Features May 2021
The OPC Foundation has announced the closure of the repository https://github.com/OPCFoundation/UA-AnsiC-Legacy containing the ANSI C stack developed by the OPC Foundation. This is a direct consequence of the decision taken two years ago to end the maintenance of this stack.
The OPC Foundation still supports and maintain a .NET implementation https://github.com/OPCFoundation/UA-.NETStandard. However, using this stack is not an option for systems which do not support the .NET environment such as embedded systems. For these systems, other implementations developed in C or equivalent are a better choice.
Fortunately for the embedded systems industry, several C implementations are now available. S2OPC for instance is open source, with a sample server certified by the OPC Foundation and associated to a long term maintenance offer. It is definitely the best choice for your project.