why "Reentrant covers for OS subroutines" use global errno?

[David Paterson]

Oh, I expect you've probably seen these, but a couple of resources I
found useful
when I started porting Newlib are :-

http://www.embecosm.com/download/ean9.html

and

http://neptune.billgatliff.com/newlib.html

(There's a downloadable PDF of the latter, but I can't find a link to
it at the moment.)

Cheers,

David P.