Am Fr., 26. Juni 2020 um 04:39 Uhr schrieb John Cowan <xxxxxx@ccil.org>:

The Scheme runtime system may change the errno value outside of
user-controlled code, so the errno of SRFI 199 cannot map directly to
the errno of the C library.

For example, the Scheme system may call a number of C procedures
setting errno while compiling code dynamically or run a GC. And both
can happen, in principle, happen between `set-errno!' and `errno'.

Thus, as currently written, the specification doesn't help too much.
SRFI 199's errno has to be abstracted (or SRFI 199 has to require that
the runtime system of supporting implementations has to be rewritten
in such a way that any change to errno is not visible to user code).