libxo - A Library for Generating Text, XML, JSON, and HTML Output

The libxo library allows an application to generate text, XML, JSON,
and HTML output using a common set of function calls. The application
decides at run time which output style should be produced. The
application calls a function "xo_emit" to product output that is
described in a format string. A "field descriptor" tells libxo what
the field is and what it means.

WWW: https://github.com/Juniper/libxo