sniffer: fix mangled newlines on stdout output (attempt #2)

src/sniffer.cpp

@@ -11,6 +11,7 @@
 #include <filesystem>
 #include <iostream>
 #include <optional>
+#include <syncstream>
 #include <thread>
 
 namespace fs = std::filesystem;
@@ -70,14 +71,15 @@ void Sniffer::start() noexcept {
   std::cout << (Reply::csv_header() + "\n");
   auto handler = [this](Tins::Packet &packet) {
     auto reply = Parser::parse(packet);
+    auto stdout = std::osyncstream(std::cout);
 
     if (reply && (!integrity_check_ || reply->is_valid(caracal_id_))) {
       spdlog::trace(reply.value());
       statistics_.icmp_messages_all.insert(reply->reply_src_addr);
       if (reply->is_time_exceeded()) {
         statistics_.icmp_messages_path.insert(reply->reply_src_addr);
       }
-      std::cout << (reply->to_csv(meta_round_.value_or("1")) + "\n");
+      stdout << reply->to_csv(meta_round_.value_or("1")) << "\n";
     } else {
       auto data = packet.pdu()->serialize();
       spdlog::trace("invalid_packet_hex={:02x}", fmt::join(data, ""));