iob_write - send I/O batch through callback

       iob_write  sends the (rest of) b through the callback cb, passing s as first argument.  cb is expected to
       behave like io_trywrite(2).

       This interface is intended to send an I/O batch through a filter, for example to encrypt or compress  it.
       If you just want to send an I/O batch to a socket, use iob_send instead.

       iob_write  returns the number of bytes written, 0 if there were no more bytes to be written in the batch,
       -1 for EAGAIN, or -3 for a permanent error (for example "connection reset by peer").

       The normal usage pattern is using io_wait to know  when  a  descriptor  is  writable,  and  then  calling
       iob_write until it returns 0, -1 or -3.

       If  iob_write  returns 0, terminate the loop (everything was written OK).  If it returns -1, call io_wait
       again.  If it returned -3, signal an error.

       The callback is supposed to behave like write(2), i.e. return the number of bytes written, 0 for EOF,  -1
       for  error  (iob_write will return -3 then).  Return -1 with errno==EAGAIN if using non-blocking I/O when
       we need to wait for the next write event.  iob_write will then return -1.

       iob_write - send I/O batch through callback

       iob_write will continue to call your callback until it returns an error.   So  if  you  are  in  a  state
       machine,  for  example  a  web server using this for SSL support, make sure to write at most n bytes at a
       time (e.g. 64k) and the next time you are called return -1.  Otherwise iob_write might not  return  until
       the whole file is served.

iob_send(3)

                                                                                                    iob_write(3)

#include<libowfat/iob.h>

       typedef int64 (*io_write_callback)(int64 s,const void* buf,uint64 n);

       int64 iob_write(int64 s,io_batch* b,io_write_callback cb);