During the weekend, I've found some time to improve documentation for Gammu. One of user accessible things which did lack documentation are native file formats used by Gammu.
There is not that much of them as Gammu supports standard formats for most data (like vCard, vCalendar, iCalendar), but there are things where standard simply does not exists and we need to store that data as well. This is the case for backups (both data and SMS), where SMS backup format is used as well for files backend in Gammu SMSD.
So I went through the SMS backup internals and wrote the documentation. While reading the code I of course discovered quite obvious bug, which was for some reason undiscovered for very long time.