cmark is the C reference implementation of CommonMark, a rationalized
version of Markdown syntax with a spec.

It provides a shared library (libcmark) with functions for parsing
CommonMark documents to an abstract syntax tree (AST), manipulating
the AST, and rendering the document to HTML, groff man, LaTeX,
CommonMark, or an XML representation of the AST.  It also provides a
command-line program (cmark) for parsing and rendering CommonMark
documents.