If dest is not a NULL pointer, the wcstombs function converts the wide-character string src to a multibyte string starting at dest. At most n bytes are written to dest. The conversion starts in the initial state. 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 (size_t)(-1) is returned.
2. The length limit forces a stop. In this case the number of bytes written to dest is returned, but the shift state at this point is lost.
3. The wide-character string has been completely converted, including the terminating L'0'. In this case the conversion ends in the initial state. The number of bytes written to dest, excluding the terminating '0' byte, is returned.
The programmer must ensure that there is room for at least n bytes at dest.
If dest is NULL, n 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.
The behaviour of wcstombs depends on the LC_CTYPE category of the current locale.