[cairo] [PATCH] Improve error message when using cairo_image_surface_create_from_png with an invalid file.

Cyril Roelandt tipecaml at gmail.com
Wed Mar 23 21:29:44 UTC 2016

On 03/23/2016 02:37 PM, Uli Schlachter wrote:

> I don't like "grepping" through error messages like this (what if libpng changes 
> the exact wording?) and wondered if there is no better API for this. For the 
> first time in my life I looked at the libpng API (and the API docs) and I hope 
> that I will never do that again...

I don't like grepping either, that's why I called this "dirty" :)

> So second try was to search for "Not a PNG file" in debian codesearch where I 
> found the following:
> http://sources.debian.net/src/vtk/5.10.1+dfsg-2/IO/vtkPNGReader.cxx/?hl=58#L58
> This reads the first 8 bytes of the file (handles short reads correctly!) and 
> uses png_sig_cmp() to check that these bytes are correct (and then it uses 
> png_set_sig_bytes() in line 101 to compensate for these missing bytes).

Couldn't we read the first 8 bytes of the file in
cairo_image_surface_create_from_png() before calling read_png() ? If the
file is not a PNG, then we could return an error code
(CAIRO_STATUS_INVALID_FILE or whatever); if it is a PNG, then we'd pass
it to libpng, which would not fail.

> Having seen this code, I like libpng's API even less, but apparently this would 
> be the "official" way to handle this.
> Perhabs checking the error message isn't so bad after all... I'm unsure.
>  > Maybe we could even have a CAIRO_STATUS_NOT_A_PNG error status. What do
>  > you think?
> The PNG-API in cairo is part of the toy interfaces, which exist to simplify 
> writing examples and tests. They aren't meant to offer all possibilities (use 
> something like gdk-pixbuf for that).
> Thus, I don't think we should have special error codes for the toy API (but I 
> don't mind if someone thinks otherwise and adds them anyway).

I'll trust you on that since I do not really know cairo :)


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