#include <iconv.h>
size_t iconv(iconv_t cd,
char **inbuf, size_t *inbytesleft,
char **outbuf, size_t *outbytesleft);
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 *outbytesleft by the number of converted output bytes, and it updates the conversion state contained in cd. If the character encoding of the input is stateful, the iconv() function can also convert a sequence of input bytes to an update to the conversion state without producing any output bytes; such input is called a shift sequence. 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, that is, *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 *outbytesleft by the number of bytes written.
A third case 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.