C frontend: support -D<name>[=<value>] command-line macro defines

• Type: feature (C frontend / driver) — Track C (compiler/cpreproc.inc predefines
  • the driver arg parse).
• Status: done

What

pxx does not honour -D on the command line for C. During the duktape bring-up, -D__STDC_VERSION__=199901L had no effect (a predefined-macro probe still reported it undefined), so there was no way to inject or override a preprocessor macro without editing cpreproc.inc and rebuilding the compiler.

Why it matters

-D is the standard knob for configuring third-party C: feature flags, DUK_USE_* / SQLITE_* / -DNDEBUG-style build options, and quick bisection of macro-gated code paths during corpus bring-up. Every corpus target (sqlite, tcc, duktape, …) is normally built with a handful of -Ds; today we can only approximate that by baking defaults into cpreproc.inc.

Scope

• Parse -D<name> (value defaults to 1) and -D<name>=<value> in the C driver, and seed them as predefined macros in CPreprocess alongside the built-in __STDC__ / __x86_64__ / __LP64__ / __STDC_VERSION__ literals (see [[bug-c-preproc-missing-stdc-version-predefine]]).
• Optionally -U<name> to undefine.
• Check whether the Pascal frontend already accepts -D (conditional-define switches): if so, mirror the plumbing rather than adding a parallel path.

Not urgent — no corpus target is blocked solely on this today (duktape's two real walls were the predefine + the macro-arg string bug, both fixed directly). Nice-to-have that pays off across the whole C corpus.

[[bug-c-preproc-missing-stdc-version-predefine]] · [[feature-c-corpus-duktape]]

RESOLVED 2026-07-10

-D<name>[=<value>] and -U<name> now seed the C preprocessor. The driver already parsed -D/-U but routed them only to the Pascal name-only define table; added a parallel raw capture (CCmdDefRaw/CCmdUndefRaw, defs.inc) applied in CPreprocess via CPApplyCmdlineDefines — after the built-in __STDC__/__STDC_VERSION__/… literals (so -D overrides a builtin, newest-wins) and undefs last (so -U beats a same-name -D). -D<name> with no = defaults to 1 (cpp convention); values are strings (-DBAR=42, -D__STDC_VERSION__=199901L). -U reuses CPUndef (kills all stacked entries), so it can remove a predefined builtin.

Verified: -DFOO→1, -DBAR=42, -DNDEBUG, -U__STDC_VERSION__ drops the builtin. Regression test/cdefine_flag_b239.c (Makefile test-core, built with -DGUARD=42 -DON -DOFFME -UOFFME, exit 42). Self-host byte-identical; testmgr quick GREEN; c-testsuite 220/220.

Log

• 2026-07-10 — resolved, commit fb1baedf.