Subj : sexyz: SIGFPE (divide by zero) when the ZMODEM transmit window is smal
To   : GitLab note in main/sbbs
From : Rob Swindell
Date : Fri Jul 24 2026 21:39:05

https://gitlab.synchro.net/main/sbbs/-/issues/1197#note_9694

### Precise trigger condition

Swept it. The stall is **not specific to `-w8192`** — it happens for **any `-w`
value less than or equal to the effective block size**, and nothing else is
relevant.

Threshold tracks the block size, not an absolute 8192:

| Block size flag | Stalls at `-w` | Works at `-w` |
|---|---|---|
| `-8` (8 KB) | ≤ 8192 | 8193 (→ 3.0 MB/s) |
| `-4` (4 KB) | ≤ 4096 | 4097 |
| `-2` (2 KB) | ≤ 2048 | 2049 |
| default (1 KB) | ≤ 1024 | — |

One byte over the block size clears it. Structural reason: with
`window <= block_size` exactly one block fills the window, so only a single
unacked block can be outstanding — pure lockstep, which is the case that hits
the missed-window-opening-ACK 1-second timeout. With `window > block_size` a
second block starts before the first is acked, the ack lands mid-flight, and it
pipelines.

Not relevant: CRC mode (`-o`/CRC-16 still stalls), file size (the low MB/s on a
small file is just startup), telnet/escaping. Only `-w` and the block-size flag
matter.

Relationship to the SIGFPE fixed in `04b47329aa`: that was the wider
`window < 4*block` range (the quarter-window divisor hitting zero). This stall
is the tighter `window <= block` sub-range. Between them —
`block < window < 4*block`, e.g. `-8 -w16384` — the ack-interval is clamped to 1
but it still pipelines and works.

*Note by Claude (Anthropic).*
--- SBBSecho 3.37-Linux
 * Origin: Vertrauen - [vert/cvs/bbs].synchro.net (1:103/705)

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