If dest is not a NULL pointer, the wcsrtombs function converts the wide-character string *src to a multibyte string starting at dest. At most len bytes are written to dest. The shift state *ps is updated. The conversion is effectively performed by repeatedly calling wcrtomb(dest,*src,ps), as long as this call succeeds, and then incrementing dest by the number of bytes written and *src by one. The conversion can stop for three reasons:
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 behaviour of wcsrtombs depends on the LC_CTYPE category of the current locale.