#include <stdlib.h> size_t wcstombs(char *dest, const wchar_t *src, size_t n);
1. A wide character has been encountered that can not be represented as a multibyte sequence (according to the current locale). In this case, (size_t) -1 is returned.
2. The length limit forces a stop. In this case, the number of bytes written to dest is returned, but the shift state at this point is lost.
3. The wide-character string has been completely converted, including the terminating null wide character (L'\0'). In this case, the conversion ends in the initial state. The number of bytes written to dest, excluding the terminating null byte ('\0'), is returned.
The programmer must ensure that there is room for at least n bytes at dest.
If dest is NULL, n is ignored, and the conversion proceeds as above, except that the converted bytes are not written out to memory, and no length limit exists.
The function wcsrtombs(3) provides a thread safe interface to the same functionality.