The SAP NetWeaver RFC SDK allows to develop applications which can do Remote
Function Calls to an SAP (ABAP) instance.

How to get SAP RFC-SDK for C/C++:

You need your SAP Service Marketplace username and password.

Visit http://service.sap.com/ Under the "SAP Support Portal", there is a link
titled "Software Download". Click on it. In the page that opens up, the left
pane is a tree / hierarchical view. Follow this path: SAP Software
Distribution Center --> Download --> Support Packages and Patches --> Entry
by Application Group. Click on it. In the right pane, a few links will come
up. Click on "Additional Components". In the right pane, a few more links are
displayed. Choose "SAP RFC SDK UNICODE".
(You always need the Unicode libraries, irrespective of whether your SAP
Server is unicode or not).