#include <stdio.h>
size_t wcrtomb(char *restrict s, wchar_t wc,
mbstate_t *restrict ps);
If s is a null pointer, the wcrtomb() function shall be equivalent to the call:
wcrtomb(buf, L'\0', ps)
where buf is an internal buffer.
If s is not a null pointer, the wcrtomb() function shall determine the number of bytes needed to represent the character that corresponds to the wide character given by wc (including any shift sequences), and store the resulting bytes in the array whose first element is pointed to by s. At most {MB_CUR_MAX} bytes are stored. If wc is a null wide character, a null byte shall be stored, preceded by any shift sequence needed to restore the initial shift state. The resulting state described shall be the initial conversion state.
If ps is a null pointer, the wcrtomb() function shall use its own internal mbstate_t object, which is initialized at program start-up to the initial conversion state. Otherwise, the mbstate_t object pointed to by ps shall be used to completely describe the current conversion state of the associated character sequence. The implementation shall behave as if no function defined in this volume of IEEE Std 1003.1-2001 calls wcrtomb().
If the application uses any of the _POSIX_THREAD_SAFE_FUNCTIONS or _POSIX_THREADS functions, the application shall ensure that the wcrtomb() function is called with a non-NULL ps argument.
The behavior of this function shall be affected by the LC_CTYPE category of the current locale.
The wcrtomb() function shall return the number of bytes stored in the array object (including any shift sequences). When wc is not a valid wide character, an encoding error shall occur. In this case, the function shall store the value of the macro [EILSEQ] in errno and shall return (size_t)-1; the conversion state shall be undefined.
The wcrtomb() function may fail if:
The following sections are informative.
mbsinit() , the Base Definitions volume of IEEE Std 1003.1-2001, <wchar.h>