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CPrepInclude0..15 should be an array

What exists

compiler/defs.inc:3616:

  CPrepInclude0 : AnsiString;
  CPrepInclude1 : AnsiString;
  ...
  CPrepInclude15: AnsiString;

and in compiler/cpreproc.inc, two ladders that exist only to index them:

procedure CPLoadInclude(depth: Integer);
begin
  case depth of
    0: LoadFile(CPrepPath, CPrepInclude0);
    ...
    15: LoadFile(CPrepPath, CPrepInclude15);
  end;
end;

function CPIncludeLength(depth: Integer): Integer;   { the same sixteen, again }

Plus a third case in CPProcessInclude. Three ladders, sixteen arms each, all saying "the buffer at depth".

What has already been fixed, so this ticket is not urgent

bug-c-an-include-nested-deeper-than-16-is-silently-dropped (frankC, 2026-08-30) closed the dangerous half within Track C's files:

So the silent wrong value is gone: 18-deep nesting used to print 0 where gcc prints 17, and now reports "C include nesting too deep (the preprocessor has 16 include buffers; this include is at level 17)".

What is left, and why it is worth doing anyway

The limit is still 16, and it is close. A model of the recursion over this box's real header sets — deliberately over-estimating, since it counts #includes inside conditional blocks the preprocessor skips — puts gtk/gtk.h, the deepest set this repo compiles against, at 15. One level. Nothing hits the ceiling today and the failure is now loud rather than silent, but the margin is a single header.

And this is a textbook normalise-dont-special-case case. Sixteen globals and three parallel ladders are one datum wearing sixteen names; the missing else was exactly the kind of defect a second (third) copy of a dispatch harbours, and it went unnoticed for as long as the arms happened to agree. An array deletes all three ladders and makes the limit one constant.

Shape

CPrepInclude: array[0..MAX_CPREP_INCLUDES-1] of AnsiString in defs.inc, with the three case ladders in cpreproc.inc collapsing to CPrepInclude[depth]. Then MAX_CPREP_INCLUDES becomes the true limit and the extra guard added by the ticket above can be deleted — it is deliberately the only line that needs removing.

Watch the memory: sixteen AnsiStrings is nothing, 128 is still nothing, but these hold whole preprocessed files and the array is global, so they are never freed. If that matters, size the array to a real limit rather than to 128.

What a fix must assert

Log

THIS NOW BLOCKS REAL-WORLD C — measured, frankC, 2026-08-30

The ticket reads as a tidiness refactor ("one datum wearing sixteen names"). It is not. The 16-deep cap is a hard wall against real C code, and it is the FIRST thing hit — before a single line of a program is parsed.

Minimal repro, four lines, against upstream busybox 1.36.1 (1a64f6a20):

#define _GNU_SOURCE 1
#include "autoconf.h"
#include "libbb.h"
int main(void){ return 0; }



pascal26:1: error: C include nesting too deep
              (the preprocessor has 16 include buffers; this include is at level 17)

libbb.h is busybox's aggregation header — every applet includes it, so this blocks all 145 translation units of even a cat-only busybox build, not one file. gcc compiles the same tree clean.

It is cumulative depth, not one pathological header. I checked the obvious suspects individually and none of them reaches the cap on its own:

stdio.h  dirent.h  endian.h  byteswap.h  paths.h  libgen.h  sys/stat.h
   -- all compile fine, depth-limit-hit=0 for each

So there is no header to blame and no include to restructure. Real aggregation headers simply nest deeper than 16 once glibc's own chains stack under them, and busybox is not unusual in this.

Why the obvious workaround is the wrong move

The depth could be shaved by giving lib/crtl its own dirent.h, paths.h, libgen.h, byteswap.h etc. so the host glibc chain is never entered (the compile emits a host-fallback warning for exactly those). That is the microfix: it buys one corpus target, leaves the cap in place, and the next real program hits it again a little deeper in. The cap is the root cause and the array is the fix — as this ticket already says, and as cpreproc.inc's own comment predicted ("if the buffers ever become an array this line is the only one to delete").

Track C is standing on this and cannot fix it

CPrepInclude0..15 are declared in compiler/defs.inc:3765 — Track A's shared file. Under Track C's rules ("anything in lexer.inc, ir*.inc, symtab.inc, defs.inc, the backends → file a Track A ticket, do not edit it under Track C") this is A's to make, so I am reporting rather than taking it. The two case ladders in cpreproc.inc are mine and I will convert them the moment the storage is an array; that half is mechanical.

Priority is not hand-edited here — [[feature-c-corpus-busybox-applet]] (p60) is now marked blocked-by this ticket, so the ranker propagates 60 down the edge on its own, which is the mechanism CLAUDE.md describes for exactly this.

A THIRD defect under this ticket, and raising the cap was the TRIGGER, not the bug

Found by frankS while implementing, 2026-08-30. The ticket has now been re-scoped twice — filed as tidiness, corrected by frankC to "blocks all 145 busybox translation units" — and this is the third layer.

cpreproc.inc carried a second depth-bounded table:

CPPathAtDepth: array[0..17] of AnsiString;          { declaration }
if depth + 1 <= 17 then CPPathAtDepth[depth + 1] := CPrepPath;   { write site }
CPCurPath := CPPathAtDepth[depth];                  { read site — UNGUARDED }

The <= 17 clamp on the write is dead code: nothing could reach depth 17 while the include buffers capped nesting at 16. The read at CPProcessText has no bound test at all, and did not need one for the same reason.

So the array's bound was correct only by the accident of a smaller bound somewhere else. Raise MAX_CPREP_INCLUDES to the real limit — which is the entire point of this ticket — and CPPathAtDepth[depth] becomes an out-of-bounds read at depth 18, on the path that runs for every single #include. The dead clamp above it would silently stop recording paths from 18 on, so -g line markers would name the wrong file before the read went out of bounds. Neither has a diagnostic.

Raising the cap was the trigger, not the bug. The bug was already there, in the shape of a bound held up by an unrelated constant. This is why the fix sizes CPPathAtDepth to MAX_CPREP_INCLUDES and deletes the clamp rather than raising 17 to some new number: one limit, spelled once, so the class cannot recur here rather than this instance of it being repaired.

frankA's phrasing, which is better than mine: correct by convention, not by construction — and it fails silently at the exact moment someone does the obvious right thing.

Two stages, because the compiler cannot compile its own new source

LoadFile(CPrepPath, CPrepInclude[depth]) did not compile: LoadFile expects string variables in IR codegen. Not a language limit — an ordinary procedure F(var s: AnsiString) accepts arr[1], runs correctly and matches FPC. The restriction was in the intrinsic (specialId = 100), which matched its destination as a SYMBOL only.

CORRECTION to this ticket's justification: it does NOT unblock busybox

Verified with a stage-2 build (fixedpoint f812166486cd), 2026-08-30, frankS.

Every assertion this ticket asks for holds:

assertion result
16-deep still compiles and gives gcc's values 0 15, matches gcc
a depth between 17 and the new limit now WORKS 18, 40, 128 all compile and match gcc
one past the new limit is the NESTING diagnostic level 129 → "the preprocessor has 128 include buffers"
self-host fixedpoint converged, 1 round

But the busybox repro still fails — now at level 129 instead of 17. The failure depth tracks the cap, which is runaway recursion, not depth. libbb.h:55 reaches <sys/param.h>, and that header alone recurses to whatever the cap is; with -I/usr/include/x86_64-linux-gnu it compiles fine, as does every one of its own includes separately and all of them together. Filed as [[bug-c-the-preprocessor-runs-away-on-sys-param-h-resolved-from-the-host-fallback]] [C p60] with the five controls and a first move for whoever takes it.

The earlier "cumulative depth, not one pathological header" conclusion came from a sweep of seven headers that did not include sys/param.h. No criticism of the sweep — it is the right instinct and it eliminated the obvious suspects. It just did not contain the one that mattered, and "no single header reaches the cap" reads identically whether the set was exhaustive or not.

So this ticket reverts to what it was originally filed as: a real normalise-dont-special-case cleanup that removes three ladders, one datum wearing sixteen names, and a latent out-of-bounds read — worth landing on those merits, at that priority. It is not the busybox blocker. [[feature-c-corpus-busybox-applet]] should be blocked-by the new ticket instead.

Also needed in stage 2, found while verifying

Makefile ~6871 asserts that 18-deep nesting is refused:

elif grep -q 'C include nesting too deep' $(TESTTMP)/cnest18.err; then \
  echo "ok: 18-deep include nesting reports the limit"; \

With the array, 18-deep must compile — that is the point of the change — so this recipe goes red as written. It needs its depth moved past the new limit (129 generated headers), keeping both halves: one depth that must compile and match gcc, one that must produce the nesting diagnostic. The ticket's own "what a fix must assert" list already called for this ("already a test; update the depth"); recording the exact recipe so it is not missed under the pin.

STAGE 2 LANDED (frankS, 2026-08-30), on pin v398 992065f21f33

CPrepInclude: array[0..MAX_CPREP_INCLUDES-1], three case depth of 0..15 ladders collapsed to one expression each, CPPathAtDepth sized to the same constant with its dead <= 17 clamp deleted, and the sixteen-slot guard in CPInclude removed — the line its own comment said would be the only one to delete once this happened. Net −45 lines in compiler/**.

A near-miss worth recording, because it would have been silent

Stage 2 was held on disk for the duration of the pin as whole-file copies of defs.inc and cpreproc.inc. Restoring those copies over the post-pin tree would have reverted frankA's CUnitOfPascalProgram block in defs.inc and 124 changed lines in cpreproc.inc — other agents' work, deleted with no conflict and no diagnostic, because a file copy has no idea what else moved underneath it.

It surfaced only because the restored tree then failed to build with undefined variable (CUnitOfPascalProgram), which looked at first like a broken master. The check that separated the two was one command: the pinned compiler against a clean tree, which built fine. Hold a parked change as an EDIT you can re-apply, never as a copy of the file it lives in — the copy is a snapshot of a whole shared file, and every other lane's work is inside it. Re-applied as scripted edits against current HEAD, with every anchor asserted.

Verified

Makefile assertions updated, both halves kept

The old pair was "16 compiles" + "18 is refused". 18 must now compile, so: 16-deep compiles and matches gcc (kept deliberately — it was the old ceiling, so it is the cheapest guard against a regression that reinstates one), 18-deep compiles and matches gcc (new, and red before this change), and 129 headers report the limit. That last number has to move with MAX_CPREP_INCLUDES, which is noted in the recipe.

Not the busybox blocker

Recorded above and unchanged: the busybox wall is [[bug-c-the-preprocessor-runs-away-on-sys-param-h-resolved-from-the-host-fallback]]. This ticket closes on its original merits.