Google-protobuf: How to create unions using Google protocol buffer

In protobuf there is a dedicated structure for that (I'm using it in C++ though, not sure if it will work in pure C):

message MyUnion {
    oneof MyUnionOneof {
        bool booleanValue = 1;
        string stringValue = 2;
    }
}

Check out this link: https://developers.google.com/protocol-buffers/docs/proto#oneof