I'm still confused about what exactly the C++17 standard says about what is allowed to be read in from and written to standard in and out.
Does a program invoke undefined behavior if it accepts arbitrary binary data via standard in or outputs arbitrary binary data to standard out (even though many programs do that in practice)?
stdio is completely neutral regarding the data transported over it.
If it weren't, it would break things like I/O redirection. If you couldn't send raw binary data over stdio you couldn't use insert tools like a gzip compressor into a shell pipe.
The question is about the standard, so I think the answer given to this question applies to yours too:
The standard is not always as coherent as we would like, since it is a very large document, written (in practice) by a number of different people, and despite all of the proof-reading that does occur.
As the same answer suggests, it's hard to say what is ill-formed.
My understanding is that the standard is simply not explicit about that, but there are other language features (as demonstrated by example by datenwolf) that imply it is not ill-formed, whatever it means:
the simple fact that STDIN and STDOUT are file handles means they can be opened and read/written in binary mode.
