flatudp: the source, kind, mock sender and stream probes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CMakeLists.txt

@@ -248,6 +248,8 @@ add_executable(udpscope
   src/source/e1608/e1608_kind.cpp
   src/source/e1608/e1608_panels.cpp
   src/source/flatudp/flatudp_signals.cpp
+  src/source/flatudp/flatudp_source.cpp
+  src/source/flatudp/flatudp_kind.cpp
   src/plot_host.cpp
   src/export.cpp
   src/search.cpp

docs/claude/build-and-run.md

@@ -135,6 +135,25 @@ python3 tools/mock_e1608.py --port 54711 &
   --source-opt ai0=BIP10V --source-opt ai1=BIP5V --source-opt plot0=1
 ```
 
+`--source flatudp` receives the MARTe2 `UDPSender` datasource (one datagram per cycle: the signals
+concatenated in declaration order, no header, no padding). The datagram carries **no description**, so the
+settings are the whole contract: `port` 44488, `bind`, `multicast` and `iface` (a group to join and the local
+interface for it), `rate` (the sender's cycle rate in Hz; **required**, it sizes the ring and the ingest
+decimation and spaces the samples within a frame), `endian` `little|big`, and `signals`
+`name:type[count],...` with types `uint8/16/32/64`, `int8/16/32/64`, `float32`, `float64` (a count above 1
+makes a vector signal: one channel whose `count` values per datagram are consecutive samples, as a UDPStreamer
+block signal), e.g. `Trigger:uint8,Ia:float32,Wave:float32[16]`. A datagram of any other size than
+the list implies is dropped and counted as stale; each distinct wrong size is reported once on stderr as
+`[flatudp] ...`. Loss is never reported: the stream has no counter. `tools/mock_flatudp.py` is a sender for
+a signal spec (`--rate`, `--signals`, `--big-endian`, `--gap S,D`, `--bad-every S`, `--multicast G --mcast-if A`,
+`--log FILE`).
+
+```sh
+python3 tools/mock_flatudp.py --port 44588 --rate 1000 --signals Trigger:uint8,Ia:float32,Wave:int16[4] &
+./build/udpscope --source flatudp --source-opt port=44588 --source-opt rate=1000 \
+  --source-opt signals=Trigger:uint8,Ia:float32,Wave:int16[4]
+```
+
 `tools/mock_udpstreamer.py` has waveform-injection options used by the probes:
 
 - `--square-harmonics N` shapes `square`: `N` odd harmonics of the ideal step's Fourier

docs/claude/probe-readings.md

@@ -1827,9 +1827,10 @@ are unrealized and measure 0x0.
 ## `[source-kinds]`
 
 ```
-[source-kinds] kinds=udps,e1608,probe
+[source-kinds] kinds=udps,e1608,flatudp,probe
 [source-kinds] form udps: round_trip=1 valid=1
 [source-kinds] form e1608: round_trip=1 valid=1
+[source-kinds] form flatudp: round_trip=1 valid=1
 [source-kinds] form probe: round_trip=1 valid=1
 [source-kinds] panel disconnected: panels=1 present=1 sensitive=0
 [source-kinds] panel connected: panels=1 present=1 sensitive=1
@@ -1841,10 +1842,10 @@ are unrealized and measure 0x0.
 source-plugin path from the registry to the producer thread. The variable also registers the test-only `probe`
 kind, so a run without it never shows that kind. The `form` line appears once per registered kind.
 
-- `kinds=udps,e1608,probe`: a missing kind means `register_builtin_sources()` was not called before `parse_args()`,
+- `kinds=udps,e1608,flatudp,probe`: a missing kind means `register_builtin_sources()` was not called before `parse_args()`,
   or the env var gate is wrong.
 - `round_trip=0`: a form's `write` and `read` disagree about a key -- a renamed `SourceConfig` key, or an
-  absent key that was not defaulted. `valid=0` is a default configuration the form refuses. The e1608 form's sample sets address, port, scan_rate, mode, ai1, trigger, dir1 and plot0; a mismatch names one of those.
+  absent key that was not defaulted. `valid=0` is a default configuration the form refuses. The e1608 form's sample sets address, port, scan_rate, mode, ai1, trigger, dir1 and plot0; a mismatch names one of those. The flatudp sample sets port, bind, multicast, iface, rate, endian and signals.
 - `panel disconnected: sensitive=0 present=1`: **disable, don't hide.** `present=0` is a panel that was hidden
   or never added; `sensitive=1` is a panel that stays live with no source behind it.
 - `panel connected: sensitive=1`: anything else means `apply_source_panels` acted on the wrong widget (it must
@@ -1907,6 +1908,54 @@ the one `[e1608] handshake with ...: Connection refused` line, `stream: ... sign
   the scan (it also reads 0 when `connects` is still 1, since a frame count that grows on a link that never dropped
   proves nothing).
 
+## `[flatudp]`
+
+```
+[flatudp] codec: ok=1
+[flatudp] datagram of 22 bytes, expected 21, check the signal list
+[flatudp] stream: connected=1 signals=4 names=Trigger,Ia,Cnt,Wave expect=Trigger,Ia,Cnt,Wave frames=3735 rate=1000.000 ok=1
+[flatudp] channels: count=4 expect=4 ok=1
+[flatudp] continuity: ch0_breaks=0 ch1_breaks=0 ok=1
+[flatudp] wrong size: count=3 message='datagram of 22 bytes, expected 21, check the signal list' ok=1
+[flatudp] capture button: present=1 sensitive=1
+[flatudp] state: valid=3735 wrong=3
+[flatudp] silence: connected=0 model_connected=0 ok=1
+[flatudp] resumed: connected=1 frames_before=4990 frames_after=7147 ok=1
+```
+
+`UDPSCOPE_PROBE_FLATUDP=2500` (the first lines; counts vary) and `UDPSCOPE_PROBE_FLATUDP_GAP=8000` (the last two)
+run as `--source flatudp` against `tools/mock_flatudp.py` with the options in `verification.md`. The second line is
+the source's own stderr message, printed once per distinct wrong size, not a probe line. The `continuity` line
+above is the scalar-list run (`Trigger,Ia,Cnt`, big endian); the mixed-list run with `Wave:int16[4]` reads
+`continuity: skipped (...)`. With no sender the stream lines read `connected=0 ... frames=0 ... ok=0`, `continuity`
+is skipped and the capture button reads `sensitive=0`: a reading, not a failure. A run without `--bad-every` reads
+`wrong size: count=0 ... ok=0` for the same reason.
+
+- `codec: ok=1`: the `signals` text round-trips, a hand-built 24-byte datagram decodes to the hand-computed
+  values in both byte orders (`uint8` 254, `int16` -2, `float32` 1.5, `uint64[2]` 1 and 4294967298, `int8` -1)
+  and the validity rules refuse what they should. It needs no source, so `--source probe` shows it too.
+  `ok=0` is a codec defect: a swapped byte order, a wrong width or a rule that does not fire.
+- `stream: ... ok=1`: the model's signals are the list's rows in order, at the declared rate. `names` out of
+  order is a decode that mixed rows; `rate` of 1000 against a different declared rate is a `sampling_rate`
+  that did not reach the model (the ring and the decimation are then sized for the wrong cadence).
+- `channels: count=4`: one channel per signal. A vector signal is **not** one channel per element: the model
+  reads its `count` values as consecutive samples of one channel, as for a UDPStreamer block signal.
+- `continuity: ch0_breaks=0 ch1_breaks=0`: consecutive samples of the mock's first two signals step by 1 and
+  by 2 modulo 100. A break is a lost or reordered datagram, a batch flushed with the wrong number of samples,
+  a wrong byte order or ingest decimation that is not 1:1. It is skipped for a list with a vector row: the
+  model spans each frame by its largest element block, so the scalar channels of a mixed list are padded
+  with zeros (`Frame::value` reads 0 out of range) and a continuity check on them would read nearly every
+  step as a break.
+- `wrong size: ok=1`: wrong-size datagrams were counted and the message names the received and expected size.
+  The message must also be the only stderr line of its kind: one per distinct size, however many arrive.
+- `capture button: present=1 sensitive=1`: the kind reports datagrams, so the capture UI is live (contrast
+  `[source-kinds]`, where the probe kind leaves it disabled). With no stream it reads `sensitive=0`.
+- `silence: connected=0 model_connected=0`: 2 s without a valid datagram raised `Disconnected`. `connected=1`
+  is the timeout not firing, wrong-size datagrams refreshing the silence clock (they must not), or the mock's
+  gap not covering the probe (see the timing note in `verification.md`).
+- `resumed: ok=1`: `Connected` was raised again and frames resumed. `connected=0` here means the source never
+  recovered after the sender came back.
+
 ## `[prefs-probe]`
 
 ```

docs/claude/verification.md

@@ -423,7 +423,7 @@ precedent, not an exception.
 - `UDPSCOPE_PROBE_SOURCE_KINDS=2000` with `UDPSCOPE_EXIT_AFTER_MS=5000`, run as `--source probe --source-opt rate=500`
   -- the source-plugin path end to end. The variable's presence also registers the test-only `probe` kind (a
   synthetic sine with one panel), so it is the only way a run sees that kind; its value is the delay in ms.
-  Eight `[source-kinds]` lines (one `form` line per registered kind: `udps`, `e1608`, `probe`); the
+  Nine `[source-kinds]` lines (one `form` line per registered kind: `udps`, `e1608`, `flatudp`, `probe`); the
   reading's own entry is in `probe-readings.md`.
 - `UDPSCOPE_PROBE_E1608=1500` with `UDPSCOPE_EXIT_AFTER_MS`, run as `--source e1608` against
   `tools/mock_e1608.py` -- the MCC E-1608 end to end: panels present and sensitive, a DAC write, a
@@ -441,6 +441,24 @@ precedent, not an exception.
   - **a rejected code:** mock `--code 1234`, app `code=7`: one error line, the retries stop.
 - `UDPSCOPE_PROBE_E1608_RECONNECT=8000` -- reads the source after the mock dropped the link and 500 ms
   later checks the frame count moved on. The reading's own entry is in `probe-readings.md`.
+- `UDPSCOPE_PROBE_FLATUDP=2500` with `UDPSCOPE_EXIT_AFTER_MS`, run as `--source flatudp` against
+  `tools/mock_flatudp.py` -- the flat UDP source end to end: a hand-computed codec and decode check (it
+  needs no source), the stream's signal names and declared rate, the channel count (one per signal), value
+  continuity across the first two signals (all-scalar lists only), the wrong-size accounting and the capture
+  button. It enables the signals itself, so do **not** add `UDPSCOPE_ENABLE_ALL_MS`. Runs, each short and
+  serial, with the mock killed afterwards (quote the `signals` value: zsh globs the `[4]`):
+  - **mixed list, gap and bad datagrams:** mock `--rate 1000 --signals Trigger:uint8,Ia:float32,Cnt:uint64,Wave:int16[4]
+    --gap 9,5 --bad-every 1 --log /tmp/flatudp-mock.log` started 1 s before the app, the same `signals` as a
+    `--source-opt`, with `UDPSCOPE_PROBE_FLATUDP=2500 UDPSCOPE_PROBE_FLATUDP_GAP=8000 UDPSCOPE_EXIT_AFTER_MS=14500`;
+    stderr carries exactly one `datagram of 22 bytes, expected 21` line. The mock's clock starts about 4 s
+    before the app's probe clock (`xvfb-run` and the app's own startup), so a gap of 9 s to 14 s on the mock's
+    clock covers the app's 8 s probe (silence) and ends before its 12 s one (resumed); a mock gap measured in
+    the app's own seconds would miss both;
+  - **scalar list, big endian (batching, continuity):** mock `--signals Trigger:uint8,Ia:float32,Cnt:uint64 --big-endian`,
+    app `--source-opt endian=big`, `UDPSCOPE_PROBE_FLATUDP=2500 UDPSCOPE_EXIT_AFTER_MS=5000`;
+  - **no sender:** `connected=0` throughout, a reading and not a failure.
+- `UDPSCOPE_PROBE_FLATUDP_GAP=8000` -- reads the source inside the mock's silence (2 s after its last datagram
+  the source is disconnected) and 4 s later checks it reconnected and the stream moved on.
 - `UDPSCOPE_PROBE_SEARCH_MS` — runs a buffer search for the armed condition, cross-checks the live
   trigger's own `t0` against the nearest hit (`agree=1` is the load-bearing reading), and jumps to
   hit 0. Reports the resulting `mode=`, `axis=` and `marks placed=`.

src/source/flatudp/flatudp_kind.cpp

@@ -0,0 +1,206 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#include "source/flatudp/flatudp_source.hpp"
+#include "source/registry.hpp"
+
+#include <adwaita.h>
+
+#include <cstdio>
+#include <functional>
+#include <memory>
+#include <string>
+
+namespace udpscope {
+namespace {
+
+using namespace flatudp;
+using ChangeCb = std::shared_ptr<std::function<void()>>;
+
+std::string num(double v) {
+  char b[48];
+  std::snprintf(b, sizeof b, "%.10g", v);
+  return b;
+}
+
+GtkWidget* entry_row(const char* title) {
+  GtkWidget* r = adw_entry_row_new();
+  adw_preferences_row_set_title(ADW_PREFERENCES_ROW(r), title);
+  adw_preferences_row_set_use_underline(ADW_PREFERENCES_ROW(r), TRUE);
+  return r;
+}
+
+GtkWidget* spin_row(const char* title, double lo, double hi, double step, double def, int digits) {
+  GtkWidget* r = GTK_WIDGET(adw_spin_row_new_with_range(lo, hi, step));
+  adw_preferences_row_set_title(ADW_PREFERENCES_ROW(r), title);
+  adw_preferences_row_set_use_underline(ADW_PREFERENCES_ROW(r), TRUE);
+  adw_spin_row_set_digits(ADW_SPIN_ROW(r), static_cast<guint>(digits));
+  adw_spin_row_set_value(ADW_SPIN_ROW(r), def);
+  return r;
+}
+
+GtkWidget* combo_row(const char* title, const char* const* items) {
+  GtkWidget* r = adw_combo_row_new();
+  adw_preferences_row_set_title(ADW_PREFERENCES_ROW(r), title);
+  adw_preferences_row_set_use_underline(ADW_PREFERENCES_ROW(r), TRUE);
+  adw_combo_row_set_model(ADW_COMBO_ROW(r), G_LIST_MODEL(gtk_string_list_new(items)));
+  return r;
+}
+
+GtkWidget* expander(const char* title, std::initializer_list<GtkWidget*> rows) {
+  GtkWidget* e = adw_expander_row_new();
+  adw_preferences_row_set_title(ADW_PREFERENCES_ROW(e), title);
+  for (GtkWidget* r : rows) adw_expander_row_add_row(ADW_EXPANDER_ROW(e), r);
+  return e;
+}
+
+std::string text_of(GtkWidget* w) { return gtk_editable_get_text(GTK_EDITABLE(w)); }
+double value_of(GtkWidget* w) { return adw_spin_row_get_value(ADW_SPIN_ROW(w)); }
+guint sel_of(GtkWidget* w) { return adw_combo_row_get_selected(ADW_COMBO_ROW(w)); }
+
+void mark(GtkWidget* w, bool bad) {
+  if (bad) {
+    gtk_widget_add_css_class(w, "error");
+  } else {
+    gtk_widget_remove_css_class(w, "error");
+  }
+}
+
+/// The form's widgets, shared by every signal closure and every `ConfigForm` closure.
+struct Form {
+  GtkWidget *port, *bind, *multicast, *iface, *rate, *endian, *signals;
+  ChangeCb changed = std::make_shared<std::function<void()>>();
+  bool updating = false;
+  bool alive = true;  ///< cleared when the chooser destroys the rows; handlers then touch nothing
+
+  /// A bad address is marked with the `error` style; the rest of the problem is `valid()`'s.
+  void refresh() {
+    mark(bind, !text_of(bind).empty() && !valid_ipv4(text_of(bind)));
+    mark(multicast, !text_of(multicast).empty() && !valid_multicast(text_of(multicast)));
+    mark(iface, !text_of(iface).empty() && !valid_ipv4(text_of(iface)));
+  }
+
+  SourceConfig read() const {
+    SourceConfig c;
+    c.set("port", num(value_of(port)));
+    if (!text_of(bind).empty()) c.set("bind", text_of(bind));
+    if (!text_of(multicast).empty()) c.set("multicast", text_of(multicast));
+    if (!text_of(iface).empty()) c.set("iface", text_of(iface));
+    c.set("rate", num(value_of(rate)));
+    c.set("endian", sel_of(endian) == 1 ? "big" : "little");
+    c.set("signals", text_of(signals));
+    return c;
+  }
+
+  void write(const SourceConfig& cfg) {
+    const FlatUdpSettings s = FlatUdpSettings::from(cfg);
+    updating = true;
+    adw_spin_row_set_value(ADW_SPIN_ROW(port), s.port);
+    gtk_editable_set_text(GTK_EDITABLE(bind), s.bind.c_str());
+    gtk_editable_set_text(GTK_EDITABLE(multicast), s.multicast.c_str());
+    gtk_editable_set_text(GTK_EDITABLE(iface), s.iface.c_str());
+    if (s.rate > 0.0) adw_spin_row_set_value(ADW_SPIN_ROW(rate), s.rate);
+    adw_combo_row_set_selected(ADW_COMBO_ROW(endian), s.big_endian ? 1 : 0);
+    gtk_editable_set_text(GTK_EDITABLE(signals), cfg.get("signals").c_str());
+    updating = false;
+    refresh();
+  }
+
+  bool valid() const { return FlatUdpSettings::from(read()).problem().empty(); }
+};
+
+using FormPtr = std::shared_ptr<Form>;
+
+void on_notify(GObject*, GParamSpec*, gpointer user) {
+  Form& f = **static_cast<FormPtr*>(user);
+  if (!f.alive) return;
+  f.refresh();
+  if (!f.updating && f.changed && *f.changed) (*f.changed)();
+}
+
+void on_edited(GtkEditable*, gpointer user) {
+  Form& f = **static_cast<FormPtr*>(user);
+  if (!f.alive) return;
+  f.refresh();
+  if (!f.updating && f.changed && *f.changed) (*f.changed)();
+}
+
+void free_form(gpointer d, GClosure*) { delete static_cast<FormPtr*>(d); }
+
+/// "destroy" on a row group: the siblings the handlers reach by raw pointer may already be gone.
+void on_form_destroy(GtkWidget*, gpointer user) { (**static_cast<FormPtr*>(user)).alive = false; }
+
+void hook_destroy(GtkWidget* w, const FormPtr& f) {
+  g_signal_connect_data(w, "destroy", G_CALLBACK(on_form_destroy), new FormPtr(f), free_form,
+                        static_cast<GConnectFlags>(0));
+}
+
+void hook_notify(GtkWidget* w, const char* signal, const FormPtr& f) {
+  g_signal_connect_data(w, signal, G_CALLBACK(on_notify), new FormPtr(f), free_form,
+                        static_cast<GConnectFlags>(0));
+}
+
+void hook_entry(GtkWidget* w, const FormPtr& f) {
+  g_signal_connect_data(w, "changed", G_CALLBACK(on_edited), new FormPtr(f), free_form,
+                        static_cast<GConnectFlags>(0));
+}
+
+class FlatUdpKind final : public SourceKind {
+ public:
+  std::string id() const override { return "flatudp"; }
+  std::string display_name() const override { return "Flat UDP (MARTe2 UDPSender)"; }
+  bool has_datagrams() const override { return true; }
+
+  std::string identity(const SourceConfig& cfg) const override {
+    return FlatUdpSettings::from(cfg).identity();
+  }
+
+  std::unique_ptr<DataSource> make(const SourceConfig& cfg) const override {
+    return std::make_unique<FlatUdpSource>(cfg);
+  }
+
+  ConfigForm make_config_form() const override {
+    auto f = std::make_shared<Form>();
+    static const char* const kEndian[] = {"Little endian", "Big endian", nullptr};
+
+    f->port = spin_row("_Port", 1, 65535, 1, kDefaultPort, 0);
+    f->bind = entry_row("_Bind address");
+    f->multicast = entry_row("_Multicast group");
+    f->iface = entry_row("Multicast _interface");
+    f->rate = spin_row("Cycle _rate (Hz)", 0.001, 1000000, 1, 1000, 3);
+    f->endian = combo_row("_Byte order", kEndian);
+    f->signals = entry_row("_Signals");
+
+    GtkWidget* net = expander("Network", {f->port, f->bind, f->multicast, f->iface});
+    GtkWidget* timing = expander("Timing and format", {f->rate, f->endian});
+    GtkWidget* sig = expander("Signals", {f->signals});
+    adw_expander_row_set_expanded(ADW_EXPANDER_ROW(net), TRUE);
+    adw_expander_row_set_expanded(ADW_EXPANDER_ROW(sig), TRUE);
+
+    for (GtkWidget* w : {net, timing, sig}) hook_destroy(w, f);
+    for (GtkWidget* w : {f->bind, f->multicast, f->iface, f->signals}) hook_entry(w, f);
+    for (GtkWidget* w : {f->port, f->rate}) hook_notify(w, "notify::value", f);
+    hook_notify(f->endian, "notify::selected", f);
+    f->refresh();
+
+    ConfigForm out;
+    out.rows = {net, timing, sig};
+    out.description =
+        "The signal list must match the sender's exactly, in order: the datagrams carry no "
+        "description. Write name:type[count], comma separated, e.g. Trigger:uint8,Ia:float32,"
+        "Wave:float32[16]";
+    out.focus = f->signals;
+    out.write = [f](const SourceConfig& c) { f->write(c); };
+    out.read = [f]() { return f->read(); };
+    out.valid = [f]() { return f->valid(); };
+    ChangeCb cb = f->changed;
+    out.on_change = [cb](std::function<void()> fn) { *cb = std::move(fn); };
+    return out;
+  }
+};
+
+}  // namespace
+
+std::unique_ptr<SourceKind> make_flatudp_kind() { return std::make_unique<FlatUdpKind>(); }
+
+}  // namespace udpscope

src/source/flatudp/flatudp_source.cpp

@@ -0,0 +1,286 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#include "source/flatudp/flatudp_source.hpp"
+
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <poll.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+#include <algorithm>
+#include <cctype>
+#include <cerrno>
+#include <cmath>
+#include <cstdio>
+#include <cstring>
+#include <stdexcept>
+#include <thread>
+
+namespace udpscope::flatudp {
+namespace {
+
+using namespace std::chrono_literals;
+
+std::string lower(std::string s) {
+  for (char& c : s) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
+  return s;
+}
+
+std::string trim(const std::string& s) {
+  std::size_t a = 0, b = s.size();
+  while (a < b && std::isspace(static_cast<unsigned char>(s[a]))) ++a;
+  while (b > a && std::isspace(static_cast<unsigned char>(s[b - 1]))) --b;
+  return s.substr(a, b - a);
+}
+
+double epoch_seconds() {
+  return std::chrono::duration<double>(std::chrono::system_clock::now().time_since_epoch()).count();
+}
+
+constexpr std::size_t kMaxReportedSizes = 8;  ///< a sender of random sizes must not flood stderr
+
+}  // namespace
+
+bool valid_ipv4(const std::string& s) {
+  in_addr a{};
+  return inet_pton(AF_INET, s.c_str(), &a) == 1;
+}
+
+bool valid_multicast(const std::string& s) {
+  in_addr a{};
+  if (inet_pton(AF_INET, s.c_str(), &a) != 1) return false;
+  return (ntohl(a.s_addr) >> 28) == 0xE;
+}
+
+// --- FlatUdpSettings ------------------------------------------------------------------------
+
+FlatUdpSettings FlatUdpSettings::from(const SourceConfig& c) {
+  FlatUdpSettings s;
+  s.port = static_cast<int>(c.get_int("port", kDefaultPort));
+  s.bind = trim(c.get("bind"));
+  s.multicast = trim(c.get("multicast"));
+  s.iface = trim(c.get("iface"));
+  s.rate = c.get_double("rate", 0.0);
+  const std::string e = lower(trim(c.get("endian", "little")));
+  s.endian_ok = e == "little" || e == "big";
+  s.big_endian = e == "big";
+  s.signals_error = parse(c.get("signals"), s.rows);
+  return s;
+}
+
+std::string FlatUdpSettings::problem() const {
+  if (port < 1 || port > 65535) return "the port must be 1 to 65535";
+  if (!signals_error.empty()) return signals_error;
+  const std::string p = flatudp::problem(rows);
+  if (!p.empty()) return p;
+  if (!(rate > 0.0) || !std::isfinite(rate)) return "give the sender's cycle rate in Hz (rate)";
+  if (!endian_ok) return "endian must be little or big";
+  if (!bind.empty() && !valid_ipv4(bind)) return "the bind address '" + bind + "' is not an IPv4 address";
+  if (!multicast.empty() && !valid_multicast(multicast))
+    return "'" + multicast + "' is not a multicast group (224.0.0.0 to 239.255.255.255)";
+  if (!iface.empty() && !valid_ipv4(iface))
+    return "the interface address '" + iface + "' is not an IPv4 address";
+  return {};
+}
+
+std::string FlatUdpSettings::identity() const {
+  return (multicast.empty() ? std::string() : multicast + ":") + std::to_string(port);
+}
+
+std::size_t FlatUdpSettings::block() const {
+  if (!all_scalar(rows) || !(rate > 0.0) || !std::isfinite(rate)) return 1;
+  const long b = std::lround(rate / 100.0);
+  return static_cast<std::size_t>(std::min(1024L, std::max(1L, b)));
+}
+
+// --- FlatUdpSource --------------------------------------------------------------------------
+
+FlatUdpSource::FlatUdpSource(const SourceConfig& cfg) : cfg_(FlatUdpSettings::from(cfg)) {}
+
+FlatUdpSource::~FlatUdpSource() { close_fd(); }
+
+std::string FlatUdpSource::last_message() const {
+  std::lock_guard<std::mutex> lk(msg_mu_);
+  return last_message_;
+}
+
+void FlatUdpSource::note(const std::string& msg) {
+  {
+    std::lock_guard<std::mutex> lk(msg_mu_);
+    last_message_ = msg;
+  }
+  std::fprintf(stderr, "[flatudp] %s\n", msg.c_str());
+  std::fflush(stderr);
+}
+
+void FlatUdpSource::close_fd() {
+  if (fd_ >= 0) {
+    ::close(fd_);
+    fd_ = -1;
+  }
+}
+
+void FlatUdpSource::open(SourceSink& sink) {
+  const std::string p = cfg_.problem();
+  if (!p.empty()) throw std::runtime_error(p);
+  sink_ = &sink;
+
+  signals_.clear();
+  for (const Row& r : cfg_.rows) {
+    SignalInfo s;
+    s.name = r.name;
+    s.type_code = static_cast<std::uint8_t>(r.type);
+    s.rows = r.count;
+    s.cols = 1;
+    s.time_mode = kTimePacket;
+    s.sampling_rate = cfg_.rate;
+    signals_.push_back(s);
+  }
+  expected_ = datagram_size(cfg_.rows);
+  block_ = cfg_.block();
+  pending_.assign(cfg_.rows.size(), {});
+  pending_n_ = 0;
+  rx_.assign(65536, 0);
+
+  const auto fail = [this](const std::string& what) {
+    const std::string msg = what + ": " + std::strerror(errno);
+    close_fd();
+    throw std::runtime_error(msg);
+  };
+
+  fd_ = ::socket(AF_INET, SOCK_DGRAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
+  if (fd_ < 0) fail("cannot create a UDP socket");
+  // Receivers of one multicast group share its port; a unicast port is the one app's.
+  if (!cfg_.multicast.empty()) {
+    const int one = 1;
+    ::setsockopt(fd_, SOL_SOCKET, SO_REUSEADDR, &one, sizeof one);
+  }
+  const int rcvbuf = 4 << 20;  // best effort: a 1 kHz sender at 64 KiB would not fit a default one
+  ::setsockopt(fd_, SOL_SOCKET, SO_RCVBUF, &rcvbuf, sizeof rcvbuf);
+
+  sockaddr_in addr{};
+  addr.sin_family = AF_INET;
+  addr.sin_port = htons(static_cast<std::uint16_t>(cfg_.port));
+  addr.sin_addr.s_addr = htonl(INADDR_ANY);
+  if (!cfg_.bind.empty()) inet_pton(AF_INET, cfg_.bind.c_str(), &addr.sin_addr);
+  if (::bind(fd_, reinterpret_cast<const sockaddr*>(&addr), sizeof addr) < 0)
+    fail("cannot bind UDP port " + std::to_string(cfg_.port));
+
+  if (!cfg_.multicast.empty()) {
+    ip_mreq mreq{};
+    inet_pton(AF_INET, cfg_.multicast.c_str(), &mreq.imr_multiaddr);
+    if (cfg_.iface.empty()) {
+      mreq.imr_interface.s_addr = htonl(INADDR_ANY);
+    } else {
+      inet_pton(AF_INET, cfg_.iface.c_str(), &mreq.imr_interface);
+    }
+    if (::setsockopt(fd_, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof mreq) < 0)
+      fail("cannot join multicast group " + cfg_.multicast);
+  }
+}
+
+void FlatUdpSource::close() {
+  close_fd();
+  connected_.store(false, std::memory_order_relaxed);
+}
+
+SourceStats FlatUdpSource::stats() const {
+  SourceStats s;
+  s.packets = valid_.load(std::memory_order_relaxed) + wrong_.load(std::memory_order_relaxed);
+  s.frames = frames_;
+  s.stale = wrong_.load(std::memory_order_relaxed);
+  s.reconnects = reconnects_;
+  s.connected = connected_.load(std::memory_order_relaxed);
+  return s;
+}
+
+void FlatUdpSource::poll(int timeout_ms) {
+  drain_commands();
+  // Never wait long: the partial-block flush and the silence timeout are checked on this cadence.
+  const int wait = std::clamp(timeout_ms, 0, 20);
+  if (fd_ < 0 || sink_ == nullptr) {
+    std::this_thread::sleep_for(std::chrono::milliseconds(wait));
+    return;
+  }
+  if (!configured_) {
+    configured_ = true;
+    sink_->on_config(signals_, 1);
+  }
+  pollfd p{};
+  p.fd = fd_;
+  p.events = POLLIN;
+  if (::poll(&p, 1, wait) > 0 && (p.revents & POLLIN)) drain_socket();
+
+  const auto now = Clock::now();
+  if (pending_n_ > 0 && now - batch_start_ >= 100ms) flush();  // a stopped sender holds nothing back
+  if (connected_.load(std::memory_order_relaxed) && now - last_valid_ >= 2s) {
+    connected_.store(false, std::memory_order_relaxed);
+    sink_->on_event(SourceEvent::Disconnected, "no datagram for 2 s");
+  }
+}
+
+void FlatUdpSource::drain_socket() {
+  for (int i = 0; i < 8192; ++i) {  // bounded, so a flood cannot starve the timers
+    const ssize_t n = ::recv(fd_, rx_.data(), rx_.size(), 0);
+    if (n < 0) {
+      if (errno == EINTR) continue;
+      break;  // EAGAIN: drained
+    }
+    const double t = epoch_seconds();
+    const std::size_t len = static_cast<std::size_t>(n);
+    if (tap_) tap_(rx_.data(), len, t);
+    if (len == expected_) {
+      accept(rx_.data());
+    } else {
+      reject(len);
+    }
+  }
+}
+
+void FlatUdpSource::accept(const std::uint8_t* data) {
+  valid_.fetch_add(1, std::memory_order_relaxed);
+  last_valid_ = Clock::now();
+  if (!connected_.load(std::memory_order_relaxed)) {
+    if (ever_connected_) ++reconnects_;
+    ever_connected_ = true;
+    connected_.store(true, std::memory_order_relaxed);
+    sink_->on_event(SourceEvent::Connected, "");
+  }
+  decode_datagram(cfg_.rows, data, cfg_.big_endian, pending_);
+  if (pending_n_ == 0) batch_start_ = last_valid_;
+  if (++pending_n_ >= block_) flush();
+}
+
+void FlatUdpSource::reject(std::size_t len) {
+  wrong_.fetch_add(1, std::memory_order_relaxed);
+  if (reported_sizes_.size() >= kMaxReportedSizes || !reported_sizes_.insert(len).second) return;
+  const std::string msg = "datagram of " + std::to_string(len) + " bytes, expected " +
+                          std::to_string(expected_) + ", check the signal list";
+  note(msg);
+  sink_->on_event(SourceEvent::Error, msg.c_str());
+}
+
+void FlatUdpSource::flush() {
+  const std::size_t n = signals_.size();
+  views_.resize(n);
+  for (std::size_t i = 0; i < n; ++i)
+    views_[i] = SignalValues{pending_[i].data(), static_cast<std::uint32_t>(pending_[i].size())};
+  Frame f;
+  f.counter = frame_counter_++;
+  f.lost = 0;
+  f.late = false;
+  f.recv_time = epoch_seconds();
+  f.publish_mode = 1;
+  f.num_samples = static_cast<std::uint32_t>(pending_n_);
+  f.num_signals = static_cast<std::uint32_t>(n);
+  f.signals = signals_.data();
+  f.values = views_.data();
+  ++frames_;
+  sink_->on_data(f);
+  for (auto& c : pending_) c.clear();
+  pending_n_ = 0;
+}
+
+}  // namespace udpscope::flatudp

src/source/flatudp/flatudp_source.hpp

@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#pragma once
+
+#include "source/data_source.hpp"
+#include "source/flatudp/flatudp_signals.hpp"
+#include "source/source_config.hpp"
+
+#include <atomic>
+#include <chrono>
+#include <cstdint>
+#include <mutex>
+#include <set>
+#include <string>
+#include <vector>
+
+namespace udpscope::flatudp {
+
+constexpr int kDefaultPort = 44488;  ///< `UDPSender`'s own default
+
+bool valid_ipv4(const std::string& s);
+/// An IPv4 address in 224.0.0.0/4.
+bool valid_multicast(const std::string& s);
+
+/// Everything a configured flat UDP source needs, parsed once from the flat `SourceConfig`.
+struct FlatUdpSettings {
+  int port = kDefaultPort;
+  std::string bind;       ///< local address; empty is any
+  std::string multicast;  ///< group to join; empty is plain unicast
+  std::string iface;      ///< local interface address for the join; empty is the system's choice
+  double rate = 0.0;      ///< the sender's cycle rate in Hz; 0 means not given
+  bool big_endian = false;
+  bool endian_ok = true;  ///< false when `endian` is neither "little" nor "big"
+  std::vector<Row> rows;
+  std::string signals_error;  ///< why the `signals` text did not parse; `rows` is empty then
+
+  static FlatUdpSettings from(const SourceConfig& cfg);
+  /// The first reason this cannot connect, in words for the user; empty when it can.
+  std::string problem() const;
+  /// `[multicast:]port`; names the session file.
+  std::string identity() const;
+  /// Datagrams per frame: 1 for a list with a vector, else `clamp(round(rate / 100), 1, 1024)`.
+  std::size_t block() const;
+};
+
+class FlatUdpSource final : public DataSource {
+ public:
+  explicit FlatUdpSource(const SourceConfig& cfg);
+  ~FlatUdpSource() override;
+
+  void open(SourceSink& sink) override;
+  void poll(int timeout_ms) override;
+  void close() override;
+  SourceStats stats() const override;
+  bool connected() const override { return connected_.load(std::memory_order_relaxed); }
+  void set_datagram_tap(DatagramTap tap) override { tap_ = std::move(tap); }
+
+  const FlatUdpSettings& settings() const { return cfg_; }
+  /// Datagrams of the expected size / of any other size; for the probe, readable from any thread.
+  std::uint64_t valid_datagrams() const { return valid_.load(std::memory_order_relaxed); }
+  std::uint64_t wrong_size_datagrams() const { return wrong_.load(std::memory_order_relaxed); }
+  std::string last_message() const;
+
+ private:
+  using Clock = std::chrono::steady_clock;
+
+  void close_fd();
+  void drain_socket();
+  void accept(const std::uint8_t* data);
+  void reject(std::size_t len);
+  void flush();
+  void note(const std::string& msg);
+
+  FlatUdpSettings cfg_;
+  SourceSink* sink_ = nullptr;
+  DatagramTap tap_;
+  int fd_ = -1;
+  bool configured_ = false;
+  bool ever_connected_ = false;
+  std::size_t expected_ = 0;  ///< bytes of a valid datagram
+  std::size_t block_ = 1;
+
+  std::vector<SignalInfo> signals_;
+  std::vector<std::uint8_t> rx_;
+  std::vector<std::vector<double>> pending_;  ///< decoded, not yet framed: [signal][value]
+  std::vector<SignalValues> views_;
+  std::size_t pending_n_ = 0;  ///< datagrams in `pending_`
+  Clock::time_point batch_start_;
+  Clock::time_point last_valid_;
+  std::set<std::size_t> reported_sizes_;
+
+  std::atomic<bool> connected_{false};
+  std::atomic<std::uint64_t> valid_{0};
+  std::atomic<std::uint64_t> wrong_{0};
+  mutable std::mutex msg_mu_;
+  std::string last_message_;
+
+  std::uint32_t frame_counter_ = 0;
+  std::uint64_t frames_ = 0;
+  std::uint64_t reconnects_ = 0;
+};
+
+}  // namespace udpscope::flatudp

src/source/registry.cpp

@@ -51,6 +51,7 @@ void register_builtin_sources() {
   done = true;
   register_kind(make_udps_kind());
   register_kind(make_e1608_kind());
+  register_kind(make_flatudp_kind());
   if (std::getenv("UDPSCOPE_PROBE_SOURCE_KINDS") != nullptr) register_kind(make_probe_kind());
 }
 

src/source/registry.hpp

@@ -21,10 +21,11 @@ const std::vector<SourceKind*>& kinds();
 /// may drop. A later `dlopen` loader would only call `register_kind()`.
 void register_builtin_sources();
 
-/// The built-in kinds, each defined in its own file (`udps_kind.cpp`, `probe_source.cpp`, `e1608/e1608_kind.cpp`).
+/// The built-in kinds, each defined in its own file (`udps_kind.cpp`, `probe_source.cpp`, `e1608/e1608_kind.cpp`, `flatudp/flatudp_kind.cpp`).
 std::unique_ptr<SourceKind> make_udps_kind();
 std::unique_ptr<SourceKind> make_probe_kind();
 std::unique_ptr<SourceKind> make_e1608_kind();
+std::unique_ptr<SourceKind> make_flatudp_kind();
 
 /// The (host, port) pair that names a source's session file. UDPS keeps its legacy
 /// `server`/`port`; any other kind is `"<kind>-<identity>"` (characters outside `[A-Za-z0-9._-]`

src/ui.cpp

@@ -6121,6 +6121,9 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   if (const char* env = g_getenv("UDPSCOPE_PROBE_FLATUDP")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), flatudp_probe, &app);
   }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_FLATUDP_GAP")) {
+    g_timeout_add(static_cast<guint>(std::atoi(env)), flatudp_probe_gap, &app);
+  }
 
   // Our own options were consumed by parse_args(); GTK would reject them, so only
   // the program name is handed to g_application_run().

src/ui_app.hpp

@@ -1884,8 +1884,13 @@ gboolean e1608_probe_report(gpointer user);
 gboolean e1608_probe_reconnect(gpointer user);
 gboolean e1608_probe_resumed(gpointer user);
 /// The flat UDP (MARTe2 UDPSender) source: the signal-list codec and a hand-computed decode, then,
-/// against `tools/mock_flatudp.py`, the stream, the wrong-size accounting and the silence timeout.
+/// against `tools/mock_flatudp.py`, the stream, the value continuity, the wrong-size accounting and
+/// the capture button (`_report`, a second later). `_gap` reads the source inside the mock's silence;
+/// `_resumed` shows the stream came back.
 gboolean flatudp_probe(gpointer user);
+gboolean flatudp_probe_report(gpointer user);
+gboolean flatudp_probe_gap(gpointer user);
+gboolean flatudp_probe_resumed(gpointer user);
 gboolean pan_probe(gpointer user);
 gboolean arm_probe(gpointer user);
 gboolean trigrenum_probe_step3(gpointer user);

src/ui_probe.cpp

@@ -9,6 +9,7 @@
 #include "source/probe_source.hpp"
 #include "source/e1608/e1608_source.hpp"
 #include "source/flatudp/flatudp_signals.hpp"
+#include "source/flatudp/flatudp_source.hpp"
 #include "source/registry.hpp"
 #include "ui_app.hpp"
 
@@ -1938,6 +1939,14 @@ gboolean source_kinds_probe(gpointer user) {
       c.set("trigger", "rising");
       c.set("dir1", "o");
       c.set("plot0", "1");
+    } else if (id == "flatudp") {
+      c.set("port", "44600");
+      c.set("bind", "10.0.0.8");
+      c.set("multicast", "239.1.2.3");
+      c.set("iface", "10.0.0.9");
+      c.set("rate", "2500");
+      c.set("endian", "big");
+      c.set("signals", "Trigger:uint8,Ia:float32,Wave:int16[4]");
     }
     return c;
   };
@@ -2102,6 +2111,31 @@ bool flatudp_codec_check() {
   return true;
 }
 
+flatudp::FlatUdpSource* flatudp_source(App* a) {
+  return a->acq != nullptr ? dynamic_cast<flatudp::FlatUdpSource*>(a->acq->source()) : nullptr;
+}
+
+std::uint64_t g_flatudp_frames_before = 0;
+
+/// The mock's signal k carries (i * (k + 1) + element) % 100 in packet i, so two consecutive
+/// samples of a scalar signal differ by k + 1 modulo 100. Returns the number of steps that do not,
+/// over a window clear of the head the producer is writing; -1 when it cannot be read.
+long flatudp_breaks(App* a, std::size_t channel, long step) {
+  if (channel >= a->model->channels().size()) return -1;
+  const ChannelBuffer& ch = a->model->live().channel(channel);
+  const u64 n = ch.size();
+  const u64 want = 400;
+  if (n <= want + 50) return -1;
+  std::vector<f32> v(static_cast<std::size_t>(want));
+  ch.raw(n - want - 50, n - 50, v.data());
+  long breaks = 0;
+  for (std::size_t i = 1; i < v.size(); ++i) {
+    const long d = std::lround(double(v[i])) - std::lround(double(v[i - 1]));
+    if (((d % 100) + 100) % 100 != step) ++breaks;
+  }
+  return breaks;
+}
+
 }  // namespace
 
 /// Phase 2, 1 s after the controls were driven: the stream, the calibration, the capture button,
@@ -2247,14 +2281,112 @@ gboolean e1608_probe_reconnect(gpointer user) {
   return G_SOURCE_REMOVE;
 }
 
-/// The flat UDP source's probe. For now only the pure codec check; it needs no flatudp source.
+/// After the mock's gap: the stream's frame count moved on and the source is connected again.
+gboolean flatudp_probe_resumed(gpointer user) {
+  App* a = static_cast<App*>(user);
+  if (a->window == nullptr) return G_SOURCE_REMOVE;
+  flatudp::FlatUdpSource* src = flatudp_source(a);
+  const std::uint64_t after = a->model->stats().frames.load(std::memory_order_relaxed);
+  const bool up = src != nullptr && src->connected() &&
+                  a->model->stats().connected.load(std::memory_order_relaxed);
+  std::fprintf(stderr, "[flatudp] resumed: connected=%d frames_before=%" G_GUINT64_FORMAT
+                       " frames_after=%" G_GUINT64_FORMAT " ok=%d\n",
+               up ? 1 : 0, g_flatudp_frames_before, after,
+               up && after > g_flatudp_frames_before ? 1 : 0);
+  std::fflush(stderr);
+  return G_SOURCE_REMOVE;
+}
+
+/// Inside the mock's gap: 2 s of silence must have raised `Disconnected`.
+gboolean flatudp_probe_gap(gpointer user) {
+  App* a = static_cast<App*>(user);
+  if (a->window == nullptr) return G_SOURCE_REMOVE;
+  flatudp::FlatUdpSource* src = flatudp_source(a);
+  if (src == nullptr) return G_SOURCE_REMOVE;
+  const bool src_up = src->connected();
+  const bool model_up = a->model->stats().connected.load(std::memory_order_relaxed);
+  g_flatudp_frames_before = a->model->stats().frames.load(std::memory_order_relaxed);
+  std::fprintf(stderr, "[flatudp] silence: connected=%d model_connected=%d ok=%d\n", src_up ? 1 : 0,
+               model_up ? 1 : 0, !src_up && !model_up ? 1 : 0);
+  std::fflush(stderr);
+  g_timeout_add(4000, flatudp_probe_resumed, a);
+  return G_SOURCE_REMOVE;
+}
+
+/// Phase 2, 1 s after the signals were enabled: the stream, its channels, the value continuity, the
+/// wrong-size accounting and the capture button.
+gboolean flatudp_probe_report(gpointer user) {
+  App* a = static_cast<App*>(user);
+  if (a->window == nullptr) return G_SOURCE_REMOVE;
+  flatudp::FlatUdpSource* src = flatudp_source(a);
+  if (src == nullptr) return G_SOURCE_REMOVE;
+  const flatudp::FlatUdpSettings& s = src->settings();
+  const bool up = a->model->stats().connected.load(std::memory_order_relaxed);
+
+  const std::vector<SignalInfo>& sigs = a->model->signals();
+  std::string names, expect_names;
+  std::size_t expect_channels = 0;  // one channel per signal: a vector's elements are consecutive samples
+  for (const SignalInfo& si : sigs) names += (names.empty() ? "" : ",") + si.name;
+  for (const flatudp::Row& r : s.rows) {
+    expect_names += (expect_names.empty() ? "" : ",") + r.name;
+    ++expect_channels;
+  }
+  const double rate = sigs.empty() ? 0.0 : sigs.front().sampling_rate;
+  const std::uint64_t frames = a->model->stats().frames.load(std::memory_order_relaxed);
+  const bool stream_ok = up && names == expect_names && std::fabs(rate - s.rate) < 1e-6 && frames > 0;
+  std::fprintf(stderr,
+               "[flatudp] stream: connected=%d signals=%zu names=%s expect=%s frames=%" G_GUINT64_FORMAT
+               " rate=%.3f ok=%d\n",
+               up ? 1 : 0, sigs.size(), names.c_str(), expect_names.c_str(), frames, rate,
+               stream_ok ? 1 : 0);
+
+  const std::size_t channels = a->model->channels().size();
+  std::fprintf(stderr, "[flatudp] channels: count=%zu expect=%zu ok=%d\n", channels, expect_channels,
+               channels == expect_channels ? 1 : 0);
+
+  // Only the first two signals are checked, and only in an all-scalar list: a vector row makes the
+  // model span each frame by its elements, and the scalar channels are then padded with zeros.
+  if (up && s.rows.size() >= 2 && flatudp::all_scalar(s.rows)) {
+    const long b0 = flatudp_breaks(a, 0, 1);
+    const long b1 = flatudp_breaks(a, 1, 2);
+    std::fprintf(stderr, "[flatudp] continuity: ch0_breaks=%ld ch1_breaks=%ld ok=%d\n", b0, b1,
+                 b0 == 0 && b1 == 0 ? 1 : 0);
+  } else {
+    std::fprintf(stderr, "[flatudp] continuity: skipped (needs a connected all-scalar list of two or more)\n");
+  }
+
+  const std::uint64_t wrong = src->wrong_size_datagrams();
+  const std::string msg = src->last_message();
+  std::fprintf(stderr, "[flatudp] wrong size: count=%" G_GUINT64_FORMAT " message='%s' ok=%d\n", wrong,
+               msg.c_str(), wrong >= 1 && msg.find("expected") != std::string::npos ? 1 : 0);
+
+  std::fprintf(stderr, "[flatudp] capture button: present=%d sensitive=%d\n",
+               a->capture_button != nullptr ? 1 : 0,
+               a->capture_button != nullptr && gtk_widget_get_sensitive(a->capture_button) ? 1 : 0);
+  std::fprintf(stderr, "[flatudp] state: valid=%" G_GUINT64_FORMAT " wrong=%" G_GUINT64_FORMAT "\n",
+               src->valid_datagrams(), wrong);
+  std::fflush(stderr);
+  return G_SOURCE_REMOVE;
+}
+
+/// Phase 1: the codec check (it needs no flatudp source), then every signal enabled in one structural
+/// change, and the report a second later.
 gboolean flatudp_probe(gpointer user) {
   App* a = static_cast<App*>(user);
   if (a->window == nullptr) return G_SOURCE_REMOVE;
   std::fprintf(stderr, "[flatudp] codec: ok=%d\n", flatudp_codec_check() ? 1 : 0);
+  if (flatudp_source(a) == nullptr) {
+    std::fprintf(stderr, "[flatudp] no flatudp source (run with --source flatudp)\n");
     std::fflush(stderr);
     return G_SOURCE_REMOVE;
   }
+  std::vector<std::size_t> all(a->model->signals().size());
+  for (std::size_t i = 0; i < all.size(); ++i) all[i] = i;
+  if (!all.empty()) a->model->set_signals_enabled(all, true);
+  std::fflush(stderr);
+  g_timeout_add(1000, flatudp_probe_report, a);
+  return G_SOURCE_REMOVE;
+}
 
 gboolean pan_probe(gpointer user) {
   App* a = static_cast<App*>(user);

tools/mock_flatudp.py

@@ -0,0 +1,129 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2026 Martino Ferrari
+"""A stand-in for the MARTe2 UDPSender datasource, for the flatudp source's self-check.
+
+One datagram is one cycle: the signals concatenated in declaration order, no padding, in the
+chosen byte order. Packet i carries, for the signal of index k (from 0) and its element e,
+the value (i * (k + 1) + e) % 100, so consecutive datagrams are predictable.
+
+  --signals     name:type[count],...   default Trigger:uint8,Ia:float32,Cnt:uint64,Wave:int16[4]
+  --rate        cycles per second
+  --big-endian  pack big endian (default little)
+  --gap S,D     send nothing from S to S+D seconds after the start (a sender that stops)
+  --bad-every S one datagram one byte too long every S seconds (the wrong-size path)
+  --multicast G --mcast-if A   send to group G out of interface A instead of --host
+  --log FILE    one line per event
+"""
+import argparse
+import socket
+import struct
+import sys
+import time
+
+CODES = {"uint8": "B", "int8": "b", "uint16": "H", "int16": "h", "uint32": "I", "int32": "i",
+         "uint64": "Q", "int64": "q", "float32": "f", "float64": "d"}
+
+
+def parse_signals(text):
+    out = []
+    for tok in text.split(","):
+        name, _, ty = tok.strip().partition(":")
+        count = 1
+        if "[" in ty:
+            ty, _, n = ty.partition("[")
+            count = int(n.rstrip("]"))
+        out.append((name.strip(), CODES[ty.strip()], count))
+    return out
+
+
+def make_packer(sigs, big):
+    st = struct.Struct((">" if big else "<") + "".join(code * count for _, code, count in sigs))
+
+    def pack(i):
+        vals = []
+        for k, (_, code, count) in enumerate(sigs):
+            for e in range(count):
+                v = (i * (k + 1) + e) % 100
+                vals.append(float(v) if code in "fd" else v)
+        return st.pack(*vals)
+
+    return pack, st.size
+
+
+def main():
+    ap = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawTextHelpFormatter)
+    ap.add_argument("--host", default="127.0.0.1")
+    ap.add_argument("--port", type=int, default=44488)
+    ap.add_argument("--rate", type=float, default=1000.0)
+    ap.add_argument("--signals", default="Trigger:uint8,Ia:float32,Cnt:uint64,Wave:int16[4]")
+    ap.add_argument("--big-endian", action="store_true")
+    ap.add_argument("--gap", default="")
+    ap.add_argument("--bad-every", type=float, default=0.0)
+    ap.add_argument("--multicast", default="")
+    ap.add_argument("--mcast-if", default="127.0.0.1")
+    ap.add_argument("--log", default="")
+    args = ap.parse_args()
+
+    logf = open(args.log, "w") if args.log else None
+
+    def log(msg):
+        if logf:
+            logf.write("%.3f %s\n" % (time.monotonic() - t0, msg))
+            logf.flush()
+
+    sigs = parse_signals(args.signals)
+    pack, size = make_packer(sigs, args.big_endian)
+    gap = tuple(float(x) for x in args.gap.split(",")) if args.gap else None
+
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    dest = (args.host, args.port)
+    if args.multicast:
+        dest = (args.multicast, args.port)
+        sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 1)
+        sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_LOOP, 1)
+        sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_IF, socket.inet_aton(args.mcast_if))
+
+    period = 1.0 / args.rate
+    t0 = time.monotonic()
+    log("start %s rate=%g size=%d %s" % (args.signals, args.rate, size, "big" if args.big_endian else "little"))
+    next_t = t0
+    next_bad = t0 + args.bad_every
+    in_gap = False
+    i = 0
+    try:
+        while True:
+            now = time.monotonic()
+            if now < next_t:
+                time.sleep(min(next_t - now, 0.001))
+                continue
+            elapsed = now - t0
+            if gap and gap[0] <= elapsed < gap[0] + gap[1]:
+                if not in_gap:
+                    in_gap = True
+                    log("gap begins")
+                next_t = now + period
+                time.sleep(0.01)
+                continue
+            if in_gap:
+                in_gap = False
+                log("gap ends")
+            if args.bad_every > 0 and now >= next_bad:
+                next_bad = now + args.bad_every
+                sock.sendto(pack(i) + b"\x00", dest)
+                log("bad datagram of %d bytes" % (size + 1))
+            burst = 0
+            while next_t <= now and burst < 64:  # catch up a late timer, but bounded
+                sock.sendto(pack(i), dest)
+                i += 1
+                next_t += period
+                burst += 1
+            if next_t < now - 0.5:
+                next_t = now
+    except KeyboardInterrupt:
+        pass
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())