[cairo] Implementing cairo_read_func_t/cairo_write_func_t properly

[Bernhard Fischer]

On Monday 04 January 2016 18:26:28 Behdad Esfahbod wrote:
> On 16-01-04 06:24 PM, Bill Spitzak wrote:
> > On Sun, Jan 3, 2016 at 11:50 PM, Bernhard Fischer <bf at abenteuerland.at
> > 
> > <mailto:bf at abenteuerland.at>> wrote:
> >     On Sunday 03 January 2016 14:29:57 Behdad Esfahbod wrote:
> >     > On 16-01-02 09:34 PM, Bill Spitzak wrote:
> >     > > On 01/02/2016 01:27 PM, Bernhard Fischer wrote:
> >     > >> On Saturday 02 January 2016 13:01:39 Bill Spitzak wrote:
> >     > >>> I think the api has to be changed to return number of bytes
> >     > >>> read, and a
> >     > >>> negative number for eof.
> >     > >> 
> >     > >> The return value should be the number of bytes read and on eof it
> >     > >> is
> >     > >> something between 0 and length. If it is called again at/after
> >     > >> eof it
> >     > >> returns 0 again. A negative value (-1) is only returned in case
> >     > >> of error
> >     > >> (but not EOF). This is exactly like read(3) behaves.
> >     > > 
> >     > > Yes you are correct, I made a typo, I meant to say "error" where I
> >     > > wrote
> >     > > "eof". I fully agree with your description.
> >     > 
> >     > Given that we cannot change the signature now, I suggest we hack it
> >     > up to
> >     > interpret negative numbers returned from the callback to be
> >     > interpreted as a short read; the absolute value of the returned
> >     > negative number is the number of bytes read.
> >     > 
> >     > Cairo itself won't need to be updated, but clients should be
> >     > encouraged to
> >     > implement the new behaviour.
> >     > 
> >     > b
> >     
> >     I agree, that is a backwards compatible solution.
> > 
> > I think you need to make negative number mean "it was this short". Ie if a
> > request to read 10 bytes only reads 8, it returns -2.
> > 
> > The reason is so that reading the entire block still returns
> > CAIRO_STATUS_SUCCESS.
> That works as well.  I was thinking, negative number returned only if not a
> full read.


I have a new idea to stay backwards compatible:
The read function shall return the negative number of bytes which have NOT 
been read, i.e. - (length - bytes_read).

This is that a full read will return 0 which is CAIRO_STATUS_SUCCESS.
Truncated reads return something between -1 and -length.
And on error, of course CAIRO_READ_ERROR is returned.


Bernhard