---
address: 0x821B6DF4
classification: msvc_eh_catch_handler
confidence: high
last_audit: 060
aliases:
  - "AUDIT-058 caller-ladder top (FALSIFIED)"
---

# sub_821B6DF4 — MSVC C++ catch-handler thunk

## Synopsis

A C++ catch-handler thunk emitted by the MSVC PowerPC C++ runtime. Dispatched by the EH machinery (`_CxxFrameHandler3` equivalent) when a matching exception type is thrown — NOT a normal `bl` callee. AUDIT-058 mistakenly treated it as the top of a "static caller ladder" for `sub_825070F0`'s activation; AUDIT-060 falsified that by reading the prolog and the `.rdata` reference context.

**This is the canonical "MSVC EH FuncInfo metadata mistaken for call edges" case. Always check the prolog before assuming a 0-caller fn is a missing activator.**

## Evidence

- Disasm at `0x821B6DF4` opens with the canonical MSVC catch-handler prolog: `subi r31, r12, 112; mflr r12; stwu r1, -96(r1); ...`. The use of `r12` (parent-frame pointer offset) and `mflr r12` is signature MSVC EH-handler shape.
- Address `0x821B6DF4` appears as a u32 value in only two places in the binary:
  - `.rdata:0x820C1994` — embedded inside an MSVC FuncInfo struct. Bracketing bytes: `FFFFFFFF 821B6DF4 19930522 00000001 820C1990 ...`. `0x19930522` is the MSVC FuncInfo magic.
  - `.pdata:0x8211C678` — exception-unwind metadata.
- AUDIT-060 Probe C-Win Windows Debug canary: `--log_lr_on_pc=0x821B6DF4`, runs at 120s and 240s wallclock → **0 fires both runs**. The matching exception is not thrown at this boot horizon.
- AUDIT-060 Probe O ours: `--ctor-probe=0x821B6DF4 -n 500M` → **0 fires**.
- Body: single `bl 0x82183B78` (an EH support routine) then return.

## Activation

C++ exception runtime dispatch. Fires iff a try-block protected by the FuncInfo at `0x820C1990` catches a thrown object whose type matches the catch's CatchTypeInfo. Neither engine throws this exception at the probed horizon.

## Static graph

- Static callers: **0** — and this is correct (0 callers does not imply dead; it implies "not a bl target").
- Callees: `sub_82183B78` (EH support routine).
- xrefs in DB will show `kind=indirect` or absent entries; the `.rdata` reference at `0x820C1994` is the FuncInfo binding, not a call edge.

## Audit log

- **AUDIT-060 (2026-05-12)** — disassembled body; identified MSVC catch-handler prolog; cross-referenced `.rdata` bytes to find FuncInfo magic `0x19930522`; probed in both engines at 240s/-n500M → 0 fires both sides. AUDIT-058's "caller ladder" framing falsified. New reading-error class #16 logged. [confirmed]
- **AUDIT-058 (2026-05-10)** — claimed as "top of static caller ladder" for `sub_825070F0` activation, walked: `sub_825070F0 ← sub_824F7800 ← sub_824F7CD0 ← sub_824F8398 ← sub_821B55D8 ← sub_821B6DF4`. All 6 fire 0× in ours; framed as missing activation. [STATUS: falsified by AUDIT-060 — the entire 6-fn chain is C++ EH unwind metadata; none of them are normal call edges; they fire only on specific exception throws.]

## Open questions

- What exception type-id activates this catch? Parse the FuncInfo struct at `0x820C1990`:
  - TryBlockMap entries → CatchTypeArray pointer → CatchType records (each has type_info* + handler ptr).
  - The type_info string would identify the C++ class being caught.
- Is the matching throw site reachable in either engine at *any* boot horizon? If yes, when?
- Are the other 5 fns in the AUDIT-058 ladder ALL catch-handler thunks? Spot-check `sub_821B55D8`, `sub_824F8398`, `sub_824F7CD0`, `sub_824F7800`, `sub_825070F0`. (`sub_825070F0` DOES fire 1× per AUDIT-058 — so at least it's not pure-EH; could be the actual throw site or a normal-call leaf.)

## Cross-references

- FuncInfo location: `.rdata:0x820C1990` (start of struct), `0x820C1994` contains this fn's pointer.
- `.pdata` unwind: `0x8211C678`.
- Body callee: `sub_82183B78` (EH support).
- Companion ladder fns (need separate dossiers): `sub_821B55D8`, `sub_824F8398`, `sub_824F7CD0`, `sub_824F7800`, [sub_825070F0](sub_825070F0.md).
- Audits: 058, 060.
- Artifacts: `audit-runs/audit-060-fnptr-array-bootstrap/canary-sub821B6DF4-120s.log`, `canary-sub821B6DF4-240s.log`, `ours-summary.md`.