FreeBSD's __sflags reads the access character, then b, +, x, e and v, and
stops at the first other character without failing, so "rt" opens for
reading. On Windows fopen inserted b after the first character and passed
the rest on, so "rt" became "rbt", the CRT rejected it with EINVAL, and
fopen threw. vkQuake opens history.txt with "rt" at startup. freopen
accepted only a fixed list of modes.
Both now parse the mode like __sflags. An unknown access character, or x
on a read-only stream, returns NULL with EINVAL; x on an existing file
returns NULL with EEXIST; e and v are accepted and ignored.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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