The argument cd must be a conversion descriptor created using the
function iconv_open.
The main case is when inbuf is not NULL and *inbuf is not NULL.
In this case, the iconv function converts the multibyte sequence
starting at *inbuf to a multibyte sequence starting at *outbuf.
At most *inbytesleft bytes, starting at *inbuf, will be read.
At most *outbytesleft bytes, starting at *outbuf, will be written.
The iconv function converts one multibyte character at a time, and for
each character conversion it increments *inbuf and decrements
*inbytesleft by the number of converted input bytes, it increments
*outbuf and decrements *outbytesptr by the number of converted
output bytes, and it updates the conversion state contained in cd.
The conversion can stop for four reasons:
1. An invalid multibyte sequence is encountered in the input. In this case
it sets errno to EILSEQ and returns (size_t)(-1). *inbuf
is left pointing to the beginning of the invalid multibyte sequence.
2. The input byte sequence has been entirely converted, i.e. *inbytesleft
has gone down to 0. In this case iconv returns the number of
non-reversible conversions performed during this call.
3. An incomplete multibyte sequence is encountered in the input, and the
input byte sequence terminates after it. In this case it sets errno to
EINVAL and returns (size_t)(-1). *inbuf is left pointing to the
beginning of the incomplete multibyte sequence.
4. The output buffer has no more room for the next converted character. In
this case it sets errno to E2BIG and returns (size_t)(-1).
A different case is when inbuf is NULL or *inbuf is NULL, but
outbuf is not NULL and *outbuf is not NULL. In this case, the
iconv function attempts to set cd's conversion state to the
initial state and store a corresponding shift sequence at *outbuf.
At most *outbytesleft bytes, starting at *outbuf, will be written.
If the output buffer has no more room for this reset sequence, it sets
errno to E2BIG and returns (size_t)(-1). Otherwise it increments
*outbuf and decrements *outbytesptr by the number of bytes
written.
A third state is when inbuf is NULL or *inbuf is NULL, and
outbuf is NULL or *outbuf is NULL. In this case, the iconv
function sets cd's conversion state to the initial state.