[cairo] Implementing cairo_read_func_t/cairo_write_func_t properly

[Behdad Esfahbod]

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.