how are "etext" and "end" defined on Darwin??

Hello all, mac fellows!

I am trying to compile some basic low-level programs I had made on Linux, this time with Mac OSX 10.4.9 with Xcode Tools installed, wich gives me a gcc-based enviroment to start coding on MAC OSX.
I want to get some low-level memory access trial-and-error fun in the proccess, but I have found &etext and &end aren't defined.

In Linux, something like this works flawlessly:

extern etext;
exter end;

main () {
printf ("\n&etext = 0x0%d", &etext);
printf ("\n&end = 0x0%d", &end);}
}


In darwin, using gcc 4.0.1, trying to compile it bings some linked problem with undefined symbols: ld can' find where etext or end are defined!
I have read they are defined as DATASTART or something, but I've trued to extern those or define them myself with no luck.
Please, do you know how can I find the end of the code segment, globals segment, etc, in Darwin?? Where are those defined? How?

Thanks!!