Work around Windows ftell() bug as per Microsoft engineering's suggestion
See
@@ -237,6 +237,15 @@ static long file_ctrl(BIO *b, int cmd, long num, void *ptr)
_setmode(fd, _O_TEXT);
else
_setmode(fd, _O_BINARY);
+ /*
+ * Reports show that ftell() isn't trustable in text mode.
+ * This has been confirmed as a bug in the Universal C RTL, see
+ * https://developercommunity.visualstudio.com/content/problem/425878/fseek-ftell-fail-in-text-mode-for-unix-style-text.html
+ * The suggested work-around from Microsoft engineering is to
+ * turn off buffering until the bug is resolved.
+ */
+ if ((num & BIO_FP_TEXT) != 0)
+ setvbuf((FILE *)ptr, NULL, _IONBF, 0);
# elif defined(OPENSSL_SYS_MSDOS)
int fd = fileno((FILE *)ptr);
/* Set correct text/binary mode */
