I want to run a websocket server with mongoose how do I solve the error? - c++

Question

https://github.com/cesanta/mongoose/blob/master/examples/websocket-server/main.c
#include "mongoose.h"
static const char *s_listen_on = "ws://localhost:80020";
static const char *s_web_root = ".";
// 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!
struct mg_ws_message *wm = (struct mg_ws_message *) ev_data;
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;
}
I want to create a websocket server, but I am getting an error while running this project.
I tried on different ports but the result did not change.
Error is:
mongoose.c:2774:mg_listen Failed: ws://localhost:80020, errno 0
Failed