[FFmpeg-devel] Re: [PATCH 1/2] avcodec/movtextenc: Check for too many styles

[Anton Khirnov]

Quoting Guo, Yejun (2021-02-21 06:31:37)
> 
> 
> > >
> > FYI: The C standard does not mandate the sizes for most types (the uintx_t
> > types are an exception). It also gives the compiler absolute freedom on how
> > much padding to add to a structure. A compiler could very well add megabytes
> > of padding to StyleBox. The numbers you provide only pertain to one (or
> > actually none, see below) implementation, not to all of them. It is like
> > someone claiming to prove a general theorem by only checking it for one
> > example.
> > 
> > Several of your statements are btw not only false for a hypothetical system
> > compliant with the C standard. They are false on common systems:
> > "SIZE_MAX / sizeof(*s->style_attributes) is always larger than uint_max"
> > is wrong on ordinary 32bit systems (where SIZE_MAX and UINT_MAX typically
> > coincide). And "sizeof(*s->style_attributes) is 12". Due to padding it is normally
> > 16; it could be reduced to 12 (for ordinary
> > systems) by rearranging its elements.
> > 
> 
> thanks for the lucid explanation, yes, you're right. 
> (and first to know that megabytes of padding is possible)
> 
> btw, just as a learning, is there possible a real system where uint_max < uint32_max?
> If no, we might say that: size_max >= uint_max >= uint32_max > uint16_max.

We assume int (and hence also unsigned int) is at least 32bit.
But AFAIK the only limit on size_t is that it's at least 16bit,
otherwise it can be smaller than int.

-- 
Anton Khirnov