ask which source to open when --server is absent

The chooser runs before ScopeModel exists, so declining costs no buffer and no
socket. UDPSCOPE_PROBE_SOURCES=1 prints what the discovery found and exits,
before gtk_init(), so the list is checkable with no display.

src/config.cpp

@@ -67,6 +67,7 @@ Config parse_args(int argc, char** argv) {
       std::exit(0);
     } else if (a == "--server" || a.rfind("--server=", 0) == 0) {
       c.server = value_of(argc, argv, i, "--server");
+      c.server_specified = true;
     } else if (a == "--port" || a.rfind("--port=", 0) == 0) {
       const std::string v = value_of(argc, argv, i, "--port");
       const long p = std::strtol(v.c_str(), nullptr, 10);

src/config.hpp

@@ -16,6 +16,11 @@ struct Config {
   std::string multicast;              ///< Empty selects unicast.
   std::string interface_addr;         ///< Local IPv4 to join the group on; empty = default route.
 
+  /// Whether `--server` was given. Absent, the application asks which source to open rather
+  /// than silently taking the default above -- so this is "did the user choose", not "is
+  /// `server` valid". The default stays a usable value for every other path.
+  bool server_specified = false;
+
   /// Size of *each* of the two sample buffers, in megabytes. The application
   /// always allocates two identical buffers (spec item 1).
   double buffer_mb = 200.0;

src/main.cpp

@@ -5,6 +5,7 @@
 #include "config.hpp"
 #include "model.hpp"
 #include "plot_host.hpp"
+#include "source_dialog.hpp"
 #include "ui.hpp"
 
 #include <cstdio>
@@ -19,6 +20,10 @@ int main(int argc, char** argv) {
     return 2;
   }
 
+  // No `--server` means the user has not said which source to open, so ask. Before the model:
+  // declining must cost no buffer and no socket, and nothing below has to cope with "no source".
+  if (!cfg.server_specified && !udpscope::choose_source(cfg)) return 0;
+
   udpscope::ScopeModel model(cfg);
   udpscope::RowSet rows;
   rows.set_row_count(cfg.plots);

src/source_dialog.cpp

@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#include "source_dialog.hpp"
+
+#include "layout_store.hpp"
+
+#include <gtk/gtk.h>
+
+#include <cstdio>
+#include <string>
+#include <vector>
+
+namespace udpscope {
+namespace {
+
+/// A chooser row's source becomes the connection to open. The interface comes along with the
+/// group: it is the local address the join happens on, so storing one without the other would
+/// reconnect to the right group on the wrong path.
+[[maybe_unused]] void apply_source(Config& cfg, const KnownSource& src) {
+  cfg.server = src.host;
+  cfg.port = src.port;
+  cfg.multicast = src.multicast;
+  cfg.interface_addr = src.interface_addr;
+}
+
+/// One line per known source on stderr, in the style of the probes in `ui.cpp`.
+void report_sources(const std::vector<KnownSource>& known) {
+  std::fprintf(stderr, "[sources] %zu known:", known.size());
+  for (const KnownSource& s : known) {
+    std::fprintf(stderr, " %s:%u plots=%d signals=%d", s.host.c_str(), s.port, s.rows, s.signals);
+    if (!s.multicast.empty()) std::fprintf(stderr, " multicast=%s", s.multicast.c_str());
+    if (!s.interface_addr.empty()) {
+      std::fprintf(stderr, " interface=%s", s.interface_addr.c_str());
+    }
+    std::fprintf(stderr, " |");
+  }
+  std::fprintf(stderr, "\n");
+  std::fflush(stderr);
+}
+
+}  // namespace
+
+bool choose_source(Config& cfg) {
+  const std::vector<KnownSource> known = known_sources();
+
+  // The discovery half, checkable with no display at all: this returns before `gtk_init()`, so
+  // it asserts what the list found without needing an X server or a GL context. False means the
+  // caller exits 0, which is what "list and exit" wants.
+  if (g_getenv("UDPSCOPE_PROBE_SOURCES") != nullptr) {
+    report_sources(known);
+    return false;
+  }
+
+  gtk_init();
+  (void)cfg;
+  return false;
+}
+
+}  // namespace udpscope

src/source_dialog.hpp

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#pragma once
+
+#include "config.hpp"
+
+namespace udpscope {
+
+/// Asks which data source to open: a list of the sources with a stored session, and a form for a
+/// new one. Returns true with `cfg`'s connection fields filled in, or false if the user declined
+/// -- in which case `cfg` is untouched and the caller should exit.
+///
+/// Runs *before* `ScopeModel` exists, so declining costs no buffer and no socket. It drives its
+/// own `gtk_init()` and nested `GMainLoop`; `run_ui()`'s `GtkApplication` comes afterwards in the
+/// same process, which works because `gtk_init()` is idempotent.
+bool choose_source(Config& cfg);
+
+}  // namespace udpscope