If dest is not a NULL pointer, the mbstowcs function converts the multibyte string src to a wide-character string starting at dest. At most n wide characters are written to dest. The conversion starts in the initial state. The conversion can stop for three reasons:
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.
The behaviour of mbstowcs depends on the LC_CTYPE category of the current locale.