flatudp: the signal-row editor form with a per-row Vector as drop-down, its probe line, README and architecture notes

Multicast was verified on a loopback group (239.255.42.99 on 127.0.0.1); a real network was not exercised.

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

README.md

@@ -145,11 +145,41 @@ Errors (no device, a rejected connection code, a timeout, the scan socket closin
 `[e1608] ...` once per distinct message (an overrun is logged each time it happens); the window shows only that
 the source is disconnected (the status popover reads `waiting`, and the panels go insensitive), not why.
 
+The third built-in kind is **Flat UDP**, a receiver for the MARTe2 `UDPSender` datasource: one datagram per
+cycle, the signals concatenated in declaration order with no header, no padding and no timestamp, in the
+sender's byte order. The datagram describes nothing, so **the signal list in the connection form is the
+whole contract and is your responsibility**: one row per signal (a name, a type from `uint8` to `float64`,
+and an element count), in the sender's order, including the `Trigger` byte an IndependentThread `UDPSender`
+puts first. A row with a count above 1 is a vector, and a "Vector as" drop-down on that row says how to
+show it: as **Channels** (the default), where `Wave:float32[16]` becomes 16 scalar channels `Wave[0]` ..
+`Wave[15]` of one sample per cycle each, or as a **Waveform**, one channel that receives a block of 16
+consecutive samples per cycle (its declared rate is the cycle rate times the count). The waveform is written
+`Wave:float32[16]w` in the `signals` text, and the same text is what a session file and `--source-opt
+signals=` carry. A list with a waveform row may hold **only waveform rows, all of the same count** (a
+scalar or a channels row beside one, or two lengths, would need zero padding or held values, so the form
+refuses it and says so), and a list may make **at most 256 channels** (a scalar is one, a channels vector is
+`count`, a waveform is one). The form's subtitle shows the channel total and the size the list implies
+("6 channels, 13 bytes per datagram"), or the first rule the list breaks; a datagram of any other size is
+dropped, counted as stale, and reported once per distinct size on stderr as `[flatudp] ...`. The form also
+takes the port (44488, the sender's own default), an optional bind address, an optional multicast group with
+the interface to join it on, the byte order, and the sender's **cycle rate in Hz, which is required**: it
+sizes the ring and the ingest decimation and spaces the samples within a frame. Time is stamped by the
+existing pipeline from arrival and that declared rate (and `--relative-time` gives a counted clock); there is
+no timestamp in the stream to prefer. **Loss is never reported** -- with no counter in the datagram, a missed
+cycle is indistinguishable from a slow one -- and neither is a changed sender: a list that does not match
+reads as "everything dropped". Source-address filtering, detecting the list from the traffic and importing a
+MARTe2 `.cfg` are not provided. A list with no waveform row is delivered in frames of about 10 ms
+(`round(rate / 100)` cycles, at most 1024; a partial frame is flushed after 100 ms); a list of waveforms
+delivers one frame per datagram. The source is connected on the first valid datagram and disconnected after
+2 s without one. It is verified against `tools/mock_flatudp.py`, not against a MARTe2 application. Multicast
+was verified on a loopback group (`239.255.42.99` joined on `127.0.0.1`, although this machine's `lo` does not
+carry the `MULTICAST` flag); a real network, its switches and IGMP were not exercised.
+
 A session file written before kinds existed has no `source_kind`, and is read as UDPS; a UDPS file is
 written exactly as before. A kind's capability panels (for instance a source's own controls) appear as
 pages of the side stack, and are insensitive rather than hidden while no source is connected.
 
-Planned, each as its own sub-project: a flat UDP kind and ABScope. ABScope has more than one
+Planned, as its own sub-project: ABScope. It has more than one
 timebase, so it will change the frame and the data path together rather than fit the present single-timebase
 frame.
 

docs/claude/architecture.md

@@ -61,6 +61,33 @@ What feeds `Acquisition` is a `DataSource`, one of the registered *kinds*.
   only through `post()`. `Acquisition::on_event` ignores the detail string and does nothing for `Error`,
   so the source also writes each new error to stderr as `[e1608] ...` (once per distinct message, not per
   retry); that line is where a user sees why a connection failed.
+- **The flat UDP kind is three files and no thread.** `flatudp_signals` is the `signals` string codec,
+  the validity rules and the decoders, with no GTK and no sockets (the probe round-trips it);
+  `flatudp_source` is `FlatUdpSettings` plus the `DataSource`; `flatudp_kind` is the form, a signal-row
+  editor (a name, a type, a count and, while the count is above 1, a "Vector as" drop-down per row). The
+  datagram has no description, so a list that is wrong is invisible except as dropped datagrams: the size
+  check is the only defence, and it is why a wrong-size datagram is counted (`stale`) and reported once per
+  size.
+- A vector row is shown one of two ways, chosen per row (`Row::wave`, the `w` suffix in the text). As
+  **channels** it is `count` scalar channels of one sample per cycle, so the list batches like an
+  all-scalar one. As a **waveform** it is one channel with `rows = count` and
+  `sampling_rate = rate * count`, one datagram per `Frame` (`num_samples = 1`). The two cannot share a
+  list because the model spans a frame by its largest element block and `Frame::value` is 0 past a scalar's
+  one value: a scalar beside a vector block would be zero-padded, so `problem()` demands that a list with a
+  waveform row be waveform rows of one count, and the form shows that message in the subtitle. The channel
+  total is capped at 256.
+- Its frames are **fixed-size blocks** like the E-1608's for the same reason (`build_stamps` warns when the
+  sample count varies): `round(rate / 100)` datagrams for a list with no waveform row, a partial block
+  flushed after 100 ms. `rate` is required because `sampling_rate = 0` makes the model size the ring and
+  the ingest decimation for 1 kHz; there is no time-base switch, since `acquisition.cpp` has no stamping
+  path that ignores the declared cadence.
+- `Disconnected` after 2 s without a *valid* datagram (wrong-size ones do not refresh the clock); it resets
+  `Acquisition`'s per-session state, so the next datagram re-anchors rather than reading as a gap, and the
+  history survives (`CONFIG` is sent once, from the first `poll()`).
+- The form's row editor removes a row from inside its own button's `clicked` emission (GTK keeps the emitter
+  alive to the end of the emission, and the handler copies the form and the row out of its context first);
+  every handler tests `Form::alive`, which the groups' `destroy` clears, because the chooser may have
+  destroyed the sibling widgets the handlers reach by raw pointer.
 
 ### Data path
 

docs/claude/probe-readings.md

@@ -1912,6 +1912,7 @@ the one `[e1608] handshake with ...: Connection refused` line, `stream: ... sign
 
 ```
 [flatudp] codec: ok=1
+[flatudp] form: initial_valid=0 good_valid=1 size_ok=1 dup_valid=0 dup_msg=1 oversize_valid=0 add=1 remove=1 wave_valid=1 wave_sub=1 wave_roundtrip=1 mixed_valid=0 mixed_msg=1 length_valid=0 length_msg=1 cap_valid=0 cap_msg=1 cap_wave_valid=1 vec_shown=1 vec_drive=1 vec_reset=1 ok=1
 [flatudp] datagram of 22 bytes, expected 21, check the signal list
 [flatudp] stream: connected=1 signals=7 names=Trigger,Ia,Cnt,Wave[0],Wave[1],Wave[2],Wave[3] expect=Trigger,Ia,Cnt,Wave[0],Wave[1],Wave[2],Wave[3] frames=368 rate=1000.000 ok=1
 [flatudp] channels: count=7 expect=7 ok=1
@@ -1945,6 +1946,22 @@ same reason.
   rules refuse what they should (a waveform of count 1, a waveform mixed with another row or of another length,
   257 channels). 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.
+- `form: ... ok=1`: driven through the widgets, not the settings. `initial_valid=0` -- an empty list is not
+  connectable (`1` is a form that lets a source with no signals through). `good_valid=1 size_ok=1` -- a
+  13-byte list is valid and its subtitle says `13 bytes` and `6 channels` (`W:int16[4]` as channels).
+  `dup_valid=0 dup_msg=1` -- a duplicate name is refused *and the subtitle says why* ("used twice").
+  `oversize_valid=0` -- two `float64[4096]` rows (65536 bytes) exceed a datagram. `add=1` -- two clicks on
+  "Add signal" append `S1` and `S2`; `remove=1` -- the first row's "Remove" button takes that row out. A `0`
+  on `add`/`remove` with a `Gtk-CRITICAL` is a row handled after it was removed. The vector fields:
+  `wave_valid=1 wave_sub=1 wave_roundtrip=1` -- two waveform rows of one length are valid, the subtitle says
+  `2 channels`, and `signals` reads back with its `w`; `mixed_valid=0 mixed_msg=1` -- a waveform beside a
+  scalar is refused and the subtitle names the waveform rule; `length_valid=0 length_msg=1` -- waveforms of
+  two lengths are refused ("same length"); `cap_valid=0 cap_msg=1 cap_wave_valid=1` -- 300 channels are over
+  the 256 cap and the subtitle says `256`, while two `[200]w` waveforms (2 channels) are fine; `vec_shown=1`
+  -- the "Vector as" drop-down (`flatudp-vector`) is shown on a count above 1 and hidden on a scalar row;
+  `vec_drive=1` -- setting it to Waveform writes `Row::wave` (one waveform beside a channels row, then beside
+  a scalar, are both still invalid, and the text gains its `w`); `vec_reset=1` -- a row whose count returns
+  to 1 hides its drop-down and returns to Channels. Like `codec`, it needs no flatudp source.
 - `stream: ... ok=1`: the model's signals are the list's channels in order, each at its declared rate: `rate`
   for a scalar or a `channels` element (named `Name[k]`), `rate * count` for a waveform. `names` out of order is a
   decode that mixed rows; a rate that is not the expected one is a `sampling_rate` that did not reach the model

docs/claude/verification.md

@@ -442,8 +442,8 @@ precedent, not an exception.
 - `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; it covers the `w` suffix, the waveform and 256-channel rules, the channel decode and the
+  `tools/mock_flatudp.py` -- the flat UDP source end to end: a hand-computed codec and decode check and the connect form's validity rules and signal-row editor (both
+  need no source; it covers the `w` suffix, the waveform and 256-channel rules, the channel decode and the
   float64/uint16/int32/uint32/int64 types in both byte orders), the stream's channel names (`Wave[0]`..
   for a `channels` vector) and declared rates (`rate * count` for a waveform), the channel count, value
   continuity per row (and the offset between a vector's elements, or a waveform's block phase), the

src/source/flatudp/flatudp_kind.cpp

@@ -6,10 +6,14 @@
 
 #include <adwaita.h>
 
+#include <algorithm>
+#include <cctype>
+#include <cstdint>
 #include <cstdio>
 #include <functional>
 #include <memory>
 #include <string>
+#include <vector>
 
 namespace udpscope {
 namespace {
@@ -23,6 +27,13 @@ std::string num(double v) {
   return b;
 }
 
+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);
+}
+
 GtkWidget* entry_row(const char* title) {
   GtkWidget* r = adw_entry_row_new();
   adw_preferences_row_set_title(ADW_PREFERENCES_ROW(r), title);
@@ -66,18 +77,70 @@ void mark(GtkWidget* w, bool bad) {
   }
 }
 
-/// The form's widgets, shared by every signal closure and every `ConfigForm` closure.
-struct Form {
-  GtkWidget *port, *bind, *multicast, *iface, *rate, *endian, *signals;
+/// One signal's editing widgets; `row` is the `AdwEntryRow` that holds the others. `vec` ("Vector as":
+/// Channels or Waveform) is shown only while `count` is above 1.
+struct SigRow {
+  GtkWidget *row, *type, *count, *vec;
+};
+
+/// The form's widgets, shared by every signal closure and every `ConfigForm` closure. The raw
+/// widget pointers are valid while the chooser keeps the rows.
+struct Form : std::enable_shared_from_this<Form> {
+  GtkWidget *port, *bind, *multicast, *iface, *rate, *endian, *group;
+  std::vector<SigRow> sigs;
   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.
+  std::vector<Row> rows_now() const {
+    std::vector<Row> rows;
+    for (const SigRow& s : sigs) {
+      Row r;
+      r.name = trim(text_of(s.row));
+      r.type = static_cast<Type>(std::min<guint>(gtk_drop_down_get_selected(GTK_DROP_DOWN(s.type)), 9));
+      r.count = static_cast<std::uint32_t>(gtk_spin_button_get_value_as_int(GTK_SPIN_BUTTON(s.count)));
+      r.wave = r.count > 1 && gtk_drop_down_get_selected(GTK_DROP_DOWN(s.vec)) == 1;
+      rows.push_back(r);
+    }
+    return rows;
+  }
+
+  /// The first `S<n>` that no row uses.
+  std::string next_name() const {
+    const std::vector<Row> rows = rows_now();
+    for (int n = 1;; ++n) {
+      const std::string name = "S" + std::to_string(n);
+      bool used = false;
+      for (const Row& r : rows) used = used || r.name == name;
+      if (!used) return name;
+    }
+  }
+
+  /// Bad addresses are marked with the `error` style; the group's subtitle carries the signal list's
+  /// own state, the first rule it breaks or the size it implies.
   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)));
+    // "Vector as" belongs to a vector row only; a row back at one element returns to Channels.
+    for (const SigRow& s : sigs) {
+      const bool vec = gtk_spin_button_get_value_as_int(GTK_SPIN_BUTTON(s.count)) > 1;
+      gtk_widget_set_visible(s.vec, vec);
+      if (!vec && gtk_drop_down_get_selected(GTK_DROP_DOWN(s.vec)) != 0)
+        gtk_drop_down_set_selected(GTK_DROP_DOWN(s.vec), 0);
+    }
+    const std::vector<Row> rows = rows_now();
+    const std::string p = flatudp::problem(rows);
+    std::string sub = p;
+    if (p.empty()) {
+      sub = std::to_string(channel_count(rows)) + (channel_count(rows) == 1 ? " channel, " : " channels, ") +
+            std::to_string(datagram_size(rows)) + " bytes per datagram";
+    }
+    adw_expander_row_set_subtitle(ADW_EXPANDER_ROW(group), sub.c_str());
+  }
+
+  void fire() {
+    if (!updating && changed && *changed) (*changed)();
   }
 
   SourceConfig read() const {
@@ -88,10 +151,13 @@ struct Form {
     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));
+    c.set("signals", format(rows_now()));
     return c;
   }
 
+  void add_row(const Row& r);
+  void remove_row(GtkWidget* row);
+
   void write(const SourceConfig& cfg) {
     const FlatUdpSettings s = FlatUdpSettings::from(cfg);
     updating = true;
@@ -101,7 +167,12 @@ struct Form {
     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());
+    while (!sigs.empty()) {
+      GtkWidget* row = sigs.back().row;
+      sigs.pop_back();
+      adw_expander_row_remove(ADW_EXPANDER_ROW(group), row);
+    }
+    for (const Row& r : s.rows) add_row(r);
     updating = false;
     refresh();
   }
@@ -111,21 +182,53 @@ struct Form {
 
 using FormPtr = std::shared_ptr<Form>;
 
+/// A remove button's context: which form, which row.
+struct RowCtx {
+  FormPtr form;
+  GtkWidget* row;
+};
+
 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)();
+  f.fire();
 }
 
 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)();
+  f.fire();
+}
+
+void on_spin(GtkSpinButton*, gpointer user) {
+  Form& f = **static_cast<FormPtr*>(user);
+  if (!f.alive) return;
+  f.refresh();
+  f.fire();
+}
+
+void on_add(GtkButton*, gpointer user) {
+  FormPtr f = *static_cast<FormPtr*>(user);
+  if (!f->alive) return;
+  f->add_row(Row{f->next_name(), Type::F32, 1});
+  adw_expander_row_set_expanded(ADW_EXPANDER_ROW(f->group), TRUE);
+  f->refresh();
+  f->fire();
+}
+
+void on_remove(GtkButton*, gpointer user) {
+  // Copy first: removing the row finalizes this button, and with it this context.
+  RowCtx* c = static_cast<RowCtx*>(user);
+  FormPtr f = c->form;
+  GtkWidget* row = c->row;
+  if (!f->alive) return;
+  f->remove_row(row);
 }
 
 void free_form(gpointer d, GClosure*) { delete static_cast<FormPtr*>(d); }
+void free_ctx(gpointer d, GClosure*) { delete static_cast<RowCtx*>(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; }
@@ -145,6 +248,70 @@ void hook_entry(GtkWidget* w, const FormPtr& f) {
                         static_cast<GConnectFlags>(0));
 }
 
+void Form::add_row(const Row& r) {
+  FormPtr self = shared_from_this();
+  GtkWidget* row = adw_entry_row_new();
+  adw_preferences_row_set_title(ADW_PREFERENCES_ROW(row), "Name");
+  gtk_editable_set_text(GTK_EDITABLE(row), r.name.c_str());
+
+  const char* names[11];
+  for (std::size_t i = 0; i < 10; ++i) names[i] = type_name(static_cast<Type>(i));
+  names[10] = nullptr;
+  GtkWidget* type = gtk_drop_down_new_from_strings(names);
+  gtk_drop_down_set_selected(GTK_DROP_DOWN(type), static_cast<guint>(r.type));
+  gtk_widget_set_valign(type, GTK_ALIGN_CENTER);
+  gtk_widget_set_name(type, "flatudp-type");
+
+  // The spin clamps to 1..4096, so a hand-edited session with a count outside it is repaired here.
+  GtkWidget* count = gtk_spin_button_new_with_range(1, kMaxCount, 1);
+  gtk_spin_button_set_value(GTK_SPIN_BUTTON(count), r.count);
+  gtk_widget_set_valign(count, GTK_ALIGN_CENTER);
+  gtk_widget_set_tooltip_text(count, "Elements: 1 is a scalar signal, more make a vector signal");
+  gtk_widget_set_name(count, "flatudp-count");
+
+  static const char* const kVec[] = {"Channels", "Waveform", nullptr};
+  GtkWidget* vec = gtk_drop_down_new_from_strings(kVec);
+  gtk_drop_down_set_selected(GTK_DROP_DOWN(vec), r.wave && r.count > 1 ? 1 : 0);
+  gtk_widget_set_valign(vec, GTK_ALIGN_CENTER);
+  gtk_widget_set_visible(vec, r.count > 1);
+  gtk_widget_set_tooltip_text(vec,
+                              "Vector as: Channels makes one channel per element, one sample per cycle; "
+                              "Waveform makes one channel of all the elements per cycle");
+  gtk_widget_set_name(vec, "flatudp-vector");
+
+  GtkWidget* rm = gtk_button_new_with_label("Remove");
+  gtk_widget_set_valign(rm, GTK_ALIGN_CENTER);
+  gtk_widget_set_name(rm, "flatudp-remove");
+
+  adw_entry_row_add_suffix(ADW_ENTRY_ROW(row), type);
+  adw_entry_row_add_suffix(ADW_ENTRY_ROW(row), count);
+  adw_entry_row_add_suffix(ADW_ENTRY_ROW(row), vec);
+  adw_entry_row_add_suffix(ADW_ENTRY_ROW(row), rm);
+
+  hook_entry(row, self);
+  hook_notify(type, "notify::selected", self);
+  hook_notify(vec, "notify::selected", self);
+  g_signal_connect_data(count, "value-changed", G_CALLBACK(on_spin), new FormPtr(self), free_form,
+                        static_cast<GConnectFlags>(0));
+  g_signal_connect_data(rm, "clicked", G_CALLBACK(on_remove), new RowCtx{self, row}, free_ctx,
+                        static_cast<GConnectFlags>(0));
+
+  adw_expander_row_add_row(ADW_EXPANDER_ROW(group), row);
+  sigs.push_back(SigRow{row, type, count, vec});
+}
+
+void Form::remove_row(GtkWidget* row) {
+  for (auto it = sigs.begin(); it != sigs.end(); ++it) {
+    if (it->row == row) {
+      sigs.erase(it);
+      adw_expander_row_remove(ADW_EXPANDER_ROW(group), row);
+      break;
+    }
+  }
+  refresh();
+  fire();
+}
+
 class FlatUdpKind final : public SourceKind {
  public:
   std::string id() const override { return "flatudp"; }
@@ -169,27 +336,34 @@ class FlatUdpKind final : public SourceKind {
     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});
+    f->group = adw_expander_row_new();
+    adw_preferences_row_set_title(ADW_PREFERENCES_ROW(f->group), "Signals");
     adw_expander_row_set_expanded(ADW_EXPANDER_ROW(net), TRUE);
-    adw_expander_row_set_expanded(ADW_EXPANDER_ROW(sig), TRUE);
+    adw_expander_row_set_expanded(ADW_EXPANDER_ROW(f->group), 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);
+    GtkWidget* add = gtk_button_new_with_label("Add signal");
+    gtk_widget_set_valign(add, GTK_ALIGN_CENTER);
+    gtk_widget_set_name(add, "flatudp-add");
+    adw_expander_row_add_suffix(ADW_EXPANDER_ROW(f->group), add);
+    g_signal_connect_data(add, "clicked", G_CALLBACK(on_add), new FormPtr(f), free_form,
+                          static_cast<GConnectFlags>(0));
+
+    for (GtkWidget* w : {net, timing, f->group}) hook_destroy(w, f);
+    for (GtkWidget* w : {f->bind, f->multicast, f->iface}) 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.rows = {net, timing, f->group};
     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;
+        "description, and a datagram of any other size is dropped. A vector signal is either "
+        "one channel per element or, as a waveform, one channel of all its elements";
+    out.focus = f->rate;
     out.write = [f](const SourceConfig& c) { f->write(c); };
     out.read = [f]() { return f->read(); };
     out.valid = [f]() { return f->valid(); };

src/ui_probe.cpp

@@ -2213,6 +2213,122 @@ long flatudp_breaks(App* a, std::size_t channel, long step) {
   return breaks;
 }
 
+/// Every widget named `name` at or under `root`, in tree order (a signal row's widgets repeat).
+void find_all_named(GtkWidget* root, const char* name, std::vector<GtkWidget*>& out) {
+  if (root == nullptr) return;
+  if (std::strcmp(gtk_widget_get_name(root), name) == 0) out.push_back(root);
+  for (GtkWidget* c = gtk_widget_get_first_child(root); c != nullptr;
+       c = gtk_widget_get_next_sibling(c)) {
+    find_all_named(c, name, out);
+  }
+}
+
+/// The connect form's validity rules and its signal-row editor, driven through the widgets: the
+/// empty form is not connectable; a good list is, and its subtitle names the channel total and the
+/// implied size; a duplicate name and an over-size list are not; "Add signal" appends `S1`, `S2`; a
+/// row's "Remove" button takes that row out. Then the vector rows: a waveform list is valid and
+/// round-trips with its `w`; a waveform beside a scalar (or of another length) is not, and the
+/// subtitle says so; the 256-channel cap counts a waveform as one; the "Vector as" drop-down is
+/// shown for a count above 1 only, writes `Row::wave`, and returns to Channels when the count does.
+void flatudp_form_probe() {
+  SourceKind* k = find_kind("flatudp");
+  if (k == nullptr) {
+    std::fprintf(stderr, "[flatudp] form: no flatudp kind\n");
+    return;
+  }
+  ConfigForm f = k->make_config_form();
+  const bool initial = f.valid();
+  const auto with = [&](const char* signals) {
+    SourceConfig c;
+    c.set("rate", "1000");
+    c.set("signals", signals);
+    f.write(c);
+    return f.valid();
+  };
+  const auto subtitle = [&]() {
+    const char* t = adw_expander_row_get_subtitle(ADW_EXPANDER_ROW(f.rows[2]));
+    return std::string(t != nullptr ? t : "");
+  };
+  const auto has = [](const std::string& text, const char* part) {
+    return text.find(part) != std::string::npos;
+  };
+  const bool good = with("A:uint8,B:float32,W:int16[4]");
+  const std::string good_sub = subtitle();
+  const bool size_ok = has(good_sub, "13 bytes") && has(good_sub, "6 channels");
+  const bool dup = with("A:uint8,A:float32");
+  const bool dup_ok = has(subtitle(), "twice");
+  const bool big = with("A:float64[4096],B:float64[4096]");
+  with("A:uint8,B:float32");
+
+  GtkWidget* add = find_named(f.rows[2], "flatudp-add");
+  if (add != nullptr) {
+    g_signal_emit_by_name(add, "clicked");
+    g_signal_emit_by_name(add, "clicked");
+  }
+  const bool added = f.read().get("signals") == "A:uint8,B:float32,S1:float32,S2:float32";
+  GtkWidget* rm = find_named(f.rows[2], "flatudp-remove");  // the first row's
+  if (rm != nullptr) g_signal_emit_by_name(rm, "clicked");
+  const bool removed = f.read().get("signals") == "B:float32,S1:float32,S2:float32";
+
+  // Waveform rows.
+  const bool wave_valid = with("W1:int16[4]w,W2:float32[4]w");
+  const bool wave_sub = has(subtitle(), "2 channels");
+  const bool wave_rt = f.read().get("signals") == "W1:int16[4]w,W2:float32[4]w";
+  const bool mixed = with("A:uint8,W:int16[4]w");
+  const bool mixed_msg = has(subtitle(), "waveform");
+  const bool length = with("W1:int16[4]w,W2:int16[8]w");
+  const bool length_msg = has(subtitle(), "same length");
+  const bool cap = with("A:float32[200],B:float32[100]");
+  const bool cap_msg = has(subtitle(), "256");
+  const bool cap_wave = with("A:float32[200]w,B:float32[200]w");
+
+  // The "Vector as" drop-down, driven.
+  with("A:uint8,V1:int16[4],V2:int16[4]");
+  std::vector<GtkWidget*> vec;
+  find_all_named(f.rows[2], "flatudp-vector", vec);
+  const bool vec_found = vec.size() == 3;
+  const bool vec_shown = vec_found && !gtk_widget_get_visible(vec[0]) &&
+                         gtk_widget_get_visible(vec[1]) && gtk_widget_get_visible(vec[2]);
+  bool vec_drive = false;
+  bool vec_reset = false;
+  if (vec_found) {
+    gtk_drop_down_set_selected(GTK_DROP_DOWN(vec[1]), 1);
+    const bool half_valid = f.valid();  // one waveform beside a scalar and a channels row
+    const std::string half = f.read().get("signals");
+    gtk_drop_down_set_selected(GTK_DROP_DOWN(vec[2]), 1);
+    const bool scalar_left = f.valid();  // the scalar A still breaks the all-waveform rule
+    vec_drive = !half_valid && half == "A:uint8,V1:int16[4]w,V2:int16[4]" && !scalar_left &&
+                f.read().get("signals") == "A:uint8,V1:int16[4]w,V2:int16[4]w";
+    // Bring V1 back to one element: its drop-down hides and returns to Channels.
+    std::vector<GtkWidget*> counts;
+    find_all_named(f.rows[2], "flatudp-count", counts);
+    if (counts.size() == 3) gtk_spin_button_set_value(GTK_SPIN_BUTTON(counts[1]), 1);
+    vec_reset = counts.size() == 3 && !gtk_widget_get_visible(vec[1]) &&
+                gtk_drop_down_get_selected(GTK_DROP_DOWN(vec[1])) == 0 &&
+                f.read().get("signals") == "A:uint8,V1:int16,V2:int16[4]w";
+  }
+
+  const bool ok = !initial && good && size_ok && !dup && dup_ok && !big && add != nullptr && added &&
+                  rm != nullptr && removed && wave_valid && wave_sub && wave_rt && !mixed &&
+                  mixed_msg && !length && length_msg && !cap && cap_msg && cap_wave && vec_shown &&
+                  vec_drive && vec_reset;
+  std::fprintf(stderr,
+               "[flatudp] form: initial_valid=%d good_valid=%d size_ok=%d dup_valid=%d dup_msg=%d "
+               "oversize_valid=%d add=%d remove=%d wave_valid=%d wave_sub=%d wave_roundtrip=%d "
+               "mixed_valid=%d mixed_msg=%d length_valid=%d length_msg=%d cap_valid=%d cap_msg=%d "
+               "cap_wave_valid=%d vec_shown=%d vec_drive=%d vec_reset=%d ok=%d\n",
+               initial ? 1 : 0, good ? 1 : 0, size_ok ? 1 : 0, dup ? 1 : 0, dup_ok ? 1 : 0,
+               big ? 1 : 0, added ? 1 : 0, removed ? 1 : 0, wave_valid ? 1 : 0, wave_sub ? 1 : 0,
+               wave_rt ? 1 : 0, mixed ? 1 : 0, mixed_msg ? 1 : 0, length ? 1 : 0, length_msg ? 1 : 0,
+               cap ? 1 : 0, cap_msg ? 1 : 0, cap_wave ? 1 : 0, vec_shown ? 1 : 0, vec_drive ? 1 : 0,
+               vec_reset ? 1 : 0, ok ? 1 : 0);
+  // The rows were never parented; sink and release them so the form leaves nothing behind.
+  for (GtkWidget* row : f.rows) {
+    g_object_ref_sink(row);
+    g_object_unref(row);
+  }
+}
+
 }  // namespace
 
 /// Phase 2, 1 s after the controls were driven: the stream, the calibration, the capture button,
@@ -2585,6 +2701,7 @@ 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);
+  flatudp_form_probe();
   if (flatudp_source(a) == nullptr) {
     std::fprintf(stderr, "[flatudp] no flatudp source (run with --source flatudp)\n");
     std::fflush(stderr);