#include <wchar.h> size_t wcsrtombs(char *dest, const wchar_t **src, size_t len, mbstate_t *ps);
1. A wide character has been encountered that can not be represented as a multibyte sequence (according to the current locale). In this case *src is left pointing to the invalid wide character, (size_t)(-1) is returned, and errno is set to EILSEQ.
2. The length limit forces a stop. In this case *src is left pointing to the next wide character to be converted, and the number of bytes written to dest is returned.
3. The wide-character string has been completely converted, including the terminating L'\0' (which has the side effect of bringing back *ps to the initial state). In this case *src is set to NULL, and the number of bytes written to dest, excluding the terminating '\0' byte, is returned.
If dest is NULL, len is ignored, and the conversion proceeds as above, except that the converted bytes are not written out to memory, and that no length limit exists.
In both of the above cases, if ps is a NULL pointer, a static anonymous state only known to the wcsrtombs function is used instead.
The programmer must ensure that there is room for at least len bytes at dest.
Passing NULL as ps is not multi-thread safe.