How to send a JSON REST API response with a list of options

main.c

size_t print_entries(void (*out)(char, void *), void *ptr, va_list *ap) {
  const char **names = va_arg(*ap, const char **);
  size_t i, len = 0;
  for (i = 0; names[i] != NULL; i++) {
    len += mg_xprintf(out, ptr, "%s%m",
                      i > 0 ? "," : "",   // Comma-separate entries
                      MG_ESC(names[i]));  // JSON-escape string
  }
  return len;
}

void my_reply_entries(struct mg_connection *c, struct mg_http_message *hm) {
  const char *headers =
      "Cache-Control: no-cache\r\n"
      "Content-Type: application/json\r\n";
  const char *names[] = {"aa", "bb", "cc", NULL};
  (void) hm;
  mg_http_reply(c, 200, headers, "[%M]\n", print_entries, names);
}