Re: Understanding a source code file

[Iain Hibbert <plunky%ogmig.net@localhost>]

On Fri, 21 Nov 2014, Rocky Hotas wrote:

> Hoping this is the right section for my question.

maybe, in that it is a technical question relating to a userland program..
however, I think you are asking a basic programming question really

> I would like to correctly understand the contents of a simple source
> file: it is banner.c (/src/games/banner), the version contained in

> It is the source code of a very simple program, as you know.

> In the first lines, there are two "static const int" arrays declared: it
> seems to be a mapping. I can't also understand the format of the "Table
> of stuff to print" between the two arrays.

> First of all, what does this part mean?

this is an encoded and compressed method for printing the letters. you
first look up the letter you want to print in the asc_ptr table, using its
ASCII code (which is 41) as an index (see ascii(7)). Supposing you want to
print an "A" then, you find the value 3072. You use this as the index to
the data_table. The first number is 30, which is n. Since it is not >128
or >64 you look at the next number 4 which is m. You put 4 characters at
position 30. The next number is 132 which is 128 + 4, making n=4 . >128
means print the current line n times, so you do that. The next numbers are
30 and 5, meaning 5 chars at position 30, and 129 meaning print this line
once (128+1). look at the output of /usr/games/banner A and compare it to
what I wrote so far above..

<30 spaces>####
<30 spaces>####
<30 spaces>####
<30 spaces>####
<30 spaces>#####

You carry on until you reach a value >64 when you can stop for that
character.

> I have never analyzed a NetBSD kernel or something similar. Supposing
> that I know the basics of the C language, is there a standard way to
> proceed in the analysis of such source files? (I didn't find useful
> sites talking about this topic)

you run the program in your head. if it is too difficult, then you can add
trace points (maybe edit the program to print something during the run,
maybe use a debugger of some kind). This is one reason why it is good to
split tasks up into smaller sections!

regards,
iain