primitive to fill a buffer

Hello, Please excuse me if this is the wrong forum for this post. I'm looking for help in coding primitives. If there is a better forum please let me know. I want to open a file with a parser written in C and fill a buffer with those contents. I don't understand how to do this. I haven't found any examples in the Xemacs Internals Manual. I briefly looked over fileio.c, but it's so involved. Where should I start? Are there any examples of this I could review? Thanks, Brian Willkie