The point was about adding a mutex inside libc in getenv and setenv. That way, every codebase in existence automatically gets this safety. The poster I was replying to claimed that this wouldn't help, because it would still not offer thread safety when doing multiple operations.

I pointed out that, in addition to libc setenv/getenv using a mutex internally, they could also expose new functions to allow transactional access for anyone that really needs it - though I suspect that is a vanishingly small minority.