yes, there are a number of functions to use to call an outside program. you can use exec() to call GNU tar. i think i understand what you want to do, youd like to actually write the code to uncompress the archive your self, while yes you can do that, it is (arguably) a large amount of work that you dont have to do, you already have tar. if you really want to write your own archiver/decompressor you could always look at the source of gnu tar to start you off.

let me add though, that if you just want to let a user pick an archive and read the text file inside of it from within your program then all you need to do is call tar from exec().