doxygen use sections

[platform/upstream/libwebsockets.git] / README.coding.md

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 avaiable 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:

***ONLY DO LWS_WRITE FROM THE WRITEABLE CALLBACK***

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

***YOU MUST PERFORM RX FLOW CONTROL***

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:

```
            case LWS_CALLBACK_RECEIVE:
            {
                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);
                    client->ResetMessage();
                } else
                    client->AppendMessageFragment(in, len, remaining);
            }
            break;
```

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

```
        $ cmake .. -DCMAKE_BUILD_TYPE=DEBUG
```

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

        `"RC4-MD5:RC4-SHA:AES128-SHA:AES256-SHA:HIGH:!DSS:!aNULL"`

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](test-server/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_filefd_type
        `lws_plat_file_open`(struct lws *wsi, const char *filename, unsigned long *filelen, int flags)

        static inline int
        `lws_plat_file_close`(struct lws *wsi, lws_filefd_type fd)

        static inline unsigned long
        `lws_plat_file_seek_cur`(struct lws *wsi, lws_filefd_type fd, long offset_from_cur_pos)

        static inline int
        `lws_plat_file_read`(struct lws *wsi, lws_filefd_type fd, unsigned long *amount, unsigned char *buf, unsigned long len)

        static inline int
        `lws_plat_file_write`(struct lws *wsi, lws_filefd_type fd, unsigned long *amount, unsigned char *buf, unsigned long 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.

@section ecdh ECDH Support

ECDH Certs are now supported.  Enable the CMake option

        cmake .. -DLWS_SSL_SERVER_WITH_ECDH_CERT=1 

**and** the info->options flag

        LWS_SERVER_OPTION_SSL_ECDH

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 <n> 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

        -DLWS_WITH_LIBEV=1
        -DLWS_WITH_LIBUV=1

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

        LWS_SERVER_OPTION_LIBEV
        LWS_SERVER_OPTION_LIBUV

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

```
        case LWS_CALLBACK_RECEIVE_CLIENT_HTTP:
                {
                        char buffer[1024 + LWS_PRE];
                        char *px = buffer + LWS_PRE;
                        int lenx = sizeof(buffer) - LWS_PRE;

                        lwsl_notice("LWS_CALLBACK_RECEIVE_CLIENT_HTTP\n");

                        /*
                         * 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--)
                                putchar(*px++);
                }
                break;
```

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,
                         struct lws_http_mount *mounts);
```

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 abc.com:1234, it will match on a
vhost named abc.com that is listening on port 1234.

If there is no exact match, lws will consider wildcard matches, for example
if cats.abc.com:1234 is provided by the client by SNI or Host: header, it will
accept a vhost "abc.com" 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/warmcat.com" */
                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_HTTP,
                LWSMPRO_HTTPS,
                LWSMPRO_FILE,
                LWSMPRO_CGI,
                LWSMPRO_REDIR_HTTP,
                LWSMPRO_REDIR_HTTPS,
                LWSMPRO_CALLBACK,
        };
```

 - 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 */
                NULL,
                0,
                0,
                0,
                0,
                0,
                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).