On Mon, Dec 30, 2019 at 11:24 PM Doug McIlroy <doug@cs.dartmouth.edu> wrote:

The use of %% to designate a literal % in printf is not
a recent convention. It was defined in K&R, first edition.

FWIW, I still have an old (fading) copy of the galley proofs for the first edition. Page 147 (Chapter 7 - Input and Output, Section 7.3 Formatted Output - Printf) first full paragraph on the page:

If the character after the % is not a conversion character, that character is printed: thus % may be printed by %%.

Doug

Ralph Cordery wrote:

Though that may seem odd to our modern C-standardised eyes, it's
understandable in that if it isn't a valid %f, etc., format specifier
then it's a literal percent sign.