understanding userspace includes

[cfk]

This is a linux programming question and not exactly a FC question, but please 
bear with me.

I am trying to bridge the gap in my understanding in programming in FC, and 
I'll use usb as an example by expressing what I know and asking for help in 
understanding what I dont know.

I have a program I am trying to understand. It #includes usb.h which I can 
find in /usr/include. I can go to /usr/lib and find libusb.a which I can run 
ar and nm on.

Ar tells me this library contains usb.o (amongst other things), and nm tells 
me this library contains usb_find_busses (amongst other things).

So, I go looking for usb.c using 'find / -iname usb.c' and find three usb.c's, 
all in the kernel source for FC (/usr/src/linux-2.6.10-1.14_FC2_arch).

None of these usb.c files contain the userspace stuff I am looking for. They 
all contain the kernelspace side of the usb stack.

So, where do I go to find the appropriate C source code for a library 
in /usr/lib that is included by an include from /usr/include in Fedora Core?

With Respect, Charles Krinke