The mbsrtowcs function converts a sequence of multibyte characters pointed to indirectly by src into a sequence of corresponding wide characters and stores at most len of them in the
.Vt wchar_t array pointed to by dst, until it encounters a terminating null character (\0).
If dst is NULL, no characters are stored.
If dst is not NULL, the pointer pointed to by src is updated to point to the character after the one that conversion stopped at. If conversion stops because a null character is encountered, *src is set to NULL.
.Vt mbstate_t argument, ps, is used to keep track of the shift state. If it is NULL, mbsrtowcs uses an internal, static
.Vt mbstate_t object, which is initialized to the initial conversion state at program startup.
The mbsnrtowcs function behaves identically to mbsrtowcs, except that conversion stops after reading at most nms bytes from the buffer pointed to by src.
The mbsrtowcs and mbsnrtowcs functions return the number of wide characters stored in the array pointed to by dst if successful, otherwise it returns (Vt size_t-1).
The mbsrtowcs and mbsnrtowcs functions will fail if: