libreadtags is a library for reading tags files generated by ctags. libreadtags assumes the tags file format explained in tags(5) of Universal-ctags.
See the comments at the top of readtags.h about the license and copyright. libreadtags was derived from readtags.c of Exuberant-ctags.
NEWS.md describes note worty changes.
If you are looking for command line interface for tags files, you will like readtags command shipped as part of Universal-ctags. It uses libreadtags extensively.