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PCL should assert its GTK version rather than rely on an accident

Filed 2026-08-29 out of [[feature-b-migrate-pcl-off-the-curated-gtk3-header]], at frank-coordinator's request, because the caveat is the kind that reads like a guarantee if nobody writes it down.

The situation

lib/pcl/gtk3_c.h is #include <gtk/gtk.h> now. On this box gtk-2.0 is a default system include root and gtk-3.0 is not, and both answer to that spelling — so the binding gets GTK3 only because GTK3_INC (Makefile, tools/gui_suite.sh) puts the gtk-3.0 root first.

Forget the flag and the build fails loudly:

error: undefined variable (gdk_event_get_button)
  in: lib/pcl/gtk3widgets.pas

which looks like a guard. It is not one. It is loud only because PCL happens to call gdk_event_get_button, which is GTK3-only. That is a property of today's PCL surface, not a designed check. A consumer whose own surface is entirely GTK2-compatible — and most of it is; gtk_main, gtk_main_quit, gtk_window_new, gtk_container_add and the rest exist in both — would build silently against the wrong library and only find out at runtime, or not at all.

A guard that works by accident reads exactly like one that works by design, which is the whole reason this is a ticket and not a comment.

Shape of a fix

A compile-time assertion inside the binding, so it fires at the include and not at some consumer's call site. Something on the shape of

#include <gtk/gtk.h>
#if !defined(GTK_MAJOR_VERSION) || GTK_MAJOR_VERSION < 3
#error "lib/pcl needs the GTK3 headers -- pass $(GTK3_INC) / $GTK3_INC"
#endif

Check first that pxx's C preprocessor honours #error and evaluates GTK_MAJOR_VERSION in an #if; if it does not, that is the real prerequisite and should be filed against the preprocessor rather than worked around here. A fallback that needs no #error is a declaration that only parses under GTK3 (the moral equivalent of the accident, but deliberate and commented as the assertion it is).

The runtime half already exists and should stay: gtk_version_check in tools/gui_suite.sh asserts libgtk-3.so.0 is and libgtk-x11-2.0.so.0 is not in DT_NEEDED. That covers the repo's own builds. This ticket is about the consumer who is not the repo.

[[decide-which-gtk-a-bare-gtk-gtk-h-means]] — if gtk-3.0 becomes a default include root, GTK3_INC disappears and this ticket gets simpler, not moot: the assertion is still what turns a silent wrong-version build into a diagnostic.

Resolution (frank-b, 2026-08-29)

The prerequisite holds — no Track C ticket needed

Measured against the pinned compiler (v393, 1d69760deabe) before writing any of the fix, because the ticket is right that it decides the shape:

probe result
bare #error fires, rc=1
#error inside #if 0 correctly skipped, rc=0
#if GTK_MAJOR_VERSION == 2 after #include <gtk/gtk.h> fires without GTK3_INC, silent with
#if GTK_MAJOR_VERSION == 3 fires with GTK3_INC, silent without
#elif (both arms) correct in both directions

The == 2 / == 3 pair is the point of that table. #if !defined(X) \|\| X < 3 alone could not tell "the macro evaluated to 2" from "the header was never found", and those want different error messages; the 2×2 shows the macro is genuinely being read. So the obvious fix was available, and the fallback the ticket describes (a declaration that only parses under GTK3) is not needed.

The failure is worse than filed, and in an instructive direction

The ticket says a GTK2-compatible consumer "would build silently against the wrong library". The library is never wrong. CHeaderStem maps the gtk3_c unit name to the gtk-3 stem, so libgtk-3.so.0 lands in DT_NEEDED whatever -I is in effect — only the headers follow -I. Measured with a consumer whose whole surface is gtk_main_quit (present in both versions): builds clean with no flag, links libgtk-3.so.0.

So the silent outcome is GTK2 headers against the GTK3 library — an ABI mismatch, not a version mixup. With gcc as the oracle:

sizeof(GtkWidget)   96 (gtk2 headers)   32 (gtk3)
sizeof(GtkWindow)  240                  56

That is worth flagging to whoever owns the prio: this is the silent-wrong- behaviour class, and it was filed at p30 on the milder reading.

Both existing "version" guards were decorative — this is the real finding

The ticket says the runtime half "already exists and should stay: gtk_version_check ... asserts libgtk-3.so.0 is and libgtk-x11-2.0.so.0 is not in DT_NEEDED. That covers the repo's own builds." It does not, and could not. Both are DT_NEEDED checks, and DT_NEEDED does not follow -I.

Measured, by rebuilding with the flag removed and a pre-assertion header:

Both were commented as asserting the version. That comment is the load-bearing part of the defect — a check that cannot fail is merely useless, but a comment saying it covers the version stops the next reader from looking, which is how this survived to be filed as "PCL should assert" rather than "nothing asserts". Same family as the accident the ticket is about: a guard that reads like design.

What landed

Verified

test_gtk_ffi, test_pcl_click, examples/life, the five examples/tk/*.npy and test_c_gtk3_stock all build with GTK3_INC; test_c_gtk3_stock now fails without it (it silently succeeded before). test_c_gtk_window still links libgtk-x11-2.0.so.0 — GTK2 stays authoritative, checked rather than assumed. Self-host fixedpoint converged in 2 rounds.

Nothing here touches what a bare #include <gtk/gtk.h> resolves to, so [[decide-which-gtk-a-bare-gtk-gtk-h-means]] is untouched and this does not depend on it: the assertion reports whichever answer that fork gets.

Log