A number of libc functions return a pointer to an internal thread-local buffer, which is invalidated on subsequent calls. If the function copies the environment variable's value to such a buffer while holding the mutex controlling access to the global state, then the returned value is guaranteed to remain unaffected by other threads.

There are, however, other problems (discussed elsewhere in this thread) that complicate such an API in the context of getenv().

Requiring you to hold a mutex to safely call the function is an API change

I was not suggesting the caller hold a mutex, but rather getenv(), which eould be transparent to the caller.

Don’t you need C++11?