If s is not a null pointer, mbtowc() shall determine the number of
bytes that constitute the character pointed to by s. It shall then
determine the wide-character code for the value of type wchar_t that
corresponds to that character. (The value of the wide-character code
corresponding to the null byte is 0.) If the character is valid and pwc
is not a null pointer, mbtowc() shall store the wide-character code in
the object pointed to by pwc.
The behavior of this function is affected by the LC_CTYPE category of the
current locale. For a state-dependent encoding, this function is placed into
its initial state by a call for which its character pointer argument,
s, is a null pointer. Subsequent calls with s as other than a
null pointer shall cause the internal state of the function to be altered as
necessary. A call with s as a null pointer shall cause this function to
return a non-zero value if encodings have state dependency, and 0 otherwise.
If the implementation employs special bytes to change the shift state, these
bytes shall not produce separate wide-character codes, but shall be grouped
with an adjacent character. Changing the LC_CTYPE category causes the
shift state of this function to be unspecified. At most n bytes of the
array pointed to by s shall be examined.
The implementation shall behave as if no function defined in this volume of
IEEE Std 1003.1-2001 calls mbtowc().
If s is a null pointer, mbtowc() shall return a non-zero or 0
value, if character encodings, respectively, do or do not have state-dependent
encodings. If s is not a null pointer, mbtowc() shall either
return 0 (if s points to the null byte), or return the number of bytes
that constitute the converted character (if the next n or fewer bytes
form a valid character), or return -1 and may set errno to
indicate the error (if they do not form a valid character).
In no case shall the value returned be greater than n or the value of the
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