> Support questions or not, just tell people "set _REENTRANT... the
> pthread man pages tell you to do so."

And if they want to use a third-party library like Motif, tell them
they're screwed?

Multiple worlds of incompatible objects suck.  Especially if you only
find out about mixing them in terms of subtle run-time failures.

> If you want to do the equivalent to setting _REENTRANT
> unconditionally, that adds a performance hit to explicitly
> single-threaded code (as opposed to just the guts of libraries, such
> as libc, which are compiled that way to be shareable between
> single-threaded and MT code).  See <errno.h>, for example.

If you're asking for the value of errno, you probably aren't in a
performance-critical situation.  getc and putc are much more worth
worrying about.