d909cb9 Feb 24, 2017
@lws-team @ppentchev @paroga @JoakimSoderberg
708 lines (490 sloc) 25.2 KB

Notes about coding with lws

@section dae Daemonization

There's a helper api lws_daemonize built by default that does everything you need to daemonize well, including creating a lock file. If you're making what's basically a daemon, just call this early in your init to fork to a headless background process and exit the starting process.

Notice stdout, stderr, stdin are all redirected to /dev/null to enforce your daemon is headless, so you'll need to sort out alternative logging, by, eg, syslog.

@section conns Maximum number of connections

The maximum number of connections the library can deal with is decided when it starts by querying the OS to find out how many file descriptors it is allowed to open (1024 on Fedora for example). It then allocates arrays that allow up to that many connections, minus whatever other file descriptors are in use by the user code.

If you want to restrict that allocation, or increase it, you can use ulimit or similar to change the available number of file descriptors, and when restarted libwebsockets will adapt accordingly.

@section evtloop Libwebsockets is singlethreaded

Libwebsockets works in a serialized event loop, in a single thread.

Directly performing websocket actions from other threads is not allowed. Aside from the internal data being inconsistent in forked() processes, the scope of a wsi (struct websocket) can end at any time during service with the socket closing and the wsi freed.

Websocket write activities should only take place in the LWS_CALLBACK_SERVER_WRITEABLE callback as described below.

[This network-programming necessity to link the issue of new data to the peer taking the previous data is not obvious to all users so let's repeat that in other words:


There is another network-programming truism that surprises some people which is if the sink for the data cannot accept more:


See the mirror protocol implementations for example code.

Only live connections appear in the user callbacks, so this removes any possibility of trying to used closed and freed wsis.

If you need to service other socket or file descriptors as well as the websocket ones, you can combine them together with the websocket ones in one poll loop, see "External Polling Loop support" below, and still do it all in one thread / process context.

If you insist on trying to use it from multiple threads, take special care if you might simultaneously create more than one context from different threads.

SSL_library_init() is called from the context create api and it also is not reentrant. So at least create the contexts sequentially.

@section writeable Only send data when socket writeable

You should only send data on a websocket connection from the user callback LWS_CALLBACK_SERVER_WRITEABLE (or LWS_CALLBACK_CLIENT_WRITEABLE for clients).

If you want to send something, do not just send it but request a callback when the socket is writeable using

  • lws_callback_on_writable(context, wsi) for a specific wsi, or

  • lws_callback_on_writable_all_protocol(protocol) for all connections using that protocol to get a callback when next writeable.

Usually you will get called back immediately next time around the service loop, but if your peer is slow or temporarily inactive the callback will be delayed accordingly. Generating what to write and sending it should be done in the ...WRITEABLE callback.

See the test server code for an example of how to do this.

@section otherwr Do not rely on only your own WRITEABLE requests appearing

Libwebsockets may generate additional LWS_CALLBACK_CLIENT_WRITEABLE events if it met network conditions where it had to buffer your send data internally.

So your code for LWS_CALLBACK_CLIENT_WRITEABLE needs to own the decision about what to send, it can't assume that just because the writeable callback came it really is time to send something.

It's quite possible you get an 'extra' writeable callback at any time and just need to return 0 and wait for the expected callback later.

@section closing Closing connections from the user side

When you want to close a connection, you do it by returning -1 from a callback for that connection.

You can provoke a callback by calling lws_callback_on_writable on the wsi, then notice in the callback you want to close it and just return -1. But usually, the decision to close is made in a callback already and returning -1 is simple.

If the socket knows the connection is dead, because the peer closed or there was an affirmitive network error like a FIN coming, then libwebsockets will take care of closing the connection automatically.

If you have a silently dead connection, it's possible to enter a state where the send pipe on the connection is choked but no ack will ever come, so the dead connection will never become writeable. To cover that, you can use TCP keepalives (see later in this document) or pings.

@section frags Fragmented messages

To support fragmented messages you need to check for the final frame of a message with lws_is_final_fragment. This check can be combined with libwebsockets_remaining_packet_payload to gather the whole contents of a message, eg:

            Client * const client = (Client *)user;
            const size_t remaining = lws_remaining_packet_payload(wsi);

            if (!remaining && lws_is_final_fragment(wsi)) {
                if (client->HasFragments()) {
                    client->AppendMessageFragment(in, len, 0);
                    in = (void *)client->GetMessage();
                    len = client->GetMessageLength();

                client->ProcessMessage((char *)in, len, wsi);
            } else
                client->AppendMessageFragment(in, len, remaining);

The test app libwebsockets-test-fraggle sources also show how to deal with fragmented messages.

@section debuglog Debug Logging

Also using lws_set_log_level api you may provide a custom callback to actually emit the log string. By default, this points to an internal emit function that sends to stderr. Setting it to NULL leaves it as it is instead.

A helper function lwsl_emit_syslog() is exported from the library to simplify logging to syslog. You still need to use setlogmask, openlog and closelog in your user code.

The logging apis are made available for user code.

  • lwsl_err(...)
  • lwsl_warn(...)
  • lwsl_notice(...)
  • lwsl_info(...)
  • lwsl_debug(...)

The difference between notice and info is that notice will be logged by default whereas info is ignored by default.

If you are not building with _DEBUG defined, ie, without this


then log levels below notice do not actually get compiled in.

@section extpoll External Polling Loop support

libwebsockets maintains an internal poll() array for all of its sockets, but you can instead integrate the sockets into an external polling array. That's needed if libwebsockets will cooperate with an existing poll array maintained by another server.

Four callbacks LWS_CALLBACK_ADD_POLL_FD, LWS_CALLBACK_DEL_POLL_FD, LWS_CALLBACK_SET_MODE_POLL_FD and LWS_CALLBACK_CLEAR_MODE_POLL_FD appear in the callback for protocol 0 and allow interface code to manage socket descriptors in other poll loops.

You can pass all pollfds that need service to lws_service_fd(), even if the socket or file does not belong to libwebsockets it is safe.

If libwebsocket handled it, it zeros the pollfd revents field before returning. So you can let libwebsockets try and if pollfd->revents is nonzero on return, you know it needs handling by your code.

Also note that when integrating a foreign event loop like libev or libuv where it doesn't natively use poll() semantics, and you must return a fake pollfd reflecting the real event:

  • be sure you set .events to .revents value as well in the synthesized pollfd

  • check the built-in support for the event loop if possible (eg, ./lib/libuv.c) to see how it interfaces to lws

  • use LWS_POLLHUP / LWS_POLLIN / LWS_POLLOUT from libwebsockets.h to avoid losing windows compatibility

@section cpp Using with in c++ apps

The library is ready for use by C++ apps. You can get started quickly by copying the test server

    $ cp test-server/test-server.c test.cpp

and building it in C++ like this

    $ g++ -DINSTALL_DATADIR=\"/usr/share\" -ocpptest test.cpp -lwebsockets

INSTALL_DATADIR is only needed because the test server uses it as shipped, if you remove the references to it in your app you don't need to define it on the g++ line either.

@section headerinfo Availability of header information

HTTP Header information is managed by a pool of "ah" structs. These are a limited resource so there is pressure to free the headers and return the ah to the pool for reuse.

For that reason header information on HTTP connections that get upgraded to websockets is lost after the ESTABLISHED callback. Anything important that isn't processed by user code before then should be copied out for later.

For HTTP connections that don't upgrade, header info remains available the whole time.

@section ka TCP Keepalive

It is possible for a connection which is not being used to send to die silently somewhere between the peer and the side not sending. In this case by default TCP will just not report anything and you will never get any more incoming data or sign the link is dead until you try to send.

To deal with getting a notification of that situation, you can choose to enable TCP keepalives on all libwebsockets sockets, when you create the context.

To enable keepalive, set the ka_time member of the context creation parameter struct to a nonzero value (in seconds) at context creation time. You should also fill ka_probes and ka_interval in that case.

With keepalive enabled, the TCP layer will send control packets that should stimulate a response from the peer without affecting link traffic. If the response is not coming, the socket will announce an error at poll() forcing a close.

Note that BSDs don't support keepalive time / probes / interval per-socket like Linux does. On those systems you can enable keepalive by a nonzero value in ka_time, but the systemwide kernel settings for the time / probes/ interval are used, regardless of what nonzero value is in ka_time.

@section sslopt Optimizing SSL connections

There's a member ssl_cipher_list in the lws_context_creation_info struct which allows the user code to restrict the possible cipher selection at context-creation time.

You might want to look into that to stop the ssl peers selecting a cipher which is too computationally expensive. To use it, point it to a string like


if left NULL, then the "DEFAULT" set of ciphers are all possible to select.

You can also set it to "ALL" to allow everything (including insecure ciphers).

@section clientasync Async nature of client connections

When you call lws_client_connect_info(..) and get a wsi back, it does not mean your connection is active. It just means it started trying to connect.

Your client connection is actually active only when you receive LWS_CALLBACK_CLIENT_ESTABLISHED for it.

There's a 5 second timeout for the connection, and it may give up or die for other reasons, if any of that happens you'll get a LWS_CALLBACK_CLIENT_CONNECTION_ERROR callback on protocol 0 instead for the wsi.

After attempting the connection and getting back a non-NULL wsi you should loop calling lws_service() until one of the above callbacks occurs.

As usual, see test-client.c for example code.

Notice that the client connection api tries to progress the connection somewhat before returning. That means it's possible to get callbacks like CONNECTION_ERROR on the new connection before your user code had a chance to get the wsi returned to identify it (in fact if the connection did fail early, NULL will be returned instead of the wsi anyway).

To avoid that problem, you can fill in pwsi in the client connection info struct to point to a struct lws that get filled in early by the client connection api with the related wsi. You can then check for that in the callback to confirm the identity of the failing client connection.

@section fileapi Lws platform-independent file access apis

lws now exposes his internal platform file abstraction in a way that can be both used by user code to make it platform-agnostic, and be overridden or subclassed by user code. This allows things like handling the URI "directory space" as a virtual filesystem that may or may not be backed by a regular filesystem. One example use is serving files from inside large compressed archive storage without having to unpack anything except the file being requested.

The test server shows how to use it, basically the platform-specific part of lws prepares a file operations structure that lives in the lws context.

The user code can get a pointer to the file operations struct

    LWS_VISIBLE LWS_EXTERN struct lws_plat_file_ops *
        `lws_get_fops`(struct lws_context *context);

and then can use helpers to also leverage these platform-independent file handling apis

    static inline lws_fop_fd_t
    `lws_plat_file_open`(struct lws_plat_file_ops *fops, const char *filename,
               lws_filepos_t *filelen, lws_fop_flags_t *flags)
    static inline int
    `lws_plat_file_close`(lws_fop_fd_t fop_fd)

    static inline unsigned long
    `lws_plat_file_seek_cur`(lws_fop_fd_t fop_fd, lws_fileofs_t offset)

    static inline int
    `lws_plat_file_read`(lws_fop_fd_t fop_fd, lws_filepos_t *amount,
           uint8_t *buf, lws_filepos_t len)

    static inline int
    `lws_plat_file_write`(lws_fop_fd_t fop_fd, lws_filepos_t *amount,
           uint8_t *buf, lws_filepos_t len )

The user code can also override or subclass the file operations, to either wrap or replace them. An example is shown in test server.

Changes from v2.1 and before fops

There are three changes:

1) Pre-2.2 fops directly used platform file descriptors. Current fops returns and accepts a wrapper type lws_fop_fd_t which is a pointer to a malloc'd struct containing information specific to the filesystem implementation.

2) Pre-2.2 fops bound the fops to a wsi. This is completely removed, you just give a pointer to the fops struct that applies to this file when you open it. Afterwards, the operations in the fops just need the lws_fop_fd_t returned from the open.

3) Everything is wrapped in typedefs. See lws-plat-unix.c for examples of how to implement.

@section ecdh ECDH Support

ECDH Certs are now supported. Enable the CMake option


and the info->options flag


to build in support and select it at runtime.

@section smp SMP / Multithreaded service

SMP support is integrated into LWS without any internal threading. It's very simple to use, libwebsockets-test-server-pthread shows how to do it, use -j argument there to control the number of service threads up to 32.

Two new members are added to the info struct

unsigned int count_threads;
unsigned int fd_limit_per_thread;

leave them at the default 0 to get the normal singlethreaded service loop.

Set count_threads to n to tell lws you will have n simultaneous service threads operating on the context.

There is still a single listen socket on one port, no matter how many service threads.

When a connection is made, it is accepted by the service thread with the least connections active to perform load balancing.

The user code is responsible for spawning n threads running the service loop associated to a specific tsi (Thread Service Index, 0 .. n - 1). See the libwebsockets-test-server-pthread for how to do.

If you leave fd_limit_per_thread at 0, then the process limit of fds is shared between the service threads; if you process was allowed 1024 fds overall then each thread is limited to 1024 / n.

You can set fd_limit_per_thread to a nonzero number to control this manually, eg the overall supported fd limit is less than the process allowance.

You can control the context basic data allocation for multithreading from Cmake using -DLWS_MAX_SMP=, if not given it's set to 32. The serv_buf allocation for the threads (currently 4096) is made at runtime only for active threads.

Because lws will limit the requested number of actual threads supported according to LWS_MAX_SMP, there is an api lws_get_count_threads(context) to discover how many threads were actually allowed when the context was created.

It's required to implement locking in the user code in the same way that libwebsockets-test-server-pthread does it, for the FD locking callbacks.

There is no knowledge or dependency in lws itself about pthreads. How the locking is implemented is entirely up to the user code.

@section libevuv Libev / Libuv support

You can select either or both


at cmake configure-time. The user application may use one of the context init options flags


to indicate it will use either of the event libraries.

@section extopts Extension option control from user code

User code may set per-connection extension options now, using a new api lws_set_extension_option().

This should be called from the ESTABLISHED callback like this

     lws_set_extension_option(wsi, "permessage-deflate",
                              "rx_buf_size", "12"); /* 1 << 12 */

If the extension is not active (missing or not negotiated for the connection, or extensions are disabled on the library) the call is just returns -1. Otherwise the connection's extension has its named option changed.

The extension may decide to alter or disallow the change, in the example above permessage-deflate restricts the size of his rx output buffer also considering the protocol's rx_buf_size member.

@section httpsclient Client connections as HTTP[S] rather than WS[S]

You may open a generic http client connection using the same struct lws_client_connect_info used to create client ws[s] connections.

To stay in http[s], set the optional info member "method" to point to the string "GET" instead of the default NULL.

After the server headers are processed, when payload from the server is available the callback LWS_CALLBACK_RECEIVE_CLIENT_HTTP will be made.

You can choose whether to process the data immediately, or queue a callback when an outgoing socket is writeable to provide flow control, and process the data in the writable callback.

Either way you use the api lws_http_client_read() to access the data, eg

            char buffer[1024 + LWS_PRE];
            char *px = buffer + LWS_PRE;
            int lenx = sizeof(buffer) - LWS_PRE;


             * Often you need to flow control this by something
             * else being writable.  In that case call the api
             * to get a callback when writable here, and do the
             * pending client read in the writeable callback of
             * the output.
            if (lws_http_client_read(wsi, &px, &lenx) < 0)
                return -1;
            while (lenx--)

Notice that if you will use SSL client connections on a vhost, you must prepare the client SSL context for the vhost after creating the vhost, since this is not normally done if the vhost was set up to listen / serve. Call the api lws_init_vhost_client_ssl() to also allow client SSL on the vhost.

@section vhosts Using lws vhosts

If you set LWS_SERVER_OPTION_EXPLICIT_VHOSTS options flag when you create your context, it won't create a default vhost using the info struct members for compatibility. Instead you can call lws_create_vhost() afterwards to attach one or more vhosts manually.

    LWS_VISIBLE struct lws_vhost *
    lws_create_vhost(struct lws_context *context,
             struct lws_context_creation_info *info);

lws_create_vhost() uses the same info struct as lws_create_context(), it ignores members related to context and uses the ones meaningful for vhost (marked with VH in libwebsockets.h).

    struct lws_context_creation_info {
        int port;                   /* VH */
        const char *iface;              /* VH */
        const struct lws_protocols *protocols;      /* VH */
        const struct lws_extension *extensions;     /* VH */

When you attach the vhost, if the vhost's port already has a listen socket then both vhosts share it and use SNI (is SSL in use) or the Host: header from the client to select the right one. Or if no other vhost already listening the a new listen socket is created.

There are some new members but mainly it's stuff you used to set at context creation time.

@section sni How lws matches hostname or SNI to a vhost

LWS first strips any trailing :port number.

Then it tries to find an exact name match for a vhost listening on the correct port, ie, if SNI or the Host: header provided, it will match on a vhost named that is listening on port 1234.

If there is no exact match, lws will consider wildcard matches, for example if is provided by the client by SNI or Host: header, it will accept a vhost "" listening on port 1234. If there was a better, exact, match, it will have been chosen in preference to this.

Connections with SSL will still have the client go on to check the certificate allows wildcards and error out if not.

@section mounts Using lws mounts on a vhost

The last argument to lws_create_vhost() lets you associate a linked list of lws_http_mount structures with that vhost's URL 'namespace', in a similar way that unix lets you mount filesystems into areas of your / filesystem how you like and deal with the contents transparently.

    struct lws_http_mount {
        struct lws_http_mount *mount_next;
        const char *mountpoint; /* mountpoint in http pathspace, eg, "/" */
        const char *origin; /* path to be mounted, eg, "/var/www/" */
        const char *def; /* default target, eg, "index.html" */

        struct lws_protocol_vhost_options *cgienv;

        int cgi_timeout;
        int cache_max_age;

        unsigned int cache_reusable:1;
        unsigned int cache_revalidate:1;
        unsigned int cache_intermediaries:1;

        unsigned char origin_protocol;
        unsigned char mountpoint_len;

The last mount structure should have a NULL mount_next, otherwise it should point to the 'next' mount structure in your list.

Both the mount structures and the strings must persist until the context is destroyed, since they are not copied but used in place.

.origin_protocol should be one of

    enum {
  • LWSMPRO_FILE is used for mapping url namespace to a filesystem directory and serve it automatically.

  • LWSMPRO_CGI associates the url namespace with the given CGI executable, which runs when the URL is accessed and the output provided to the client.

  • LWSMPRO_REDIR_HTTP and LWSMPRO_REDIR_HTTPS auto-redirect clients to the given origin URL.

  • LWSMPRO_CALLBACK causes the http connection to attach to the callback associated with the named protocol (which may be a plugin).

@section mountcallback Operation of LWSMPRO_CALLBACK mounts

The feature provided by CALLBACK type mounts is binding a part of the URL namespace to a named protocol callback handler.

This allows protocol plugins to handle areas of the URL namespace. For example in test-server-v2.0.c, the URL area "/formtest" is associated with the plugin providing "protocol-post-demo" like this

    static const struct lws_http_mount mount_post = {
        NULL,       /* linked-list pointer to next*/
        "/formtest",        /* mountpoint in URL namespace on this vhost */
        "protocol-post-demo",   /* handler */
        NULL,   /* default filename if none given */
        LWSMPRO_CALLBACK,   /* origin points to a callback */
        9,          /* strlen("/formtest"), ie length of the mountpoint */

Client access to /formtest[anything] will be passed to the callback registered with the named protocol, which in this case is provided by a protocol plugin.

Access by all methods, eg, GET and POST are handled by the callback.

protocol-post-demo deals with accepting and responding to the html form that is in the test server HTML.

When a connection accesses a URL related to a CALLBACK type mount, the connection protocol is changed until the next access on the connection to a URL outside the same CALLBACK mount area. User space on the connection is arranged to be the size of the new protocol user space allocation as given in the protocol struct.

This allocation is only deleted / replaced when the connection accesses a URL region with a different protocol (or the default protocols[0] if no CALLBACK area matches it).

@section dim Dimming webpage when connection lost

The lws test plugins' html provides useful feedback on the webpage about if it is still connected to the server, by greying out the page if not. You can also add this to your own html easily

  • include lws-common.js from your HEAD section

  • dim the page during initialization, in a script section on your page


  • in your ws onOpen(), remove the dimming


  • in your ws onClose(), reapply the dimming