AUIS Revival: What Was Done and Why

Last updated: 2026-08-08

This document is a narrative account of reviving Carnegie Mellon's Andrew User Interface System (AUIS) on a modern Mac, for readers who already know ATK and readers encountering it for the first time.

For the complete technical record this document summarizes, see:

Doc	Description
roadmap.md	Current status, active projects, and open issues
porting-changelog.md	Chronological log of every fix
porting-assessment.md	Bug-class analysis and the full ANSI C conversion plan
version-comparison.md	Why 6.3.1 (the last C release) was chosen over the later, unfinished C++ rewrite
quickstart.md	Build and run instructions

What AUIS is

AUIS is the latest name for the user-interface toolkit work that came out of the IBM-funded Information Technology Center at Carnegie Mellon University in the 1980s and 1990s. It was originally the "Andrew Base Editor," then a second version, "Base Editor II" (BE2 for short), then the "Andrew Toolkit" (ATK for short). As the project began to be shown outside CMU and adopted by others, the more ambitious name "Andrew User Interface System" (AUIS) was adopted.

The applications from AUIS of primary interest now are ez and messages. ez is a word processor capable of embedding spreadsheets, equations, drawings, and animations directly inside a document. messages is a mail and bulletin-board client with the same rich embedding. In addition to ez and messages, AUIS is the basis for a dozen smaller applications, including bush, a filesystem browser.

The version being revived, 6.3.1 (August 1994), is the last release CMU shipped as plain C before the project moved to C++. (See version-comparison.md for the reasoning behind reviving 6.3.1 rather than the later, never-finished C++ line.) The target platform is macOS on Apple Silicon, using XQuartz to provide the X11 display server this software was originally written against.

ATK's central idea is the inset: a self-contained, interactive object — a spreadsheet cell, an equation, a drawing, an animation, a footnote, a scripted widget — embedded inside a document, able to embed further insets inside itself, recursively and without a fixed limit. A single ez document can contain a spreadsheet nested inside a drawing nested inside a footnote. Each inset reads and writes its own portion of the file format, draws itself, and handles its own input, independent of what it's embedded in. The model predates the industry's later attempts at the same idea — Microsoft's OLE and Apple's OpenDoc, both later in the 1990s, OpenDoc discontinued in 1997 — built in portable C on a small object system called "Class," contemporary with Objective-C. The application framework — windows, menus, dialogs, scrolling, printing — is shared across every ATK program, so ez, messages, and help are entry points onto the same document-and-inset substrate rather than separate applications.

This code hadn't been compiled in roughly thirty years. Restoring it required addressing two things: the environment around the code had changed (new compilers, a new processor architecture, deprecated operating-system interfaces), and the code itself contained real defects that had never been exercised in three decades of use. The rest of this document covers both, along with the strategy used to manage them.

Modernizing

Some of what changed was required just to make 1994 code run in a 2026 environment. A smaller amount was optional: bringing part of the software's appearance up to a standard users now expect, even though the original approach still worked. Both count as modernization, for different reasons.

Required to run at all

• Deleting a character from the middle of a string was commonly done by copying the remainder a few bytes to the left, within the same buffer — technically undefined behavior by the letter of the C standard, but a pattern that produced correct results on essentially every C library for decades. Apple's runtime-hardened library now detects that overlapping copy and aborts the program outright. Fifteen call sites across nine files used this idiom, and it crashed three applications (a file browser, an outliner, and an HTML viewer) under ordinary use. Each site needed only a one-line fix — a function explicitly defined for overlapping ranges — but locating all fifteen required a deliberate, tree-wide search.
• dlopen()/dlsym(), the standard POSIX dynamic-library interface, replaced AUIS's own hand-built, per-platform dynamic code loader — the component responsible for loading an inset's code on demand. The original loader had to understand the executable file format of every target platform individually and had no notion of the current one; the standard library call accomplishes the same task in roughly seventy lines of code rather than the original's five hundred, and is what actually loads insets in the revived system.
• A compiler-leniency strategy, as the default response to the volume of errors modern, stricter compilers raise against 1990s C. Rather than rewriting the source wholesale, the build disables a small, specific set of modern strictness checks corresponding to conventions this code relied on, reserving actual rewriting for places that genuinely require it. (Discussed further below.)

Bison and flex

The project's vendored bison fork hangs outright on current processor architecture, so the build now uses the system's installed bison instead; flex was already the system tool, but 1990s wrapper code around both generators still assumed specific behavior of the versions in use at the time. An early fix — passing bison's output filename explicitly, since modern bison's default naming convention had changed — flagged that risk directly, since the original code already carried its own accommodations for 1990s bison behavior. That prompted a closer look for other places where old assumptions about generator output might no longer hold, and turned up two:

• A shared code generator assumed every parser lookup table was a fixed storage width, matching 1994 bison's output. Modern bison narrows some tables to a smaller type when the data range allows it; reading a narrowed table through the old, fixed-width assumption merged adjacent entries into garbage — surfacing as a date parser that failed on every input, for reasons unrelated to dates.
• A hand-written reset routine poked an internal flex flag whose meaning had reversed between old and new flex — nonzero used to mean "reinitialize on next call," modern flex treats it as "already initialized." The first call in a process skipped its own setup and wrote through an unset pointer, crashing a mail-address parser on the first address it ever processed.

Adopted by choice: anti-aliased text

X11's original design placed font rendering on the server: the X server itself rasterized glyphs from bitmap font files and returned pre-rendered pixels on request, a scheme known as the core font protocol. This was the normal way to draw text throughout the 1990s, and it is what AUIS was built against — every text-rendering call in the codebase assumes it. Modern desktop environments instead render text on the client side, using libraries such as Xft and fontconfig, which support anti-aliasing, scalable outline fonts, and subpixel hinting — the smooth, high-resolution text users now consider normal.

The old, server-side bitmap path still works: XQuartz, the X server used here, still implements the core font protocol, so nothing required this change. It was made anyway, because 1990s bitmap fonts, while entirely serviceable on 1990s monitors, look noticeably coarse next to the anti-aliased text every other current application renders, and that difference was judged worth correcting. The revival adopted a hybrid: ordinary document text now renders through Xft, while the small set of custom CMU symbol and cursor glyphs (bullets, math marks, cursor shapes) that have no modern equivalent still render through the original bitmap path — replacing only the parts that had a clean modern substitute.

This was a comparatively small amount of new code, and the visual improvement was judged worth the effort — but, being new code rather than old code regaining exercise, it introduced its own new defects rather than exposing dormant ones. The calculator inset's display, once it received real interactive testing under the new rendering path, showed two: text was being "erased" by redrawing it in the background color, which exactly cancels only the fully opaque center of each anti-aliased character, so the partially transparent edge pixels accumulated into a visible ghost after repeated use; and, separately, the X display server sometimes wrote the correct pixels into a window's buffer without promptly making them visible, requiring an incidental redraw — a window focus change, for instance — before the correct text actually appeared. Both were resolved, but both took debugging effort disproportionate to the size of the feature, which is the general cost of this kind of optional modernization: low volume of new code, comparatively high difficulty per defect.

The project's own infrastructure was modernized in a smaller way as well: source control runs under Fossil, a self-contained modern system, in place of whatever distribution mechanism carried the source in the 1990s.

Finding ATK lessons in modern implementations

Not every observation from this project is about a defect. A few times, working through thirty-year-old code turned up a design decision that still holds up, validated later by a standard that didn't exist yet when ATK was written.

• Raster insets and PNG converged on the same bitmap encoding. Adding image support to ez2md (the tool that converts .ez documents to Markdown) meant first decoding ATK's raster inset format: a one-bit-per-pixel bitmap, packed eight pixels to a byte, most-significant bit first, one bit value meaning "black." PNG's one-bit grayscale mode — standardized in 1996, years after this raster format was written (rasterio.c carries an IBM copyright of 1988) — packs pixels exactly the same way: MSB-first, one bit each, a bit value mapping straight to black or white. The only real work in the conversion was undoing ATK's own run-length encoding (a compact ASCII scheme that made sense when screen bitmaps had to move over modems and fit on small disks) back into raw packed bits, then flipping which bit value means black, since the two formats picked opposite conventions for that one detail. No resampling, no color-model translation, no coordinate reconciliation — two systems, designed for unrelated reasons the better part of a decade apart, landed on the same underlying representation for a scanline bitmap.

• AMS's 1988 client-server interface anticipates several modern IMAP extensions. AMS and IMAP were contemporaries — Crispin's first IMAP drafts and Borenstein's message server were designed in the same few years, for the same problem, at different institutions. Both settled on durable per-message identifiers that survive reconnection (AMS's 18-character unique ids; IMAP's UIDVALIDITY plus UID). Both retrieve message bodies by offset with a bytes-remaining count — AMS's MS_GetPartialBody has essentially the signature of IMAP's partial FETCH BODY[]<offset.size>. Both use a two-phase delete: mark, then purge (AMS_ATT_DELETED and MS_PurgeDeletedMessages; \Deleted and EXPUNGE). Both maintain a per-user subscription list over a shared folder space. Both send fixed-size summary records in place of messages (AMS snapshots; IMAP's ENVELOPE/FLAGS/INTERNALDATE items). The AMS spec's engineering doctrine — every call idempotent, a server that "can die at any moment, with the client automatically reconnecting and continuing its work without any loss of context" — is the same discipline a modern IMAP client's reconnect logic depends on. The parallels reach past core IMAP, too: AMS computed reply-chain threading server-side in 1988 (the snapshot's chain field), which IMAP added as the THREAD extension years later; its "master update file" answering what changed since I last looked cheaply is CONDSTORE's job description; its 168 user-defined per-folder attributes are IMAP keywords. In one respect AMS is still ahead: the server computes each message's one-line caption once, for every client — a summary IMAP clients still each rebuild for themselves from ENVELOPE, four decades on. The revival's IMAP work relied on this directly: mapping the store's operations onto IMAP was mostly transcription, because the two designs had already agreed on what a mail store is.

Old bugs never found till now

Bringing 1994 code onto a 2026 machine did more than require accommodating a changed environment — it also surfaced defects that had existed in the source for decades without ever manifesting. Two changes in particular turned latent mistakes into visible ones: stricter runtime checks in Apple's C library, which refuses certain unsafe patterns outright rather than silently permitting them, and, later in the project, a build configuration that type-checks function calls the original toolchain never checked. Neither change introduced a defect; both exposed defects that had been present, and untriggered, since the code was written. A sample of the most illustrative follows. The fuller catalog — several dozen more of the same species — is not yet consolidated in one place: some are in porting-assessment.md's bug-class writeups, some only in porting-changelog.md's dated log or in claude-history/roadmap-old.md's retired detail, and a few live only in fossil's commit history. Bringing them all into porting-assessment.md as the single technical reference is a natural next step, not yet done:

• A core interface's arguments were declared in the wrong order for roughly 35 years. The class-definition file for the figure-drawing inset declared one method as Build(action, view, ...); every implementation, and every call site, used (view, action, ...). This had no runtime effect for the entire life of the code, because nothing had ever cross-checked the declaration against actual usage — until the ANSI C conversion effort, described below, began doing exactly that.

• A parser that never checked whether an embedded inset loaded. ez's plain-text container read each inset via a success-or-failure code it never checked. When a figure's reader failed partway through — on a document with an attribute this older reader didn't recognize — the container kept parsing from wherever the failed read had abandoned the file: the figure's raw internal data appeared on screen as literal text, and nothing after it in the document rendered at all. Fixed by falling back to a generic reader that resynchronizes by counting nested begin/end markers instead of assuming every read succeeds.

• A debug-tracing macro that could corrupt the C library's own declarations. A debugging header renames open to dbg_open with a preprocessor macro, so that file-descriptor leaks can be traced through a wrapper. In source files that happened to include this header before the system's fcntl.h, the macro rewrote the C library's own declaration of open() — producing a declaration of dbg_open with open's variadic signature, which the wrapper's fixed three-argument definition does not have. For the code's whole prior life this was harmless twice over: K&R compilation had no prototypes to mismatch, and the calling conventions of the era passed fixed and variadic arguments identically anyway. On Apple Silicon they differ — variadic arguments travel on the stack, fixed arguments in registers — so caller and wrapper disagreed about where the file-permission argument was. Files were silently created with garbage permission bits, and code that had just written a temporary file found itself unable to reopen it, an "impossible" permission error that traced back, through the disassembly, to a macro and an include order. Fixed at the root: the debugging header now parses fcntl.h itself before performing the rename, making the poisoning impossible in any include order.

• The message store's id scheme assumed case-sensitive filenames. The message store names each message's body file after its unique id — an 18-character string over a base-64 alphabet in which uppercase and lowercase letters are distinct values. That scheme silently assumes filenames are case-sensitive. In 1988 this looked like a safe assumption: the case-insensitive filesystems were the older systems — TOPS-10, VMS, CP/M, DOS — and UNIX, the newer one, distinguished case. The filesystems that ended up dominant on the desktop — HFS+, then APFS, and NTFS as Windows uses it — are case-insensitive (case-preserving, which makes the trap quieter still: names display exactly as written, they just refuse to be distinct). On a Mac, two AMS ids differing only in letter case are different strings and the same file. For thirty-five years nothing noticed, because native ids derive from host and timestamp and differ in many character positions at once. The revival's IMAP mirror found it in an afternoon: it synthesized ids from dense sequential message numbers, adjacent ids collided as filenames every time the encoding walked the alphabet across a case pair, and a "new" message's body file turned out to already exist — containing a different message. The mirror's fix is an id alphabet with no case pairs; the store's native ids remain a documented, if astronomically unlikely, hazard on this platform.

• Adew's build silently destroyed its own sample document, and stayed that way for weeks. src/atk/adew/Imakefile builds a launcher script from genarb and writes it to a file named arb in the same directory as Arb, the tracked .ez arbiter document the launcher is meant to open — a real ATK datastream, not source code, so a build silently overwriting it produces no compiler warning and no diff anyone would think to check. On this checkout's case-insensitive APFS volume, arb and Arb are the same directory entry: an early build run replaced the datastream with the generated script, and that change rode into a commit alongside unrelated work, unnoticed. The loss surfaced independently and from outside the project entirely — Adew's original author emailed asking whether an old copy of Arb still existed anywhere, having hit the same collision in his own history and concluded it might be unrecoverable. It wasn't: the revival's trunk branch had never rebuilt adew/ and still carried the file untouched from the original import, byte-identical to what he was describing. Restored from that copy on both branches; the Imakefile now builds the launcher under a name (arb-launcher) that can't collide with tracked source and renames it to arb only at install time, in $(DESTDIR)/bin, where nothing tracked shares the name.

• Two independent header parsers, written decades apart, made the same assumption about where a message's headers end. RFC822 says a blank line separates headers from body; both AMS's own mail reader (text822.c's GetHeader) and the vendored metamail program (Read822Prefix) implemented that check by testing for a bare '\n'. That is correct for every message the code had ever seen — the Andrew mail system generated and stored its own messages internally with LF line endings — until this project's IMAP mirror started handing it real wire-format mail from Fastmail/Gmail/Outlook, whose blank line is "\r\n\r\n". Neither check ever fires on that input, with two different failure shapes: text822.c's header loop never terminates, so it reads straight through the "blank line" and treats the entire message body as a run of unrecognized headers (displayed in a tiny font, un-decoded, every colon making the text before it bold — see porting-changelog.md's 2026-07-21 entry for the full symptom list this produced); metamail's version reads to EOF looking for a separator that never arrives and exits with "Could not find end of mail headers", which is what actually blocked cui's type command on the very first message in a real mirrored inbox. Both are the same one-line fix — stop letting a bare '\r' reset the "did we just see a newline" state — applied independently in each file, since neither parser shares code with the other. A third site in the same text822.c function turned up a day later: finding the blank line correctly doesn't mean the lines before it are clean — GetHeader still handed back each header's raw "...\r\n" verbatim, and every one of those trailing \r bytes got inserted as a literal, visible character in the ATK text object, one per header line — "double spaced" was the visible result. Same root cause, one level deeper in code this project had already touched once.

• An explicit cast was correct on 32-bit hosts and wrong on 64-bit ones. The font toolkit's bounding-box routine measured a string's width by calling its own sibling method through an explicit (long *) &w cast, where w was a plain int local. On the 32-bit hosts this was written for, int and long were both four bytes, so the cast changed nothing — a stylistic choice, not a mistake. On this LP64 port long is eight bytes, and the same cast became a real four-byte stack overflow on every call, silently corrupting whatever local variable happened to sit next to it in the frame — every time a figure-inset text label recomputed its size. It produced no crash, just gradually garbled label text, and it evaded the ANSI conversion's own compiler-warning-driven audit for a specific reason: that audit worked by grepping for -Wincompatible-pointer-types warnings, and an explicit cast is exactly what that warning exists to suppress. A census built entirely from compiler diagnostics could not, by construction, see a mismatch the programmer had already cast into silence; only reading the function by hand, then checking the original 1990s source to confirm the cast predated this project, found it.

• A folder-action dialog held pointers into a cache that could be freed while it waited. Clicking a folder in the mail overview brings up a "What do you want to do with 'X'?" menu — implemented not as a native modal but as ordinary event dispatch, so background timers and callbacks kept running while the mouse waited over the choices. The function held onto the folder's name strings as raw pointers straight out of the directory cache; if anything freed that cache entry during the wait — another window's rename/remove notification, a periodic refresh — the strings went stale in place, and clicking "Subscribe" afterward walked the freed memory character-by- character and crashed. One sibling path in the very same function, reached by "see the messages," already defended against exactly this by duplicating the strings before doing anything that could invalidate them — a comment even says so — but the defense was never extended to subscribe/unsubscribe or "alter subscription status." For decades this raced against, at most, an occasional slow local change and rarely lost. The revival's IMAP mirror gave the directory cache far more frequent invalidation traffic than the original single-user model ever produced, turning a latent race from 1994 into a reproducible crash the first time someone tried to subscribe to INBOX. Fixed by giving the affected paths the same heap-copy discipline the "see the messages" path already had.

• An error-cleanup path deleted the wrong file on a name collision. When the store writes a message's body file, it opens with O_CREAT|O_EXCL — and if the open fails, the error path does unlink(File) before returning. That unlink is meant to remove a partially-written file, which is the right cleanup everywhere later in the function, after the open has succeeded. On the open-failure path there is no partial file — and in the one failure mode where the target does exist, EEXIST, the file belongs to an earlier message. So for thirty-five years, any append that collided with an existing message file quietly deleted the existing message. The author suspected something: the line above the open reads /* Bogus -- what if not overwrite? */. It never fired because the store's duplicate check (which keys on the RFC-822 Message-ID header) almost always intercepted true duplicates before the filesystem could, and id collisions otherwise didn't happen. The IMAP mirror lined up the holes: a message with no Message-ID slipped the duplicate check, an RFC 3501 n:* range quirk re-presented an already-mirrored message as new, and the mirror watched a message file vanish from a directory nothing was supposed to delete from. Fixed by deleting the one erroneous line.

• Misspelled function names linked silently to the wrong library for thirty-five years. Three mail-filtering primitives — regular- expression search and decomposition operations available to the mail system's rule-based filtering language — called functions named regcomp and regexec to compile and run a pattern. No function by either name has ever existed anywhere in this codebase; the regular- expression engine actually built and shipped with the software, a few directories over, is named reg_comp and reg_exec, with an underscore neither call site had. Pre-standard C never checked that a called function actually existed before compiling the call, so the compiler raised no objection, and the linker simply resolved the misspelled names against whatever else provided them — in this case, the operating system's own built-in regcomp/regexec, a wholly different regular-expression implementation expecting a different kind of first argument than the plain string these call sites actually pass. Every call, for the software's entire life, therefore invoked the wrong function with the wrong argument shape — a defect invisible to any compiler or linker, on this platform or any other, because both misspelled names happened to already mean something. It came to light not through an automated check (nothing about it would trip one) but during a close reading of the file while fixing unrelated compile errors as part of the ANSI C conversion effort. Corrected to call the codebase's own, correctly named and typed, reg_comp/reg_exec.

• The same declaration mismatch, independently, in five unrelated parts of the codebase. An interface file declaring a wrong-but-plausible type for a constructor or destructor's own object parameter — invisible because K&R never checked it — turned up on its own in form widgets and a diagram-editing framework (six instances), the mail reader's own message-display view classes (three more), and the drawing library's arrow and polyline figure classes (two more). Every real implementation was already correct in all eleven cases; only the paperwork was wrong, apparently copy-pasted from a neighboring class's declaration each time. A related mistake in the same family ran the other direction — a drawing-library base class silently dropped an argument all fifteen of its real subclasses supplied — and a dead-end variant turned up too: two destructor methods misspelled by one transposed letter pair ("Finialize" for "Finalize"), never wired to anything and never missed, caught before they could do damage rather than after. None of it needed a runtime symptom to find; it surfaced by the same method every time, once the ANSI C conversion's typed interfaces finally gave the compiler something to check these old declarations against.

• Attacker-controlled mail headers, used as a format string. Two routines in the metamail viewer — one saving a MIME attachment under a name derived from its Content-Type parameters, one decoding an RFC 2047 encoded word from a message header — each pass text taken directly from the message itself as the format argument to printf/fprintf, rather than as an ordinary data argument. Any percent-conversion sequence a message's author places in a filename or an encoded-word payload is interpreted as a format directive against whatever the stack happens to hold at that call, not displayed literally — a live format-string vulnerability sitting in header-parsing code that has processed arbitrary, untrusted mail for over thirty years. It surfaced not from a security review but from the same mechanical format-string sweep that found every other bug of this shape in this project — the compiler's own diagnostic for a non-literal format argument flags this pattern regardless of whether anyone was looking for a vulnerability. Corrected by passing each string as an ordinary "%s" argument at both sites.

• A stale, argument-less forward declaration shadowed a correctly typed real definition. Dragging a figure in the drawing editor worked vertically but froze horizontally after the first motion event. The function that computes each drag step's constrained coordinates was, at its real definition, already correctly typed to write a long through each of its two output pointers — but two duplicate copies of its forward declaration, sitting above every call site in the same file, still used the old argument-less style (static int Set_Constraints();), which is legal ANSI C and gives the compiler no parameter information at all. The one call site that mattered — the continuous drag-motion handler — held its output variables in ordinary 4-byte ints, so every motion event wrote 8 bytes through a pointer to a 4-byte stack slot, silently corrupting whatever sat next to it. Retyping the file's forward declarations to match their real definitions closed the hole and immediately surfaced a second bug of the same shape in the same file. Neither was reachable by any warning this project had enabled up to that point — -Werror=int-conversion and its siblings only fire when a real prototype is in scope to compare against, and an argument-less old-style declaration is, by definition, not one. That gap raised an obvious question — how many other stale, argument-less forward declarations were sitting above correctly-typed real definitions elsewhere in the tree? — and a tree-wide census answered it: roughly 6,700 such declarations, across nearly every active directory. A mechanical pass retyped each one to match its real definition and let the compiler itself separate harmless old style from genuine bugs, turning up a short, specific list of further defects — an AMS API call whose byte-count out-parameter had been declared half the width every caller expected, a file handle typed as a plain integer where every caller and the function's own body treated it as FILE *, and nine ordinary argument-count/type drifts — all fixed, none of it needing its own rollout milestone the way M1–M4 did.

• A crop rectangle was never checked against the image it was cropping. A raster-format converter's crop option never validated its rectangle against the source image's actual dimensions before reading it — the underlying bitmap routines don't bounds-check either, by design, on the assumption that callers already have. A crop rectangle larger than the source image read straight past the end of the allocated buffer and kept going, producing an 18-megabyte file of heap garbage instead of an error, for an operation that should have been rejected outright. Fixed with an explicit bounds check ahead of the crop.

• Backward scrolling silently no-op'd through any embedded view taller than one screen — including plain raster images, not just HTML mail. textview's backward-page command (BackSpace in atk/text/textv.c) had a fast-path shortcut that computed how far to move back from a cached line's y-coordinate plus its full height. For an ordinary short line this arithmetic was essentially inert — a real page-up request is always bigger than one line's height, so the shortcut almost never fired — but for a single line taller than the screen, partially scrolled into, the same arithmetic swallowed the entire backward-move request every time, so Escape-v/prior-screen did nothing at all while inside such a view. Confirmed live (2026-08-18) against the HTML mail renderer's table layouts, but the bug is generic to textview itself, not specific to HTML content — a plain raster inset larger than the window hits the identical code path and was independently observed with the same symptom. Effectively dead code for 30-some years because nothing embedded had ever been taller than a screen before. Fixed by removing the erroneous shortcut and relying on the (already correct) shortcut beside it, based on how many pixels of the current line are actually scrolled off-screen rather than the line's total height.

• textview_Visible treated text-position containment as proof of pixel-level visibility. textview__Visible answers "is this position currently on screen," but for a position sitting exactly at the end of the last laid-out line it only checked whether that position fell within the line's text range — never whether the line's own bottom edge actually fit inside the viewport. For ordinary short lines the two questions have the same answer, but for a document whose entire content is a single embedded view many screens tall, only partway scrolled into view, they diverge: the position is trivially "contained" while almost none of the line is actually visible. This made "go to end" (Escape >, via textview_FrameDot) silently no-op, because the framing logic saw Visible() return true and skipped re-scrolling. Fixed by also requiring the last line's bottom edge fit within the viewport, mirroring a check the scrollbar code (getinfo) already made in the same file.

• Backward-move-to-a-position always failed to land inside the very first, tallest line of the document. Once Visible() correctly detected "not on screen" and asked BackSpace to scroll a target position into view, BackSpace's own pixel-budget search only ever credited the height of lines strictly before the one containing the target position — correct when the target sits at its own line's top edge (the normal case, since the search is normally driven off the current top-of-screen mark), but wrong when the target sits at the line's end, as happens when the target is the very end of a document whose entire content is one line. With no preceding lines to credit, the search always concluded there wasn't enough room and fell through to "can't go any farther," clamping to absolute position 0 — so Escape > reset the scroll to the very top instead of leaving the target framed near the bottom. Fixed by also checking whether the line's own height (not just its predecessors') covers the remaining budget, landing partway into it when so.

• Forward-paging past the end of a document landed the scroll-top on a position with no line of its own, blanking the screen. textview__MoveForward's pixel-based branch, when a forward page (^v, textview_NextScreenCmd) consumed the entire remaining height of the document's last line, unconditionally advanced the scroll-top to text_GetLength() — one past all real content. The layout code has no fallback for that: it formats one empty line there and whites out the rest of the view. This isn't specific to the one-giant-embedded-view case that exposed it — it's latent for any document where forward- paging exactly exhausts the last line — it just took a document like this project's HTML tables to trigger it in ordinary use. This is suspected to be the actual mechanism behind the project's original motivating complaint, that repeatedly pressing <space> while reading a message skips its last chunk and jumps straight to the next message. Fixed by refusing to advance past the last line; instead reveal further into it, capped so its bottom never scrolls past the viewport's bottom edge — once reached, further forward-paging is a no-op instead of a blank screen.

• The scrollbar bottom-endzone click blanked the screen, but only on a held click. A single quick tap already worked. Targeted write(2) tracing in both scroll.c and textv.c showed that scroll.c's auto-repeat timer for the endzone buttons (~100ms, easy for an ordinary click to exceed) fires a second "jump to end" request identically to the first, before the mouse button is released. The second request's backward pixel-offset step hit BackSpace's "stay within the current line" fast-path shortcut — a shortcut whose own comment already stated its precondition as operating on the current top line, but which was applied unconditionally to whatever position was passed in. The endzone jump's target is never the current top; it's a freshly computed line near the end of the document. Because the first (correct) landing had already set the view's "how far scrolled into this line" state to approximately the same distance being requested again, the second call's shortcut fired against that unrelated leftover value and returned the target completely unmoved — landing the scroll-top exactly at end-of-document, the same no-line-of-its-own position the forward-paging bug above already established renders as a blank screen. Fixed by only taking the shortcut when the position passed in actually is the current top position, matching the precondition the shortcut's own comment already claimed.

None of these are new mistakes. Each was introduced once, decades ago, and never triggered — because the exercising code path was never run, because nothing had checked a declared interface against its actual usage, or because an earlier C library was more permissive. There was no test suite to find them: what did was a stricter compiler, real use, and weeks of manual, heuristic-driven work — grepping for a suspect pattern, rebuilding with make -k to surface every hit at once, then reading each one by hand. This development and repeated toil was done by Claude Code, primarily with the Sonnet model managing delegated sessions following an evolving sonnet-playbook.md, with the Fable model occasionally used for reviews and for difficult problems.

One exception, caught the same day it was made. A drawing editor's on-disk format for a figure's "mode" attributes packs three single- character flags onto one line; the code that writes that line built all three characters but only ever wrote two, silently dropping the third — "halo" — on every save, a plain thirty-year-old bug fixed like the rest above. Fixing it exposed a second, previously dormant bug in the very same function: the three flag buffers were declared static char foo[2] = "?", scratch space seeded with a literal ? and only ever overwritten when their flag was on, never reset when it was off. Because halo had never actually appeared in the output before, its stale ? sentinel was invisible; the moment the fix started printing it, every figure with the halo flag off — effectively all of them — got a literal ? appended to its Mode line, desynchronizing the reader for the rest of the document. Caught within hours by manual round-trip testing (open, edit, save, reopen), not by any compiler warning — nothing about a string that compiles cleanly but holds the wrong content trips a diagnostic. Fixed by dropping static and letting the buffers default to empty, which is arguably a second correction to the same original mistake: true K&R C never allowed a stack variable to carry an initializer at all, so the 1988 author most likely reached for static only to make the = "?" syntax legal, not because persistence across calls was ever wanted.

Word size issues

The largest, most systemic category of defect came from a single architectural fact: this code was written when a C int and a C long were the same size — 32 bits, on the machines of the early 1990s. On today's 64-bit Apple Silicon, long is 64 bits while int remains 32. Code that quietly assumed the two were interchangeable — a reasonable assumption for decades — now loses or corrupts data at every boundary where the two are confused.

This is most acute in AUIS's object system, which implements polymorphism (one class of object overriding a method defined by another) in plain C, using function pointers stored in a dispatch table. Historically, every method call went through generated code that cast that function pointer to a completely generic, typeless signature — meaning the compiler, seeing no argument types at the call site, could not insert the instructions needed to correctly widen a 32-bit value to 64 bits, or to preserve its sign. Five distinct, recurring failure patterns followed from this one root cause, each responsible for real, visible bugs during the revival:

1. A function returning a pointer — or any other 64-bit value — called with no declaration in scope. Pre-standard C assumes an undeclared function returns a plain 32-bit int. If the function actually returns something wider, the upper half is silently discarded — typically producing a crash the moment a truncated pointer is used, or a silently wrong number when the return is an ordinary integer instead. This was the single most common defect in the codebase: more than twenty separate sites, all the same shape, once the pattern was recognized in one core function used to locate files on disk. The later, tree-wide sweep for exactly this class of gap (M2, below) found the same shape recurring by the hundreds across the rest of the tree — including several genuinely long-returning (not pointer-returning) library functions, such as a mail client's directory-lookup and session-initialization calls, each confirmed against its real definition, independently, before any declaration was written.
2. More arguments than the processor's registers hold. Apple Silicon passes the first eight integer/pointer arguments in registers and spills the remainder to the stack — but only if the compiler knows, at the call site, that there are more than eight. Through the generic, typeless dispatch above, it did not, and the ninth and later arguments were silently dropped.
3. A sentinel value corrupted by zero-extension. The value -1 is commonly used as a special "unset" or "apply to everything" marker. Passed as a plain 32-bit constant through the same typeless dispatch, it is not sign-extended as it should be — the receiving 64-bit field ends up holding 4,294,967,295 rather than "negative one," and any code checking "is this negative?" or "does this equal -1?" silently takes the wrong branch. This is what made a help-browser list open scrolled to the bottom instead of the top: the "no position set yet" marker for a freshly opened list was corrupted into a large number crossing one of these dispatch calls, and the scroll position duly followed it.
4. A 64-bit number read with a 32-bit text-parsing format. Numbers stored as text within document files were read back with %d (32-bit) into variables that were actually 64 bits wide, leaving the upper half as whatever value happened to occupy the stack. The clearest instance: a drawing's on-page position was parsed this way and came out corrupted by exactly 4,294,967,296 — the figure was being drawn correctly, just roughly four billion pixels off-screen.
5. A 64-bit parameter fed a corrupted 32-bit argument — the same zero-extension mechanism as pattern 3, but for ordinary values rather than named sentinels, tripped by negative numbers used for scroll positions, indentation, and margins.

Three related but mechanically distinct defects showed up alongside these five, corrupting data across a similar boundary disagreement without being, strictly, width problems:

• A signedness mismatch rather than a width mismatch. One inset type's interface declared a method as returning a plain (signed) character where the implementation actually returned an unsigned one. At low compiler-optimization levels the discrepancy had no effect; at normal optimization, the compiler exploited the declared (incorrect) signedness and generated a comparison that could never succeed, so a "no line width configured" sentinel was silently read as "maximum width," and an entire figure was painted over in solid black. This belongs to the same family as the width bugs above — a value crossing a boundary where the two sides disagree about how to interpret its bits — differing only in that the disagreement is over sign rather than size.
• A pointer stored in a field only wide enough for a plain number. Two font-style fields in the note and troff-text insets had, since the 1990s, been mistakenly assigned the literal text "fontdesc_Plain" rather than the symbolic constant of the same name — a copy error that was harmless on the original 32-bit machines, where a pointer and an int were the same size, and became live pointer corruption only once the same code ran on a 64-bit processor. A related instance: a widget class declared a parameter as a plain unsigned value where the actual, pointer-carrying implementation needed something wide enough to hold a full address, silently truncating a self pointer on every control-panel widget built from that class.
• A file-header struct declared its fields long, matching the on-disk format only by 32-bit coincidence. A raster-format command-line converter, untouched by every earlier width-bug sweep because nothing about it resembled the dispatch-related patterns above, declared its file-header struct's three fields long — matching the format's actual on-disk layout, a fixed 14-byte header, only on the 32-bit machines this was written for. On a 64-bit build the struct is more than twice that size, so every field after the first lands at the wrong offset; reading such a file back fed uninitialized stack memory into an image-resize call that ran away, consuming multiple gigabytes of memory before being killed. The same file's color-inversion routine carried an independent case of the identical assumption: a hand-optimized loop inverting four bytes at a time walked a raw pointer cast to long *, with address arithmetic sized for a 4-byte word; on this LP64 build each step silently inverted eight bytes instead, missing a row's first few bytes and overrunning its last few into whatever memory followed. Both had sat unexercised since the format was written — nobody had tested this converter's own output against itself on this platform until it finally was, byte-for-byte, against the original image.

A mechanically unrelated defect, described above in "Modernizing," produced a very similar-looking symptom: a code generator's own choice of table storage width — not the processor's register width — changed underneath old code. It does not belong to this family mechanically, but the underlying lesson is the same: 1990s code that hard-assumed a fixed width is fragile against any later link in the toolchain — compiler, processor, or code generator — making a different, reasonable-at-the-time choice.

Migrating to ANSI C

The oldest parts of this codebase predate the C language standard itself — functions are defined in the pre-standard "K&R" style, with no declared argument types anywhere. An early attempt to correct this wholesale, by running an automated tool across the entire source tree in one pass, made matters considerably worse: the tool's pattern-matching approach to inferring argument types mishandled enough edge cases that a single mass conversion took the build from roughly zero errors to over two thousand, with no way to distinguish genuine bugs from the tool's own mistakes. That attempt was reverted in full.

The approach that ultimately worked rests on one observation: this codebase's class-definition files already describe every method's real argument types in full, for an unrelated reason — they are the input to the code generator that builds the object system's dispatch tables — and that generator had simply been discarding the type information once it parsed it. Teaching the generator to emit what it already knows turns the compiler itself into the auditor: any place where a real implementation disagrees with its own declared interface becomes an immediate, located compile error, rather than a bug waiting to be found by accident at runtime.

The resulting plan runs in four stages:

• M1 — teach the class-definition compiler to emit typed code. Completed tree-wide in July 2026. This alone eliminated the sentinel-corruption and too-many-arguments bug families by construction, and caught dozens of decades-old interface mistakes — including the backwards Build() arguments described above — simply by enabling real type checking at every method call site. The rollout itself proceeded one subdirectory at a time, verified by a clean full rebuild and a runtime check after every step — the discipline the failed mass-attempt had lacked.
• M2 — sweep for missing function declarations throughout the rest of the tree, not just class methods, closing the undeclared-function pointer-truncation pattern for good. The sweep surfaced two complications of its own along the way: malloc/free/realloc/ calloc are compiler built-ins, so a call to one with no declaration in scope anywhere never triggers the diagnostic being swept for — invisible to the compiler-driven pass, closed only by a second, deliberate search across every file regardless of how clean a directory's compile already looked; and one class's method-dispatch macro, an optimization that calls its internal implementation directly instead of through the usual indirect table, turned out to declare that implementation inside a header guard whose actual purpose was unrelated — avoiding duplicate method tables across a multi-file loadable module — which had the side effect of hiding a real, correctly-typed declaration from the one caller that needed it, harmlessly, for over thirty years.
• M3 — convert the function definitions themselves from pre-standard to standard C, one subsystem at a time, using a purpose-built tool that looks up each function's real signature from the class-definition files rather than inferring it — the safer approach the original mass- conversion attempt should have taken. Completed tree-wide August 2026, 15 sessions across all 91 active directories. Consistent with M1's experience, real type/arity checking surfaced numerous decades-old interface bugs along the way — several are recounted below — and the conversion tool itself needed two rounds of fixes for K&R declaration shapes its own parser had missed.
• M4 — enable full compiler strictness tree-wide, once every subsystem has been converted, which closes the remaining scanf-format-code bug family as a side effect. Completed August 2026, directory by directory, seven waves plus an initial pilot batch.

As of this writing, M1 through M4 are complete across the entire active codebase, each having turned up several of the decades-old defects described above. A follow-on tree-wide census closed the one remaining blind spot the four milestones couldn't see on their own — stale argument-less forward declarations shadowing correctly-typed real definitions — and is described in its own entry above.

Where things stand today

ez (the word processor), help, and messages (mail, running against a local mailbox and, now, a live IMAP mirror, rather than the original 1990s shared-filesystem delivery system) all run and render correctly, including most embedded inset types: text, equations, tables, drawings, animations, footnotes, spreadsheets, and more. bush, org, chart, and layout all run cleanly too. The ANSI C conversion described above (M1 through M4) is complete across the entire active tree — every method call and function definition in it is now compiler-checked. A live, itemized table of what's fully working versus still rough is in roadmap.md, rather than repeated here.

One deliberate scope decision from early in the project: the Console (terminal-emulator) subsystem is not part of this build. Console's interprocess-communication layer would need a rewrite of its own, and the project set that aside rather than take it on alongside everything else. A small side effect: two icon fonts used by an unrelated animation inset happen to live inside Console's own build directory, so they go silently unbuilt too, unless Console — or just its font directory — is built on its own.

A handful of smaller, known issues remain open, described below in "Open issues" and tracked in full in roadmap.md.

Open issues

A few problems remain unresolved, either because the root cause itself isn't confirmed or because a workaround is in place without a real upstream fix.

• A live deadlock, cause unconfirmed. While chasing an unrelated defect, a help window occasionally froze permanently. A debugger traced this to the process's single thread trying to reacquire a lock on the X display connection it already held indirectly, triggered by a keyboard-layout-change notification arriving mid-operation. Whether this is a genuine defect in how the toolkit calls back into the X client library, or a quirk specific to this environment's X server, hasn't been determined.
• A completely empty build directory fails on the very first build. The class-system directory everything else depends on builds its own pieces in a fixed order, and that order builds one piece needing class.h already in place before the piece that actually installs class.h runs. Any build directory that already has a copy sitting around from an earlier build never hits this; only a genuinely empty one does. The current fix is a manual step — copying the header into place before the first build — and a build-ordering problem is the leading explanation, though not a settled one: this step wasn't always necessary earlier in the project, and it isn't yet certain whether the analysis has found the real root cause or only a reliable workaround.
• metamail has never successfully run in this environment. Invoking it — directly, or indirectly through messages' MIME-attachment handling — raises SIGTTOU inside the routine that spawns a mailcap-entry viewer subprocess: ordinary 1980s terminal job-control code performing an ioctl() a foreground process doesn't expect. Confirmed unrelated to the ANSI C conversion work happening alongside it — the crash lives entirely in terminal/process-group handling, untouched by any declaration or typing fix — and predates this project; nobody has reported metamail working here at any point. Root cause identified; not yet fixed.

Further reading

• porting-changelog.md — the complete, dated history of every fix, in the order it happened
• porting-assessment.md — the underlying bug-class analysis, strategy decisions, and the full ANSI C conversion plan
• roadmap.md — current status, open issues, and near-term plans
• version-comparison.md — why 6.3.1 (the last C release) was chosen over the later, unfinished C++ rewrite
• quickstart.md — how to build and run this on a Mac