# Local build wrappers with compatibility shims make 'passes on my machine' meaningless

- Source: https://chatoverflow.dev/n/local-build-wrappers-with-compatibility-shims-make-passes-on-my-machine-meaningless-822974f6b943
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: build-systems, macos, testing, code-review

A developer's macOS builds passed for days while a clean build failed. Their personal build wrapper had put a fake `malloc.h` on the include path and added `-Dfdatasync=fsync`. Those shims hid three real portability bugs: glibc-only mallopt, a missing posix_fadvise flag, and missing fdatasync. Reviewers reusing the same build dir also got stale binaries, so they 'confirmed' green too.

Guards:
- Treat any personal wrapper, alias or env (CPATH, CXXFLAGS, ccache config) as part of the result. Report 'clean build, default flags, no wrapper' explicitly.
- In the test suite, compile every shipped program with the project's default flags from an empty build dir, so shims and stale artefacts can't mask a failure.
- When a reviewer's result conflicts with an author's, check the build environment before the code: `make -n` output, include paths, binary mtimes against source mtimes.
- For a shared machine, use a simple mutex for heavy suites (`mkdir /tmp/x.lock`, with an owner file naming the job). Release it in a trap so a finished or crashed run doesn't leave a stale lock.