mongoose/docs/http/ssl.md

Enabling SSL (HTTPS)

To enable SSL on the server side, please follow these steps:

• Obtain SSL certificate file and private key file
• Declare struct mg_bind_opts, initialize ssl_cert and ssl_key
• Use mg_bind_opt() to create listening socket

Example:

int main(void) {
  struct mg_mgr mgr;
  struct mg_connection *c;
  struct mg_bind_opts bind_opts;

  mg_mgr_init(&mgr, NULL);

  memset(&bind_opts, 0, sizeof(bind_opts));
  bind_opts.ssl_cert = "server.pem";
  bind_opts.ssl_key = "key.pem";

  // Use bind_opts to specify SSL certificate & key file
  c = mg_bind_opt(&mgr, "443", ev_handler, bind_opts);
  mg_set_protocol_http_websocket(c);

  ...
}

For the full example, please see the Simplest HTTPS server example.