4 There's a helper api lws_daemonize built by default that does everything you
5 need to daemonize well, including creating a lock file. If you're making
6 what's basically a daemon, just call this early in your init to fork to a
7 headless background process and exit the starting process.
9 Notice stdout, stderr, stdin are all redirected to /dev/null to enforce your
10 daemon is headless, so you'll need to sort out alternative logging, by, eg,
14 Maximum number of connections
15 -----------------------------
17 The maximum number of connections the library can deal with is decided when
18 it starts by querying the OS to find out how many file descriptors it is
19 allowed to open (1024 on Fedora for example). It then allocates arrays that
20 allow up to that many connections, minus whatever other file descriptors are
21 in use by the user code.
23 If you want to restrict that allocation, or increase it, you can use ulimit or
24 similar to change the avaiable number of file descriptors, and when restarted
25 libwebsockets will adapt accordingly.
28 Libwebsockets is singlethreaded
29 -------------------------------
31 Directly performing websocket actions from other threads is not allowed.
32 Aside from the internal data being inconsistent in forked() processes,
33 the scope of a wsi (struct websocket) can end at any time during service
34 with the socket closing and the wsi freed.
36 Websocket write activities should only take place in the
37 "LWS_CALLBACK_SERVER_WRITEABLE" callback as described below.
39 Only live connections appear in the user callbacks, so this removes any
40 possibility of trying to used closed and freed wsis.
42 If you need to service other socket or file descriptors as well as the
43 websocket ones, you can combine them together with the websocket ones
44 in one poll loop, see "External Polling Loop support" below, and
45 still do it all in one thread / process context.
48 Only send data when socket writeable
49 ------------------------------------
51 You should only send data on a websocket connection from the user callback
52 "LWS_CALLBACK_SERVER_WRITEABLE" (or "LWS_CALLBACK_CLIENT_WRITEABLE" for
55 If you want to send something, do not just send it but request a callback
56 when the socket is writeable using
58 - libwebsocket_callback_on_writable(context, wsi) for a specific wsi, or
59 - libwebsocket_callback_on_writable_all_protocol(protocol) for all connections
60 using that protocol to get a callback when next writeable.
62 Usually you will get called back immediately next time around the service
63 loop, but if your peer is slow or temporarily inactive the callback will be
64 delayed accordingly. Generating what to write and sending it should be done
65 in the ...WRITEABLE callback.
67 See the test server code for an example of how to do this.
70 Closing connections from the user side
71 --------------------------------------
73 When you want to close a connection, you do it by returning -1 from a
74 callback for that connection.
76 You can provoke a callback by calling libwebsocket_callback_on_writable on
77 the wsi, then notice in the callback you want to close it and just return -1.
78 But usually, the decision to close is made in a callback already and returning
81 If the socket knows the connection is dead, because the peer closed or there
82 was an affirmitive network error like a FIN coming, then libwebsockets will
83 take care of closing the connection automatically.
85 If you have a silently dead connection, it's possible to enter a state where
86 the send pipe on the connection is choked but no ack will ever come, so the
87 dead connection will never become writeable. To cover that, you can use TCP
88 keepalives (see later in this document)
94 To support fragmented messages you need to check for the final
95 frame of a message with libwebsocket_is_final_fragment. This
96 check can be combined with libwebsockets_remaining_packet_payload
97 to gather the whole contents of a message, eg:
99 case LWS_CALLBACK_RECEIVE:
101 Client * const client = (Client *)user;
102 const size_t remaining = libwebsockets_remaining_packet_payload(wsi);
104 if (!remaining && libwebsocket_is_final_fragment(wsi)) {
105 if (client->HasFragments()) {
106 client->AppendMessageFragment(in, len, 0);
107 in = (void *)client->GetMessage();
108 len = client->GetMessageLength();
111 client->ProcessMessage((char *)in, len, wsi);
112 client->ResetMessage();
114 client->AppendMessageFragment(in, len, remaining);
118 The test app llibwebsockets-test-fraggle sources also show how to
119 deal with fragmented messages.
125 Also using lws_set_log_level api you may provide a custom callback to actually
126 emit the log string. By default, this points to an internal emit function
127 that sends to stderr. Setting it to NULL leaves it as it is instead.
129 A helper function lwsl_emit_syslog() is exported from the library to simplify
130 logging to syslog. You still need to use setlogmask, openlog and closelog
133 The logging apis are made available for user code.
141 The difference between notice and info is that notice will be logged by default
142 whereas info is ignored by default.
145 External Polling Loop support
146 -----------------------------
148 libwebsockets maintains an internal poll() array for all of its
149 sockets, but you can instead integrate the sockets into an
150 external polling array. That's needed if libwebsockets will
151 cooperate with an existing poll array maintained by another
154 Four callbacks LWS_CALLBACK_ADD_POLL_FD, LWS_CALLBACK_DEL_POLL_FD,
155 LWS_CALLBACK_SET_MODE_POLL_FD and LWS_CALLBACK_CLEAR_MODE_POLL_FD
156 appear in the callback for protocol 0 and allow interface code to
157 manage socket descriptors in other poll loops.
160 Using with in c++ apps
161 ----------------------
163 The library is ready for use by C++ apps. You can get started quickly by
164 copying the test server
166 $ cp test-server/test-server.c test.cpp
168 and building it in C++ like this
170 $ g++ -DINSTALL_DATADIR=\"/usr/share\" -ocpptest test.cpp -lwebsockets
172 INSTALL_DATADIR is only needed because the test server uses it as shipped, if
173 you remove the references to it in your app you don't need to define it on
177 Availability of header information
178 ----------------------------------
180 From v1.2 of the library onwards, the HTTP header content is free()d as soon
181 as the websocket connection is established. For websocket servers, you can
182 copy interesting headers by handling LWS_CALLBACK_FILTER_PROTOCOL_CONNECTION
183 callback, for clients there's a new callback just for this purpose
184 LWS_CALLBACK_CLIENT_FILTER_PRE_ESTABLISH.
190 It is possible for a connection which is not being used to send to die
191 silently somewhere between the peer and the side not sending. In this case
192 by default TCP will just not report anything and you will never get any more
193 incoming data or sign the link is dead until you try to send.
195 To deal with getting a notification of that situation, you can choose to
196 enable TCP keepalives on all libwebsockets sockets, when you create the
199 To enable keepalive, set the ka_time member of the context creation parameter
200 struct to a nonzero value (in seconds) at context creation time. You should
201 also fill ka_probes and ka_interval in that case.
203 With keepalive enabled, the TCP layer will send control packets that should
204 stimulate a response from the peer without affecting link traffic. If the
205 response is not coming, the socket will announce an error at poll() forcing
208 Note that BSDs don't support keepalive time / probes / inteveral per-socket
209 like Linux does. On those systems you can enable keepalive by a nonzero
210 value in ka_time, but the systemwide kernel settings for the time / probes/
211 interval are used, regardless of what nonzero value is in ka_time.
213 Optimizing SSL connections
214 --------------------------
216 There's a member ssl_cipher_list in the lws_context_creation_info struct
217 which allows the user code to restrict the possible cipher selection at
218 context-creation time.
220 You might want to look into that to stop the ssl peers selecting a ciher which
221 is too computationally expensive. To use it, point it to a string like
223 "RC4-MD5:RC4-SHA:AES128-SHA:AES256-SHA:HIGH:!DSS:!aNULL"
225 if left NULL, then the "DEFAULT" set of ciphers are all possible to select.