You can of course choose your own E type so long as it has standard layout and is trivially copyable. You would declare that with API NOT FOUND IN DOCS result_c/#standardese-CXX_DECLARE_RESULT, refer to it with API NOT FOUND IN DOCS result_c/#standardese-CXX_RESULT and need to do your own decoding of errors from your E type. By using the _EC postfixed macros, you are in fact using E =

struct cxx_error_code
  int code;
  void *category;

… which is declared for you by result.h.

Outcome’s C support was designed assuming that you do not wish to penalise the performance of C++ code calling these C-compatible C++ functions, but if you don’t care about that, then always setting errno to any error in a result<T> is easily implemented and thus the natural C use pattern becomes available:

extern size_t to_string(char *buffer, size_t bufferlen, int v);
size_t len = to_string(buffer, sizeof(buffer), v);
if(errno != 0)

Implementing this via boilerplate generating script is straightforward, and is left as an exercise to the interested reader.