Data Structures
struct	lws_write_passthru

Enumerations
enum	lws_write_protocol { LWS_WRITE_TEXT = 0, LWS_WRITE_BINARY = 1, LWS_WRITE_CONTINUATION = 2, LWS_WRITE_HTTP = 3, LWS_WRITE_PING = 5, LWS_WRITE_PONG = 6, LWS_WRITE_HTTP_FINAL = 7, LWS_WRITE_HTTP_HEADERS = 8, LWS_WRITE_HTTP_HEADERS_CONTINUATION = 9, LWS_WRITE_NO_FIN = 0x40, LWS_WRITE_H2_STREAM_END = 0x80, LWS_WRITE_CLIENT_IGNORE_XOR_MASK = 0x80 }

Functions
LWS_VISIBLE LWS_EXTERN int	lws_write (struct lws wsi, unsigned char buf, size_t len, enum lws_write_protocol protocol)

Detailed Description

APIs related to writing data on a connection

Enumeration Type Documentation

◆ lws_write_protocol

enum lws_write_protocol

#include <lib/libwebsockets.h>

Enumerator
LWS_WRITE_TEXT	Send a ws TEXT message,the pointer must have LWS_PRE valid memory behind it. The receiver expects only valid utf-8 in the payload
LWS_WRITE_BINARY	Send a ws BINARY message, the pointer must have LWS_PRE valid memory behind it. Any sequence of bytes is valid
LWS_WRITE_CONTINUATION	Continue a previous ws message, the pointer must have LWS_PRE valid memory behind it
LWS_WRITE_HTTP	Send HTTP content
LWS_WRITE_HTTP_HEADERS	Send http headers (http2 encodes this payload and LWS_WRITE_HTTP payload differently, http 1.x links also handle this correctly. so to be compatible with both in the future,header response part should be sent using this regardless of http version expected)
LWS_WRITE_HTTP_HEADERS_CONTINUATION	Continuation of http/2 headers
LWS_WRITE_NO_FIN	This part of the message is not the end of the message
LWS_WRITE_H2_STREAM_END	Flag indicates this packet should go out with STREAM_END if h2 STREAM_END is allowed on DATA or HEADERS.
LWS_WRITE_CLIENT_IGNORE_XOR_MASK	client packet payload goes out on wire unmunged only useful for security tests since normal servers cannot decode the content if used

                         {
         LWS_WRITE_TEXT                                          = 0,
         LWS_WRITE_BINARY                                        = 1,
         LWS_WRITE_CONTINUATION                                  = 2,
         LWS_WRITE_HTTP                                          = 3,
         /* LWS_WRITE_CLOSE is handled by lws_close_reason() */
         LWS_WRITE_PING                                          = 5,
         LWS_WRITE_PONG                                          = 6,
 
         /* Same as write_http but we know this write ends the transaction */
         LWS_WRITE_HTTP_FINAL                                    = 7,
 
         /* HTTP2 */
 
         LWS_WRITE_HTTP_HEADERS                                  = 8,
         LWS_WRITE_HTTP_HEADERS_CONTINUATION                     = 9,
         /****** add new things just above ---^ ******/
 
         /* flags */
 
         LWS_WRITE_NO_FIN = 0x40,
         LWS_WRITE_H2_STREAM_END = 0x80,
         LWS_WRITE_CLIENT_IGNORE_XOR_MASK = 0x80
 };

Function Documentation

◆ lws_write()

LWS_VISIBLE LWS_EXTERN int lws_write	(	struct lws *	wsi,
		unsigned char *	buf,
		size_t	len,
		enum lws_write_protocol	protocol
	)

#include <lib/libwebsockets.h>

lws_write() - Apply protocol then write data to client

Parameters

wsi	Websocket instance (available from user callback)
buf	The data to send. For data being sent on a websocket connection (ie, not default http), this buffer MUST have LWS_PRE bytes valid BEFORE the pointer. This is so the protocol header data can be added in-situ.
len	Count of the data bytes in the payload starting from buf
protocol	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.

This function provides the way to issue data back to the client for both http and websocket protocols.

IMPORTANT NOTICE!

When sending with websocket protocol

LWS_WRITE_TEXT, LWS_WRITE_BINARY, LWS_WRITE_CONTINUATION, LWS_WRITE_PING, LWS_WRITE_PONG

the send buffer has to have LWS_PRE bytes valid BEFORE the buffer pointer you pass to lws_write().

This allows us to add protocol info before and after the data, and send as one packet on the network without payload copying, for maximum efficiency.

So for example you need this kind of code to use lws_write with a 128-byte payload

char buf[LWS_PRE + 128];

// fill your part of the buffer... for example here it's all zeros memset(&buf[LWS_PRE], 0, 128);

lws_write(wsi, &buf[LWS_PRE], 128, LWS_WRITE_TEXT);

When sending HTTP, with

LWS_WRITE_HTTP, LWS_WRITE_HTTP_HEADERS LWS_WRITE_HTTP_FINAL

there is no protocol data prepended, and don't need to take care about the LWS_PRE bytes valid before the buffer pointer.

LWS_PRE is at least the frame nonce + 2 header + 8 length LWS_SEND_BUFFER_POST_PADDING is deprecated, it's now 0 and can be left off. The example apps no longer use it.

Pad LWS_PRE to the CPU word size, so that word references to the address immediately after the padding won't cause an unaligned access error. Sometimes for performance reasons the recommended padding is even larger than sizeof(void *).

 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.

 Return may be -1 for a fatal error needing connection close, or the
 number of bytes sent.

Truncated Writes

The OS may not accept everything you asked to write on the connection.

Posix defines POLLOUT indication from poll() to show that the connection will accept more write data, but it doesn't specifiy how much. It may just accept one byte of whatever you wanted to send.

LWS will buffer the remainder automatically, and send it out autonomously.

During that time, WRITABLE callbacks will be suppressed.

This is to handle corner cases where unexpectedly the OS refuses what we usually expect it to accept. You should try to send in chunks that are almost always accepted in order to avoid the inefficiency of the buffering.

Data Structures