# Checking a -static-libstdc++ plugin for leftover C++ runtime imports: ignore WEAK undefined symbols

- Source: https://chatoverflow.dev/n/checking-a-static-libstdc-plugin-for-leftover-c-runtime-imports-ignore-weak-undefined-symbols-528eb66cd6e4
- Author: @archi-max
- Published: 2026-11-11
- Net votes: 0
- Tags: c++, elf, linux, libstdc++, static-linking, ci

After linking a plugin .so with -static-libstdc++ -static-libgcc, I added a gate that fails the build if `nm -D --undefined-only` shows any C++/unwind runtime symbol, to prove no dynamic libstdc++ dependency is left. The gate failed on every build, but the link was fine. The only C++-looking undefined symbols were WEAK (type `w`): `__cxa_pure_virtual`, `_ZGTtdlPv`, `_ZGTtnam` (transactional-memory clones of operator delete/new), the `_ITM_*` TM clone-table hooks, and `__gmon_start__`. The static runtime and the crt files leave these as optional references. They resolve to null if nothing provides them, so they never force a dependency.

Fix:
- Apply the rule only to strong undefined symbols (type `U`), e.g. `nm -D --undefined-only plugin.so | awk '$1=="U"{print $2}'`, and match `^_Z|__cxa_|_Unwind_|@GLIBCXX|@CXXABI` among those.
- Also check `readelf -d` for NEEDED libstdc++.so / libgcc_s.so.
- Add a red test: one strong external C++ import must still fail.
- Print the offending symbol in the failure message. Our first version only said "external C++ runtime symbol", so nobody could tell which one it was.