[cairo] SUMMARY of Implementing cairo_read_func_t/cairo_write_func_t properly

[Bill Spitzak]

On Wed, Mar 9, 2016 at 12:30 AM, Bernhard R. Fischer <bf at abenteuerland.at>
wrote:

> On 2016-03-04 19:16, Bryce Harrington wrote:
> > On Thu, Mar 03, 2016 at 03:52:12PM +0100, Bernhard R. Fischer wrote:
> >> On 2016-03-01 06:58, Bernhard R. Fischer wrote:
> >>> The conclusion of this discussion was to slightly adapt the meaning of
> >>> the return value of the read_func_t as follows:
> >>>
> >>>
> >>> 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. On error,
> >>> CAIRO_STATUS_READ_ERROR is returned (which is a positive value).
> >>>
> >>>
> >>>
> >>> If there is the consent of the core devs we should adapt the docs.
> >>>
> >>> I'll have a look at cairo_image_surface_create_from_png_stream() if
> >>> there is a change necessary.
> >>>
> >>> Bernhard
> >>>
> >>
> >>
> >>
> >> As promised, I just had a look at implementation of the PNG code of
> >> Cairo (cairo-png.c).
> >>
> >> If a read_func() returns a negative value as suggested, the whole
> >> program will be aborted because of a call to assert() in
> >> _cairo_surface_create_in_error() (cairo-surface.c) which in turn is
> >> called by the PNG error handler png_simple_error_callback() in
> cairo-png.c.
> >>
> >>
> >> I suggest the following tiny patch (to cairo-1.14.6) to fix this:
> >>
> >>
> >> *** cairo-png_orig.c    2016-03-03 15:41:35.565400515 +0100
> >> --- cairo-png.c 2016-03-03 15:42:05.301547969 +0100
> >> ***************
> >> *** 525,530 ****
> >> --- 525,531 ----
> >>       png_closure = png_get_io_ptr (png);
> >>       status = png_closure->read_func (png_closure->closure, data,
> size);
> >>       if (unlikely (status)) {
> >> +        if ((int) status < 0) status = CAIRO_STATUS_READ_ERROR;
> >>         cairo_status_t *error = png_get_error_ptr (png);
> >>         if (*error == CAIRO_STATUS_SUCCESS)
> >>             *error = status;
> >
> > This would probably need to look more like:
> >
> >     status = png_closure->read_func (png_closure->closure, data, size);
> >     if (unlikely (status)) {
> >         cairo_status_t *error;
> >         if ((int) status < 0)
> >             status = CAIRO_STATUS_READ_ERROR;
> >         error = png_get_error_ptr (png);
>
>
> Why? It makes no difference.
>

The variables have to be declared first before any statements (required by
older C compilers).
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