This is the prototypical C library for the GP3 I/O board from AWC (see http://www.awce.com/gp3.htm)
On Windows you may prefer to use the ActiveX controls provided for the kit in both control form (for VB, Delphi, VC++, etc. or as an ActiveX DLL (useful for WSH, ASP, etc.). In addition, there are older binary-only standard DLLs that are not generated from this source.
The strength of this library is that it has two parts: a portable part, and a platform-specific part. AWC provides platform-specific parts for Linux, Win32, and Cygwin. It should be relatively easy to port the library to other C platforms by simply rewriting the platform- specific part.