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The GNU programs big and complicated. The size of GNU Hello World shows that even the simplest GNU project needs a lot of code and configuration around it.

The autotools are hard to understand for a beginner, but you don't need to understand them to read the code. Even if you modify the code, most of the time you can simply run make to compile your changes.

To read code, you need a good editor (VIM, Emacs) or IDE (Eclipse) and some tools to navigate through the source. The tar project contains a src directory, that is a good place to start. A program always start with the main function, so do

grep main *.c
or use your IDE to search for this function. It is in tar.c. Now, skip all the initialization stuff, untill

/* Main command execution.  */
There, you see a switch for subcommands. If you pass -x it does this, if you pass -c it does that, etc. This is the branching structure for those commands. If you want to know what these macro's are, run

grep EXTRACT_SUBCOMMAND *.h
there you can see that they are listed in common.h.

Below EXTRACT_SUBCOMMAND you see something funny:

read_and (extract_archive);
The definition of read_and() (again obtained with grep):

read_and (void (*do_something) (void))
The single parameter is a function pointer like a callback, so read_and will supposedly read something and then call the function extract_archive. Again, grep on it and you will see this:

  if (prepare_to_extract (current_stat_info.file_name, typeflag, &fun))
    {
      if (fun && (*fun) (current_stat_info.file_name, typeflag)
      && backup_option)
    undo_last_backup ();
    }
  else
    skip_member ();
Note that the real work happens when calling fun. fun is again a function pointer, which is set in prepare_to_extract. fun may point to extract_file, which does the actual writing.

I hope I walked you a great deal through this and shown you how I navigate through source code. Feel free to contact me if you have related questions.