The btowc() function shall determine whether c constitutes a valid
(one-byte) character in the initial shift state.
The behavior of this function shall be affected by the LC_CTYPE category
of the current locale.
The btowc() function shall return WEOF if c has the value EOF or
if (unsigned char)c does not constitute a valid (one-byte)
character in the initial shift state. Otherwise, it shall return the
wide-character representation of that character.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html