[SDL] How to set an individual pixel to transparent?

[David Bruce]

Hi,

Quick question - if p points to an individual pixel in a 32-bit RGBA 
SDL_Surface, how do I set it to transparent for either a little- or 
big-endian system?

(I'm trying to write a function to round off the corners of rectangles, and I 
think I've figured the rest out, but I'm having trouble with the actual 
manipulation of the pixel data).

Thanks for any help,
-- 
David Bruce