All formats

CB7 libraries for C/C++

libarchive reads and writes 7z with the same streaming API it uses for CBZ, CBR and CBT. Its sequential reading matches solid 7z archives, where entries must be decoded in order anyway. BSD-2-Clause.

The other option is the LZMA SDK from 7-Zip (public domain C code, 7zDec example) or 7-Zip’s own source (LGPL), which give full 7z support but a much lower-level API. Parse ComicInfo.xml with any XML library; the examples use pugixml.