You mean <ctype.h>, right?

I don't use UNIX so I don't know about it's man pages.
On UNIX (or Linux, or Cygwin -- an implementation of bash for Windows), you can generally type
Code:
$ man function
and get information about that function. There are man pages for programs, functions, system calls, etc, divided into 7 sections by type. (For example, printf() has two man pages, for the program and for the function. To access the man page for the function, type man 3 printf.)