How to create checksum of tarfile?

[JakobDev]

I'm working in a lib for raeding and writing tar files. You can find the source here. The only problem, is that I don't know how to create the checksum for the header record. The other things are working. Does anybody know that?

[Lupus590]

I would look at how another (open source) implementation does this or look for the file format specification.