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.
73 To support fragmented messages you need to check for the final
74 frame of a message with libwebsocket_is_final_fragment. This
75 check can be combined with libwebsockets_remaining_packet_payload
76 to gather the whole contents of a message, eg:
78 case LWS_CALLBACK_RECEIVE:
80 Client * const client = (Client *)user;
81 const size_t remaining = libwebsockets_remaining_packet_payload(wsi);
83 if (!remaining && libwebsocket_is_final_fragment(wsi)) {
84 if (client->HasFragments()) {
85 client->AppendMessageFragment(in, len, 0);
86 in = (void *)client->GetMessage();
87 len = client->GetMessageLength();
90 client->ProcessMessage((char *)in, len, wsi);
91 client->ResetMessage();
93 client->AppendMessageFragment(in, len, remaining);
97 The test app llibwebsockets-test-fraggle sources also show how to
98 deal with fragmented messages.
104 Also using lws_set_log_level api you may provide a custom callback to actually
105 emit the log string. By default, this points to an internal emit function
106 that sends to stderr. Setting it to NULL leaves it as it is instead.
108 A helper function lwsl_emit_syslog() is exported from the library to simplify
109 logging to syslog. You still need to use setlogmask, openlog and closelog
112 The logging apis are made available for user code.
120 The difference between notice and info is that notice will be logged by default
121 whereas info is ignored by default.
124 External Polling Loop support
125 -----------------------------
127 libwebsockets maintains an internal poll() array for all of its
128 sockets, but you can instead integrate the sockets into an
129 external polling array. That's needed if libwebsockets will
130 cooperate with an existing poll array maintained by another
133 Four callbacks LWS_CALLBACK_ADD_POLL_FD, LWS_CALLBACK_DEL_POLL_FD,
134 LWS_CALLBACK_SET_MODE_POLL_FD and LWS_CALLBACK_CLEAR_MODE_POLL_FD
135 appear in the callback for protocol 0 and allow interface code to
136 manage socket descriptors in other poll loops.
139 Using with in c++ apps
140 ----------------------
142 The library is ready for use by C++ apps. You can get started quickly by
143 copying the test server
145 $ cp test-server/test-server.c test.cpp
147 and building it in C++ like this
149 $ g++ -DINSTALL_DATADIR=\"/usr/share\" -ocpptest test.cpp -lwebsockets
151 INSTALL_DATADIR is only needed because the test server uses it as shipped, if
152 you remove the references to it in your app you don't need to define it on
156 Availability of header information
157 ----------------------------------
159 From v1.2 of the library onwards, the HTTP header content is free()d as soon
160 as the websocket connection is established. For websocket servers, you can
161 copy interesting headers by handling LWS_CALLBACK_FILTER_PROTOCOL_CONNECTION
162 callback, for clients there's a new callback just for this purpose
163 LWS_CALLBACK_CLIENT_FILTER_PRE_ESTABLISH.