# Showcase 15 — IOChan over files + strict signature checks

Tags `v3-showcase15`, `v3-showcase15-summary` (this doc).
Suite **131/131**; fixpoint **OK** (2,103,316 bytes).

## What it runs

`compiler/tests/showcase15.mod` (`Showcase15`, exit 42) links a
DEFINITION/IMPLEMENTATION helper plus four stdlib modules:

```
IMPORT Showcase15Lib, IOChan, TextIO, Strings;
```

1. **Separate compilation + strict 235** — `Showcase15Lib` is a
   classic `DEFINITION`/`IMPLEMENTATION` pair; its signatures are
   verified by the new check (`InitPoint`, `SumPoint`,
   `AppendInt`).
2. **Records and VAR formals** — `Point`, `InitPoint(VAR p …)`.
3. **String concatenation/assignment** — builds the report line
   `"point " + <int>` via `Strings.Assign` + `AppendInt`.
4. **IOChan over files** — `OpenWrite` a channel, write `"total"`,
   `"point <n>"`, `Close`, then `OpenRead` and `ReadString` the lines
   back, checking both.

Output:

```
read back: point 42
showcase15 total=42
```

## Bug found and fixed (shim)

`m2freadline` originally set the descriptor's **count header** to the
number of bytes read. But `HIGH(s)` of an open `ARRAY OF CHAR` formal
reads that same header, so `HIGH(s) + 1` (used by `IOChan.ReadString`
as the capacity) collapsed to the previous line's length after the
first read — the second line was truncated (`"point"` instead of
`"point 42"`).

`m2freadline` now **leaves the capacity header intact** and returns the
byte count instead, so a caller may keep using `HIGH`. (`ReadString`
only needs the NUL-terminated content.)

Also, `max` is now declared `int` in C: Modula-2 `CARDINAL` arguments
arrive in a 32-bit register (QBE `w`), so only the low 32 bits of the
C `long` parameter are defined.

## Registration

`rm -f gen_ssa/_showcase15.txt` before and after;
`expect_run_files_out Showcase15 42 "read back: point 42" …` with the
helper and the `SysShim`/`Utf8`/`iochan`/`textio`/`strings`/
`conversions` modules.

## Files

`compiler/tests/showcase15.mod`,
`compiler/tests/showcase15lib.def`/`.mod`,
`compiler/run_tests.sh`, `runtime/syslib/shim.c`,
`docs/summary_showcase15.md`.