cpio(5) is an archive format similar to tar(5), and is compatible with the cpio(1) tool.
The cpio package API is almost identical to the archive/tar package API from the Go standard library.
This library only reads/writes the "newc" archive format. (There are many legacy cpio header formats, but cpio usage is so unusual these days that implementing all of them would likely be a colosal waste of time.)
You can read/write/list archives of this format using the cpio(1) tool.
Linux initramfs images are compressed cpio archives, so a cpio library might be useful to you if you're trying to do unholy things from afar during network boot. (That was my motivation for writing the package, anyway...)