server
</blockquote>
<hr>
<h2>callback - User server actions</h2>
<i>int</i>
<b>callback</b>
(<i>struct libwebsocket_context *</i> <b>context</b>,
<i>struct libwebsocket *</i> <b>wsi</b>,
<i>enum libwebsocket_callback_reasons</i> <b>reason</b>,
<i>void *</i> <b>user</b>,
<i>void *</i> <b>in</b>,
<i>size_t</i> <b>len</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>Websockets context
<dt><b>wsi</b>
<dd>Opaque websocket instance pointer
<dt><b>reason</b>
<dd>The reason for the call
<dt><b>user</b>
<dd>Pointer to per-session user data allocated by library
<dt><b>in</b>
<dd>Pointer used for some callback reasons
<dt><b>len</b>
<dd>Length set for some callback reasons
</dl>
<h3>Description</h3>
<blockquote>
This callback is the way the user controls what is served.  All the
protocol detail is hidden and handled by the library.
<p>
For each connection / session there is user data allocated that is
pointed to by "user".  You set the size of this user data area when
the library is initialized with libwebsocket_create_server.
<p>
You get an opportunity to initialize user data when called back with
LWS_CALLBACK_ESTABLISHED reason.
</blockquote>
<h3>LWS_CALLBACK_ESTABLISHED</h3>
<blockquote>
after the server completes a handshake with
an incoming client
</blockquote>
<h3>LWS_CALLBACK_CLIENT_ESTABLISHED</h3>
<blockquote>
after your client connection completed
a handshake with the remote server
</blockquote>
<h3>LWS_CALLBACK_CLOSED</h3>
<blockquote>
when the websocket session ends
</blockquote>
<h3>LWS_CALLBACK_BROADCAST</h3>
<blockquote>
signal to send to client (you would use
<b>libwebsocket_write</b> taking care about the
special buffer requirements
</blockquote>
<h3>LWS_CALLBACK_RECEIVE</h3>
<blockquote>
data has appeared for this server endpoint from a
remote client, it can be found at *in and is
len bytes long
</blockquote>
<h3>LWS_CALLBACK_CLIENT_RECEIVE_PONG</h3>
<blockquote>
if you elected to see PONG packets,
they appear with this callback reason.  PONG
packets only exist in 04+ protocol
</blockquote>
<h3>LWS_CALLBACK_CLIENT_RECEIVE</h3>
<blockquote>
data has appeared from the server for the
client connection, it can be found at *in and
is len bytes long
</blockquote>
<h3>LWS_CALLBACK_HTTP</h3>
<blockquote>
an http request has come from a client that is not
asking to upgrade the connection to a websocket
one.  This is a chance to serve http content,
for example, to send a script to the client
which will then open the websockets connection.
<tt><b>in</b></tt> points to the URI path requested and
<b>libwebsockets_serve_http_file</b> makes it very
simple to send back a file to the client.
</blockquote>
<h3>LWS_CALLBACK_CLIENT_WRITEABLE</h3>
<blockquote>
if you call
<b>libwebsocket_callback_on_writable</b> on a connection, you will
get this callback coming when the connection socket is able to
accept another write packet without blocking.  If it already
was able to take another packet without blocking, you'll get
this callback at the next call to the service loop function.
</blockquote>
<h3>LWS_CALLBACK_FILTER_NETWORK_CONNECTION</h3>
<blockquote>
called when a client connects to
the server at network level; the connection is accepted but then
passed to this callback to decide whether to hang up immediately
or not, based on the client IP.  <tt><b>user</b></tt> contains the connection
socket's descriptor.  Return non-zero to terminate
the connection before sending or receiving anything.
Because this happens immediately after the network connection
from the client, there's no websocket protocol selected yet so
this callback is issued only to protocol 0.
</blockquote>
<h3>LWS_CALLBACK_FILTER_PROTOCOL_CONNECTION</h3>
<blockquote>
called when the handshake has
been received and parsed from the client, but the response is
not sent yet.  Return non-zero to disallow the connection.
<tt><b>user</b></tt> is a pointer to an array of struct lws_tokens, you can
use the header enums lws_token_indexes from libwebsockets.h
to check for and read the supported header presence and
content before deciding to allow the handshake to proceed or
to kill the connection.
</blockquote>
<h3>LWS_CALLBACK_OPENSSL_LOAD_EXTRA_CLIENT_VERIFY_CERTS</h3>
<blockquote>
if configured for
including OpenSSL support, this callback allows your user code
to perform extra <b>SSL_CTX_load_verify_locations</b> or similar
calls to direct OpenSSL where to find certificates the client
can use to confirm the remote server identity.  <tt><b>user</b></tt> is the
OpenSSL SSL_CTX*
</blockquote>
<h3>LWS_CALLBACK_OPENSSL_LOAD_EXTRA_SERVER_VERIFY_CERTS</h3>
<blockquote>
if configured for
including OpenSSL support, this callback allows your user code
to load extra certifcates into the server which allow it to
verify the validity of certificates returned by clients.  <tt><b>user</b></tt>
is the server's OpenSSL SSL_CTX*
</blockquote>
<h3>LWS_CALLBACK_OPENSSL_PERFORM_CLIENT_CERT_VERIFICATION</h3>
<blockquote>
if the
libwebsockets context was created with the option
LWS_SERVER_OPTION_REQUIRE_VALID_OPENSSL_CLIENT_CERT, then this
callback is generated during OpenSSL verification of the cert
sent from the client.  It is sent to protocol[0] callback as
no protocol has been negotiated on the connection yet.
Notice that the libwebsockets context and wsi are both NULL
during this callback.  See
</blockquote>
<h3>http</h3>
<blockquote>
//www.openssl.org/docs/ssl/SSL_CTX_set_verify.html
to understand more detail about the OpenSSL callback that
generates this libwebsockets callback and the meanings of the
arguments passed.  In this callback, <tt><b>user</b></tt> is the x509_ctx,
<tt><b>in</b></tt> is the ssl pointer and <tt><b>len</b></tt> is preverify_ok
Notice that this callback maintains libwebsocket return
conventions, return 0 to mean the cert is OK or 1 to fail it.
This also means that if you don't handle this callback then
the default callback action of returning 0 allows the client
certificates.
</blockquote>
<h3>LWS_CALLBACK_CLIENT_APPEND_HANDSHAKE_HEADER</h3>
<blockquote>
this callback happens
when a client handshake is being compiled.  <tt><b>user</b></tt> is NULL,
<tt><b>in</b></tt> is a char **, it's pointing to a char * which holds the
next location in the header buffer where you can add
headers, and <tt><b>len</b></tt> is the remaining space in the header buffer,
which is typically some hundreds of bytes.  So, to add a canned
cookie, your handler code might look similar to:
<p>
char **p = (char **)in;
<p>
if (len &lt; 100)
return 1;
<p>
*p += sprintf(*p, "Cookie: a=b\x0d\x0a");
<p>
return 0;
<p>
Notice if you add anything, you just have to take care about
the CRLF on the line you added.  Obviously this callback is
optional, if you don't handle it everything is fine.
<p>
Notice the callback is coming to protocols[0] all the time,
because there is no specific protocol handshook yet.
</blockquote>
<h3>LWS_CALLBACK_CONFIRM_EXTENSION_OKAY</h3>
<blockquote>
When the server handshake code
sees that it does support a requested extension, before
accepting the extension by additing to the list sent back to
the client it gives this callback just to check that it's okay
to use that extension.  It calls back to the requested protocol
and with <tt><b>in</b></tt> being the extension name, <tt><b>len</b></tt> is 0 and <tt><b>user</b></tt> is
valid.  Note though at this time the ESTABLISHED callback hasn't
happened yet so if you initialize <tt><b>user</b></tt> content there, <tt><b>user</b></tt>
content during this callback might not be useful for anything.
Notice this callback comes to protocols[0].
</blockquote>
<h3>LWS_CALLBACK_CLIENT_CONFIRM_EXTENSION_SUPPORTED</h3>
<blockquote>
When a client
connection is being prepared to start a handshake to a server,
each supported extension is checked with protocols[0] callback
with this reason, giving the user code a chance to suppress the
claim to support that extension by returning non-zero.  If
unhandled, by default 0 will be returned and the extension
support included in the header to the server.  Notice this
callback comes to protocols[0].
<p>
The next four reasons are optional and only need taking care of if you
will be integrating libwebsockets sockets into an external polling
array.
</blockquote>
<h3>LWS_CALLBACK_ADD_POLL_FD</h3>
<blockquote>
libwebsocket deals with its <b>poll</b> loop
internally, but in the case you are integrating with another
server you will need to have libwebsocket sockets share a
polling array with the other server.  This and the other
POLL_FD related callbacks let you put your specialized
poll array interface code in the callback for protocol 0, the
first protocol you support, usually the HTTP protocol in the
serving case.  This callback happens when a socket needs to be
</blockquote>
<h3>added to the polling loop</h3>
<blockquote>
<tt><b>user</b></tt> contains the fd, and
<tt><b>len</b></tt> is the events bitmap (like, POLLIN).  If you are using the
internal polling loop (the "service" callback), you can just
ignore these callbacks.
</blockquote>
<h3>LWS_CALLBACK_DEL_POLL_FD</h3>
<blockquote>
This callback happens when a socket descriptor
needs to be removed from an external polling array.  <tt><b>user</b></tt> is
the socket desricptor.  If you are using the internal polling
loop, you can just ignore it.
</blockquote>
<h3>LWS_CALLBACK_SET_MODE_POLL_FD</h3>
<blockquote>
This callback happens when libwebsockets
wants to modify the events for the socket descriptor in <tt><b>user</b></tt>.
The handler should OR <tt><b>len</b></tt> on to the events member of the pollfd
struct for this socket descriptor.  If you are using the
internal polling loop, you can just ignore it.
</blockquote>
<h3>LWS_CALLBACK_CLEAR_MODE_POLL_FD</h3>
<blockquote>
This callback occurs when libwebsockets
wants to modify the events for the socket descriptor in <tt><b>user</b></tt>.
The handler should AND ~<tt><b>len</b></tt> on to the events member of the
pollfd struct for this socket descriptor.  If you are using the
internal polling loop, you can just ignore it.
</blockquote>
<hr>
<h2>extension_callback - Hooks to allow extensions to operate</h2>
<i>int</i>
<b>extension_callback</b>
(<i>struct libwebsocket_context *</i> <b>context</b>,
<i>struct libwebsocket *</i> <b>wsi</b>,
<i>enum libwebsocket_callback_reasons</i> <b>reason</b>,
<i>void *</i> <b>user</b>,
<i>void *</i> <b>in</b>,
<i>size_t</i> <b>len</b>)
<h3>Arguments</h3>
<dl>
<dt><b>context</b>
<dd>Websockets context
<dt><b>wsi</b>
<dd>Opaque websocket instance pointer
<dt><b>reason</b>
<dd>The reason for the call
<dt><b>user</b>
<dd>Pointer to per-session user data allocated by library
<dt><b>in</b>
<dd>Pointer used for some callback reasons
<dt><b>len</b>
<dd>Length set for some callback reasons
</dl>
<h3>Description</h3>
<blockquote>
Each extension that is active on a particular connection receives
callbacks during the connection lifetime to allow the extension to
operate on websocket data and manage itself.
<p>
Libwebsockets takes care of allocating and freeing "user" memory for
each active extension on each connection.  That is what is pointed to
by the <tt><b>user</b></tt> parameter.
</blockquote>
<h3>LWS_EXT_CALLBACK_CONSTRUCT</h3>
<blockquote>
called when the server has decided to
select this extension from the list provided by the client,
just before the server will send back the handshake accepting
the connection with this extension active.  This gives the
extension a chance to initialize its connection context found
in <tt><b>user</b></tt>.
</blockquote>
<h3>LWS_EXT_CALLBACK_CLIENT_CONSTRUCT</h3>
<blockquote>
same as LWS_EXT_CALLBACK_CONSTRUCT
but called when client is instantiating this extension.  Some
extensions will work the same on client and server side and then
you can just merge handlers for both CONSTRUCTS.
</blockquote>
<h3>LWS_EXT_CALLBACK_DESTROY</h3>
<blockquote>
called when the connection the extension was
being used on is about to be closed and deallocated.  It's the
last chance for the extension to deallocate anything it has
allocated in the user data (pointed to by <tt><b>user</b></tt>) before the
user data is deleted.  This same callback is used whether you
are in client or server instantiation context.
</blockquote>
<h3>LWS_EXT_CALLBACK_PACKET_RX_PREPARSE</h3>
<blockquote>
when this extension was active on
a connection, and a packet of data arrived at the connection,
it is passed to this callback to give the extension a chance to
change the data, eg, decompress it.  <tt><b>user</b></tt> is pointing to the
extension's private connection context data, <tt><b>in</b></tt> is pointing
to an lws_tokens struct, it consists of a char * pointer called
token, and an int called token_len.  At entry, these are
set to point to the received buffer and set to the content
length.  If the extension will grow the content, it should use
a new buffer allocated in its private user context data and
set the pointed-to lws_tokens members to point to its buffer.
</blockquote>
<h3>LWS_EXT_CALLBACK_PACKET_TX_PRESEND</h3>
<blockquote>
this works the same way as
LWS_EXT_CALLBACK_PACKET_RX_PREPARSE above, except it gives the
extension a chance to change websocket data just before it will
be sent out.  Using the same lws_token pointer scheme in <tt><b>in</b></tt>,
the extension can change the buffer and the length to be
transmitted how it likes.  Again if it wants to grow the
buffer safely, it should copy the data into its own buffer and
set the lws_tokens token pointer to it.
</blockquote>
<hr>
<h2>struct libwebsocket_protocols - List of protocols and handlers server supports.</h2>
<b>struct libwebsocket_protocols</b> {<br>
&nbsp; &nbsp; <i>const char *</i> <b>name</b>;<br>
&nbsp; &nbsp; <i>int (*</i><b>callback</b>) <i>(struct libwebsocket_context * context,struct libwebsocket *wsi,enum libwebsocket_callback_reasons reason, void *user,void *in, size_t len)</i>;<br>
&nbsp; &nbsp; <i>size_t</i> <b>per_session_data_size</b>;<br>
&nbsp; &nbsp; <i>struct libwebsocket_context *</i> <b>owning_server</b>;<br>
&nbsp; &nbsp; <i>int</i> <b>broadcast_socket_port</b>;<br>
&nbsp; &nbsp; <i>int</i> <b>broadcast_socket_user_fd</b>;<br>
&nbsp; &nbsp; <i>int</i> <b>protocol_index</b>;<br>
};<br>
<h3>Members</h3>
<dl>
<dt><b>name</b>
<dd>Protocol name that must match the one given in the client
Javascript new WebSocket(url, 'protocol') name
<dt><b>callback</b>
<dd>The service callback used for this protocol.  It allows the
service action for an entire protocol to be encapsulated in
the protocol-specific callback
<dt><b>per_session_data_size</b>
<dd>Each new connection using this protocol gets
this much memory allocated on connection establishment and
freed on connection takedown.  A pointer to this per-connection
allocation is passed into the callback in the 'user' parameter
<dt><b>owning_server</b>
<dd>the server init call fills in this opaque pointer when
registering this protocol with the server.
<dt><b>broadcast_socket_port</b>
<dd>the server init call fills this in with the
localhost port number used to forward broadcasts for this
protocol
<dt><b>broadcast_socket_user_fd</b>
<dd>the server init call fills this in ... the <b>main</b>
process context can write to this socket to perform broadcasts
(use the <b>libwebsockets_broadcast</b> api to do this instead,
it works from any process context)
<dt><b>protocol_index</b>
<dd>which protocol we are starting from zero
</dl>
<h3>Description</h3>
<blockquote>
This structure represents one protocol supported by the server.  An
array of these structures is passed to <b>libwebsocket_create_server</b>
allows as many protocols as you like to be handled by one server.
</blockquote>
<hr>
<h2>struct libwebsocket_extension - An extension we know how to cope with</h2>
<b>struct libwebsocket_extension</b> {<br>
&nbsp; &nbsp; <i>const char *</i> <b>name</b>;<br>
&nbsp; &nbsp; <i>int (*</i><b>callback</b>) <i>(struct libwebsocket_context *context,struct libwebsocket *wsi,enum libwebsocket_extension_callback_reasons reason,void *user, void *in, size_t len)</i>;<br>
&nbsp; &nbsp; <i>size_t</i> <b>per_session_data_size</b>;<br>
};<br>
<h3>Members</h3>
<dl>
<dt><b>name</b>
<dd>Formal extension name, eg, "deflate-stream"
<dt><b>callback</b>
<dd>Service callback
<dt><b>per_session_data_size</b>
<dd>Libwebsockets will auto-malloc this much
memory for the use of the extension, a pointer
to it comes in the <tt><b>user</b></tt> callback parameter
</dl>
<hr>