What is the simplest way to write to stdout in binary mode?

You can use setmode(fileno(stdout), O_BINARY)

Wrap it in an ifdef if you want to keep it compatible with Linux.

See also: https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/setmode?view=vs-2017

You can do something like that (which is sort of cross platform):

FILE *const in = fdopen(dup(fileno(stdin)), "rb");
FILE *const out = fdopen(dup(fileno(stdout)), "wb");
/* ... */
fclose(in);
fclose(out);

Or you can use write() and read() system calls directly with fileno(stdin) and fileno(stdout). Those system calls operate on lower level and don't do any conversions. But they also don't have buffering that you get from FILE streams.