`libs/libcmt/_getbuf.obj` exact-match ledger

Plain-English behavior

This object gives a C FILE stream its buffer. It first ensures the CRT's flush-at-exit path will run, then tries to allocate a 4 KiB buffer. If that allocation fails, it deliberately falls back to the two-byte character field embedded in the stream. Finally it resets the current pointer and unread-byte count so the caller can begin using the selected buffer.

Scope and provenance

Source: libs/libcmt/_getbuf.c.
January target: libs/libcmt/_getbuf.obj, object index 737.
Compiler: XDK 3911 CL 13.00.9254.1.
Flags: unchanged libcmt project flags /O1 /Gy.
Primary provenance: obj\i386\_getbuf.obj, extracted from the exact XDK 3911 libcmt.lib. Hardened comparison proves its sole runtime function is strict-identical to January.
The member's CodeView names Microsoft's crtw32\stdio\_getbuf.c, public function _getbuf, and source lines 74-114. Microsoft's historical Xbox CRT source at xerohour/xbox_leak_may_2020, commit fd00b4b3b2abb1ea6ef9ac64b755419741a3af00, independently proves the function prototype, statement order, fields, flags, and fallback behavior.

Validated baseline

The object was MISSING, unparked, and absent from all earlier source, object-log, Git-ref, and worktree history. It contains one public code COMDAT and no linked runtime data:

Function	Size	Relocs	Normalized SHA-256	Relocation destinations
`__getbuf`	68	2	`1eba03727374348f7ec18fa2438a932e8a9bdb9c6da5b480aec5467de388dbe7`	`+0x02 -> __cflush`; `+0x0c -> _malloc`

The target and XDK member agree on every byte, relocation address/type/target, one-byte-aligned external COMDAT ownership, and the absence of .rdata, .data, and .bss.

Accepted controls

The source keeps Microsoft's allocation-first if/else topology and final pointer/count reset in their attested order.
The public XDK FILE fields and _IOMYBUF/_IONBF names express the behavior directly; no anonymous offsets or unexplained flag values appear.
The private 4 KiB allocation size and two-byte wide-character fallback are named semantic constants. Both are proven by source and January immediates.
_cflush is declared as the historical CRT integer global. Its increment is not redundant: Microsoft documents it as forcing library pre-termination when a redirected standard stream first acquires a buffer here.

Experiment matrix

ID	Source shape	Size T/B	Relocs T/B	Base hash / first divergence	Siblings	Decision
E00	January target vs exact XDK archive member	68/68	2/2	hashes equal; no divergence	1/1 exact	provenance accepted
E01	Authentic Microsoft allocation/fallback topology with public `FILE` fields and named constants	68/68	2/2	target hash equal; no divergence	1/1 exact	retained

No speculative or prohibited source shape was needed. The first production build reproduced the target exactly under the unchanged flags.

Whole-object ownership and alias audit

Target and candidate each own one 68-byte, one-byte-aligned, external .text COMDAT named __getbuf, with function type 0x20 at offset zero.
Both relocations have identical addresses, types, targets, and addends: DIR32 to __cflush at +0x02 and REL32 to _malloc at +0x0c.
Neither runtime object owns .rdata, .data, or .bss; no aliases, local helpers, strings, tables, or mutable data were introduced.
Candidate .drectve, .debug$S, and .debug$F are removable compiler metadata, not linked runtime ownership.

Do-not-repeat list

Do not substitute public BUFSIZ (512) for the CRT's private 4096-byte internal buffer size.
Do not remove the _cflush increment as apparently unrelated bookkeeping; it is target-proven and required for redirected-stream shutdown behavior.
Do not replace the inline _charbuf fallback with a static buffer or a one-byte allocation. It is deliberately two bytes so one wide character fits in unbuffered mode.
Assembly, volatile, forced inline, optimizer pragmas, byte patches, undefined aliasing, compiler-flag changes, and fuzzy credit are prohibited.

House-rule, Berth-rule, readability, and original-bug audit

The reconstruction uses the public typed FILE layout, descriptive names, ordinary structured C, and provenance-backed constants.
The prototype, fields, flags, allocation size, and exact source order are supported by January bytes, the identical XDK member, CodeView, and Microsoft's source rather than decompiler invention.
No original defect is present. Allocation failure is intentionally handled by the inline-buffer fallback, so no BUG (original) annotation is needed.

Residual classification and disposition

There is no function or ownership residual. The whole object is strict exact and eligible for Matching, subject to the full validation gates below.

Final validation

A forced XDK rebuild used the unchanged /O1 /Gy /I"xbox/include" flags and reproduced 68/68 bytes, both relocation identities, and the exact target SHA-256.
Full halobetacache_build and libcmt_build both succeeded.
After rebasing onto canonical b9bad27a, regenerated progress reports 341/833 complete objects, 3,456/11,060 exact functions, 412,979/2,198,102 exact code bytes, and 1,793,834/4,176,062 exact data bytes. The libcmt category is 74/212 objects, 108/476 functions, 6,973/55,015 code bytes, and 1,368/8,637 data bytes.
Semantic audit reports 3,377 semantic-exact functions, 3,461 accepted-exact functions, 93 hidden exact functions, and zero unit errors. It explicitly places __getbuf in both semantic_exact and accepted_ledger.
Admission audit reports zero candidates and zero revocations. Its one contradiction is the pre-existing unrelated source/shell/shell_xbox baseline item.
All 179 tooling tests pass. Every configuration JSON parses, the source has no prohibited construct, and git diff --check is clean.
The clean-commit fail-closed regression result is recorded after the atomic commit and final rebase validation.

6.1 KiB Raw Blame History

libs/libcmt/_getbuf.obj exact-match ledger