Send a binary data example from server to client

[NuclearRazor]

Hi everyone!

I use server example from the master branch of the mongoose library (examples/websocket-server/main.c).

All works fine, but I want to know, how to send integers as binary data to client for example? With WEBSOCKET_OP_BINARY flag.
Or it will always be a "string" (I mean, I should always to use mg_str call).

My dummy example:

// Copyright (c) 2020 Cesanta Software Limited
// All rights reserved
//
// Example Websocket server. See https://mongoose.ws/tutorials/websocket-server/


#include "mongoose.h"

static const char *s_listen_on = "ws://localhost:8000";
static const char *s_web_root = ".";

//static struct mg_rpc *s_rpc_head = NULL;

// This RESTful server implements the following endpoints:
//   /websocket - upgrade to Websocket, and implement websocket echo server
//   /api/rest - respond with JSON string {"result": 123}
//   any other URI serves static files from s_web_root
static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
  if (ev == MG_EV_OPEN) {
    // c->is_hexdumping = 1;
  } else if (ev == MG_EV_HTTP_MSG) {
    struct mg_http_message *hm = (struct mg_http_message *) ev_data;
    if (mg_http_match_uri(hm, "/websocket")) {
      // Upgrade to websocket. From now on, a connection is a full-duplex
      // Websocket connection, which will receive MG_EV_WS_MSG events.
      mg_ws_upgrade(c, hm, NULL);
    } else if (mg_http_match_uri(hm, "/rest")) {
      // Serve REST response
      mg_http_reply(c, 200, "", "{\"result\": %d}\n", 123);
    } else {
      // Serve static files
      struct mg_http_serve_opts opts = {.root_dir = s_web_root};
      mg_http_serve_dir(c, ev_data, &opts);
    }
  } else if (ev == MG_EV_WS_MSG) {
    // Got websocket frame. Received data is wm->data. Echo it back!

    int payload = 123;
    int payload_size = 4; //len is in bytes, right?

    mg_ws_send(c, payload, payload_size, WEBSOCKET_OP_BINARY);

    //mg_ws_send(c, wm->data.ptr, wm->data.len, WEBSOCKET_OP_TEXT);
  }
  (void) fn_data;
}


int main(void) {
  struct mg_mgr mgr;  // Event manager
  mg_mgr_init(&mgr);  // Initialise event manager
  printf("Starting WS listener on %s/websocket\n", s_listen_on);
  mg_http_listen(&mgr, s_listen_on, fn, NULL);  // Create HTTP listener
  for (;;) mg_mgr_poll(&mgr, 1000);             // Infinite event loop
  mg_mgr_free(&mgr);
  return 0;
}

gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39)
GNU Make 4.2.1
Built for x86_64-redhat-linux-gnu

[scaprile]

Check the WebSocket server example and tutorial.
Check the MQTT over WebSocket client example and tutorial; when sending binary protocols, you need to "wrap" your message (add "message boundaries") so the other end can identify it as a whole message

[scaprile]

You won't send an integer, you will send bytes
Binary data (bytes) is sent using mg_ws_send()
As you can see in the docs, you specify a pointer and a length, you are not passing a pointer; your compiler should probably have issued a warning

    int payload = 123;
    mg_ws_send(c, &payload, sizeof(payload), WEBSOCKET_OP_BINARY);

Anyway, I'd use JSON and send as text

[NuclearRazor]

Hm, thanks, but can't read incoming data. Maybe I need to "decode" in some way not using just a mg_ws_message type?

Part of server code (other parts are the same as on my intial comment):

int payload = 123;
mg_ws_send(c, &payload, sizeof(payload), WEBSOCKET_OP_BINARY);

Part of client code (all parts are the same as websocket-client example, except this):

 else if (ev == MG_EV_WS_MSG) {
    // When we get echo response, print it
    struct mg_ws_message *wm = (struct mg_ws_message *) ev_data;
    printf("GOT ECHO REPLY: [%d] [%d]\n", (int) wm->data.len, (int)wm->data.ptr);
  }

Got only:
GOT ECHO REPLY: [4] data: [31725874]

[NuclearRazor]

Maybe I need to "unpack" mg_ws_message?

[NuclearRazor]

Sorry, will try with memcpy, printf will not work for binary data.

[NuclearRazor]

void *value = malloc(wm->data.len);
memcpy(value, wm->data.ptr, wm->data.len);
printf("value = %d\n",* (int*)value);

It works great on the client side, thank you very much!

[cpq]

It is a bad idea to send native data types over the network.
Your client can be 32-bit, and a server can be 64-bit. Or vice versa. Or they may have a different endianness. Don't do that.
A significantly more robust way is to encode the data. The simplest encoding is JSON, and Mongoose has native support for it.
So on the sending side, do:

mg_ws_printf(c, "{%Q: %d}", "value", 123);   // Sends a {"value": 123}  websocket message

On the receiving side,

struct mg_ws_message *wm = ev_data;
long value = mg_json_get_long(wm->data, "$.value", -1);  // value is 123

You can do a less verbose enconding - just send a number in a string form: mg_ws_printf(c, "%d", value), and decode on the receiving side as mg_json_get_long(wm->data, "$", -1)

[NuclearRazor]

Thank you very much for more example. Yeah, know about endianness, but this code would be a part of "local services" and I just wanted to know how to work with binary data. Was not so obvious for me.

[cpq]

void *value = malloc(wm->data.len);
memcpy(value, wm->data.ptr, wm->data.len);
printf("value = %d\n",* (int*)value);

That is an overkill. There is no need to copy the received data in order to parse it:

int value = * (int *) wm->data.ptr;  // Assumes that wm->data.len >= sizeof(int)