# V3 step 5.1 — foreign functions + SysIO (done 2026-09-22)

First stdlib landing: the compiler can now call the host. Suite
88/88 (45 run — incl. 4 trap-aborts, 4 sessions, 1 session-output
— + 43 reject). LL(1)-clean, zero gm2 warnings.

## EXTERNAL procedures (FFI)

- Grammar: `PROCEDURE P(...); EXTERNAL;` — a body-less procedure
  in an implementation. `MarkExternal` flags the current heading;
  its header/scope are discarded (`AbortFunc`), so no function is
  emitted.
- SymTab: `ext` + `link` on the procedure node; `MarkExternal`,
  `IsExternal`, `ProcLink`. The flag rides the node, and the
  implementation resumes the *definition's* node (`ResumeProc`), so
  clients that `IMPORT`/`FROM`-import the routine also see it as
  external.
- QbeGen: `Mangled` binds an external procedure to its link name
  (default: its own name — a valid C symbol; `m2write` etc.).
- **No static link for C callees**: `CallBegin`/`CallEnd` gained an
  `isExt` flag; the hidden static-link argument is omitted for
  external calls (the shim signature has no link).

## Runtime shim (`runtime/syslib/shim.c`)

Small C layer that knows the compiler's memory layout. A string
(`ARRAY OF CHAR`) value is a descriptor `{ LONGCARD count, bytes }`;
the compiler passes its address as an `l`. Shim exports:
`m2write(desc)`, `m2writeln()`, `m2writeint(v)`, `m2exit(code)`.
Linked into every session image by the harness.

## SysIO library (`tests/sysio.def` + `.mod`)

Classic `DEFINITION`/`IMPLEMENTATION` (separate compilation):
`Write(VAR s : ARRAY OF CHAR)`, `WriteLn`, `WriteInt(n)`, each a
thin Modula-2 wrapper over an `EXTERNAL` shim routine. This also
exercises passing a **string literal** as a `VAR ARRAY OF CHAR`
actual: literals now register their static descriptor address in
the actual-address note table.

## Tests

`Hello` → stdout `Hello, Modula-2!` then `42`, exit 0 — the first
V3 program with real output, checked by the new
`expect_run_files_out` harness (exit code + first output line).
Harness refactor: `build_session` (compile→qbe→cc with the shim)
shared by `expect_run_files` / `expect_run_files_out`.

## Deferred to 5.2+

`SYSTEM`/`Storage` (`malloc`/`free` wrappers), `Strings`,
`TextIO`, `WholeIO`, `Math`, `Files`; explicit external link names
(`EXTERNAL "c_name"`); string escapes (so literals are raw bytes —
newlines come from `WriteLn`); reading input.