the problem with the code

[Ted Lemon]

On Aug 6, 2008, at 7:33 AM, DÔ Phan-Cam-Thach wrote:

> I have a problem while reading the code. I dont understand much of  
> struct universe which is copied below.  I find that there are the  
> functions in this structure, ex:
>         struct option_cache *(*lookup_func) (struct universe *,
>                          struct option_state *,
>                          unsigned);
> I dont understand this model much (there are functions in a  
> structure). And i dont understand why dont they put these functions  
> out of the structure instead of in the structure? Besides, i dont  
> understand what this function is used for?

Putting functions in a structure is a way of doing object-oriented  
programming in C.   The idea is that you may have several different  
data objects that behave similarly, but require different underlying  
code to implement that behavior.   So you put the data for these  
objects in a common structure, and you include in that structure  
pointers to the code that implements the object's behavior.

Then a function that uses that object's data can call the  
implementation functions for the object from the structure, rather  
than having to figure out what kind of object it is, and then directly  
call that object's implementation functions.   It's a lot cleaner.

"Universe" is my navel-gazing name for a DHCP option space.   So for  
example, there's a DHCP option space for all the DHCP options.   And  
there's another one for all the Netware/IP encapsulated options.   And  
the FQDN option has its own option space.   This allows you to extract  
parts of the FQDN option when you're writing a configuration file.

If you grep in options.c for the names of the functions that are in  
the universe data structure, you can see how they're used.   They're  
all initialized in places like common/tables.c and server/stable.c (if  
I remember correctly - it's been a while since I wrote this code).