I ran a set of existing implementations under Linux with input redirected to a file containing:
(write (string-length "abc
def"))where the line break is CR+LF.
Racket, Gauche, Chicken, Bigloo, Scheme48/scsh, Guile, SISC, Chibi, STklos, SigScheme, Scheme 9 report 8.
MIT, Kawa, Chez, SCM, Ikarus, Larceny, Ypsilon, Mosh, IronScheme, Sagittarius report 7.