iob_bytesleft man page
iob_bytesleft — how many bytes are still to be sent?
uint64 iob_bytesleft(const io_batch* b);
iob_bytesleft returns the number of bytes that still need to be sent in this IO batch. If you just want to see whether you need to call iob_send again, there is no advantage in this function to calling iob_send directly and looking whether it returned 0.