Subj : sexyz: SIGFPE (divide by zero) when the ZMODEM transmit window is smal
To   : GitLab issue in main/sbbs
From : Rob Swindell
Date : Fri Jul 24 2026 11:14:53

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

`sexyz` dies with **SIGFPE** (integer divide-by-zero) as soon as it starts
sending, whenever the ZMODEM transmit window is smaller than **4x the block
size**.

### Reproduction

```
$ dd if=/dev/urandom of=test.bin bs=1M count=8
$ sexyz -8 -w8192 sz test.bin      # window 8K, block 8K
```

The sender handshakes (ZRQINIT/ZRINIT) and then dies before transferring any
file data; the process exit status is signal 8 (SIGFPE). Anything that reports
the raw wait status shows `-8`. Roughly 3 KB reaches the wire, all handshake.

Also crashes: `-8 -w16384`, `-8 -w32767`, `-2 -w4096`, and the equivalents set
via `MaxWindowSize` in `sexyz.ini`. Does **not** crash when the window is at
least 4 blocks, e.g. `-8 -w32768`.

### Cause

`src/sbbs3/zmodem.c`, in `zmodem_send_from()`:

```c
tx_type = (subpkts_sent % (zm->max_window_size / zm->block_size / 4)) == 0
        ? ZCRCQ : ZCRCG;
```

With `max_window_size == block_size == 8192` the divisor is
`8192 / 8192 / 4` = `1 / 4` = **0**, so the `%` is a division by zero. The
intent is "request an ACK every quarter-window"; a window narrower than four
blocks has no valid quarter-window interval.

### Fix

Clamp the interval to a minimum of 1, so a window smaller than four blocks
requests an ACK on every subpacket.

### Scope

`zmodem.c` is shared with SyncTERM, but **SyncTERM never sets
`max_window_size`** (it leaves it 0, which short-circuits the branch), so this
is not reachable there. The exposure is `sexyz` only, via `-w<size>` or the
`MaxWindowSize` ini key.

### Related, not fixed here

With the divide-by-zero guarded, a window exactly equal to the block size
(`-8 -w8192`) no longer crashes but is pathologically slow -- roughly one
subpacket per second, each iteration logging a one-second receive timeout
followed by `Transmit-Window management: 8192 >= 8192`. It completes, but it
cannot pipeline. That is a separate issue.

---
*Reported and fixed by Claude (Anthropic), working with Rob Swindell.*
 * Origin: Vertrauen - [vert/cvs/bbs].synchro.net (1:103/705)

.