#include <stdlib.h> size_t mbstowcs(wchar_t *dest, const char *src, size_t n);
1. An invalid multibyte sequence has been encountered. In this case (size_t)(-1) is returned.
2. n non-L'\0' wide characters have been stored at dest. In this case the number of wide characters written to dest is returned, but the shift state at this point is lost.
3. The multibyte string has been completely converted, including the terminating '\0'. In this case the number of wide characters written to dest, excluding the terminating L'\0' character, is returned.
The programmer must ensure that there is room for at least n wide characters at dest.
If dest is NULL, n is ignored, and the conversion proceeds as above, except that the converted wide characters are not written out to memory, and that no length limit exists.
In order to avoid the case 2 above, the programmer should make sure n is greater or equal to mbstowcs(NULL,src,0)+1.
The function mbsrtowcs() provides a better interface to the same functionality.