C: hand-declared libc prototypes (no #include) silently no-op in libc-free mode

• Type: bug/feature (cfront crtl binding). Track C.
• Priority: HIGH — extern int printf(const char*, ...); COMPILES, LINKS, and the call silently does NOTHING (no output, no error). Classic C89 style, c-testsuite uses it throughout; real code will too.
• Found: 2026-07-06 c-testsuite run.

Failing tests

• 00210 (also parses __attribute__((packed/stdcall)) — that part works)
• 00211, 00215, 00217 — all declare printf by hand, output empty.

Why

crtl impl auto-pull is keyed on the #include <hdr> path; a bare prototype is marked external-against-libc, and a libc-free link leaves it unbound → call is a silent nop. Either bind it or make it a HARD link error — silence is the bug.

Prior art (reverted 2026-07-06 — landed without ticket/review, pulled back out)

Previous session drafted in cparser.inc: after pass 1, scan procs still ProcExternal whose name is a known crtl libc symbol (table name→header for stdio/stdlib/string/ctype/math), synthesize the #include lines, CPreprocess + CLexAppend them, and run the appended region through the pass-1 loop before pass 2 compiles bodies; no-op under --system-libs. Approach worked (took suite 178/220) but needs proper review: table duplication vs crtl manifest, pass-1 re-entry, interaction with CHeaderMode. Rebuild it deliberately from this ticket.

Gate

Drop 00210/00211/00215/00217 from test/c-conformance/pxx.skip; runner green; make test + self-host byte-identical.

Log

• 2026-07-07 — resolved, commit 147087b0.