whprintf contains a printf-like implementation which supports aribtrary data destinations (e.g. GUI widgets, streams, or memory buffers).
Authors: many, probably. This code supposedly goes back to the early 1980's.
Current maintainer: Stephan Beal (http://wanderinghorse.net/home/stephan)
License: Public Domain.
The primary functions of interest are whprintfv() and whprintf(), which works similarly to printf() except that they take a callback function which they use to send the generated output to arbitrary destinations. e.g. one can supply a callback to output formatted text to a UI widget or a C++ stream object.
Downloading: see the downloads page