Many years ago, glibc did something along those lines for the _res variable (with preprocessor magic instead of linker magic). For the main thread, legacy _res

Many years ago, glibc did something along those lines for the _res variable (with preprocessor magic instead of linker magic). For the main thread, legacy _res (the actual global data symbol) and new thread-local _res (actually *__res_state()) are the same object, but they diverge for subsequently created threads. I don't think this would work here because it likely changes semantics too much, and not all binaries that need a thread-safe getenv/setenv combination can be rebuilt, especially since compatibility with both variants from the binaries would likely some changes to each application/library.