Thank you Phillip because, I have pored over the man pages in depth and have seen the words which Howard has pointed out. What I have not seen... not one word anywhere... that says "and these default apply to the 'C' library calls". I fully well understand that the environment variables are used to augment the file-based values. I have scripts that take advantage of that actual capability. Have had them for a while. That is not what I've been asking here.
It's not a big deal and I have followed the source code through those sections that you've mentioned. I did finally get that. It's actually a very nice feature and should be made clear in the man pages for the 'C' library calls whenever it is practical for someone to do so.
Actually, looking at the source code is a great learning experience. I encourage it. I understand the library much better now than I did just a couple weeks ago. Still much to learn though. My next learning (by looking at examples) will be how to actually use SASL EXTERNAL in "C". There is a good bit of info in the man pages but not comprehensive. I'm looking at ldapwhoami and the common.c (etc) code. Lots of good stuff there.
Thanks,
Frank