Indeed -- it's an extremely unconvincing list, because any sensible library which may require a library user to set env variables (which includes all the ones I

Indeed -- it's an extremely unconvincing list, because any sensible library which may require a library user to set env variables (which includes all the ones I checked on the list) can also be configured without setting env variables. Most of the time the env variables set fallback defaults for parameters not specified by the caller. In these cases, the sane thing to do, regardless of the thread-safety of setenv(), is simply to supply the parameter in code. The only exception is things like debug logging, which is unlikely even to work dynamically. On the other hand, setenv() is clearly broken in modern code, particularly in a library context, and the man page (at least on my Linux machine) does not make that particularly obvious -- "Thread safety: MT-Unsafe" is the only note, with a reference to attributes(7) for more information. It could definitely be made more obvious.