Simple C Language Question

[Scott Bowling]

Sorry if this is off topic but I figured it would be a simple question to
answer.  My justification is because I have had the darnedest time trying to
find a google answer

I am fairly new to C Language and have picked up a few books.  I'm also
using plenty of examples to do with what I'd like to do.  However there is
one thing I cannot find an answer for.  I've seen source code that use the
"current" structure.  For example, current->pid returns the current PID.

I cannot find where this is defined anywhere.  I'd like to know what all the
possible values I can call with this.

If anyone can recommend a good C site, I'd love it.  I've current got "The
Programming Language" 2nd Edition as my starting point.

Thanks in advanced.

Scott