[FFmpeg-devel] [PATCH] fix warning in libavutil/fifo.c

[Michael Niedermayer]

On Sun, Jan 25, 2009 at 03:58:48PM +0100, Diego Biurrun wrote:
> On Tue, Jan 13, 2009 at 12:52:01PM -0800, Baptiste Coudurier wrote:
> > 
> > Michael Niedermayer wrote:
> > > On Mon, Jan 12, 2009 at 10:33:12PM +0100, Diego Biurrun wrote:
> > >> On Mon, Jan 12, 2009 at 09:13:58PM +0100, Michael Niedermayer wrote:
> > >>> On Sun, Jan 11, 2009 at 10:17:18PM +0100, Diego Biurrun wrote:
> > >>>> Here is a patch to fix
> > >>>>
> > >>>> fifo.c:79: warning: cast discards qualifiers from pointer target type
> > >>>>
> > >>>> --- libavutil/fifo.c	(revision 16549)
> > >>>> +++ libavutil/fifo.c	(working copy)
> > >>>> @@ -74,7 +74,7 @@
> > >>>>  
> > >>>> -void av_fifo_write(AVFifoBuffer *f, const uint8_t *buf, int size)
> > >>>> +void av_fifo_write(AVFifoBuffer *f, uint8_t *buf, int size)
> > >>> I belive av_fifo_write does not change the content of buf thus const
> > >>> is correct
> > >> Umm, I beg to differ. av_fifo_write is just a wrapper for
> > >> av_fifo_generic_write, which does modify buf, hence the warning.
> > > 
> > > where does it modify the _CONTENT_OF_ buf?
> > > 
> > > It does not do this anywhere in the case where it is called through
> > > av_fifo_write thus the const is correct.
> > > av_fifo_write can be used with const arrays, if you remove the cont in the
> > > parameter of av_fifo_write() you will have a warning for every use of
> > > av_fifo_write() with a const argument, i doubt you prefer this ...
> > 
> > Would changing the prototype of 'func' to take const void * fix it ?
> 
> As in $attached?
> 
> Diego

> Index: libavutil/fifo.c
> ===================================================================
> --- libavutil/fifo.c	(revision 16768)
> +++ libavutil/fifo.c	(working copy)
> @@ -76,10 +76,10 @@
>  
>  void av_fifo_write(AVFifoBuffer *f, const uint8_t *buf, int size)
>  {
> -    av_fifo_generic_write(f, (void *)buf, size, NULL);
> +    av_fifo_generic_write(f, (const void *)buf, size, NULL);
>  }
>  
> -int av_fifo_generic_write(AVFifoBuffer *f, void *src, int size, int (*func)(void*, void*, int))
> +int av_fifo_generic_write(AVFifoBuffer *f, const void *src, int size, int (*func)(const void*, void*, int))

this breaks the API and causes undefined behavior, src is not a const
in av_fifo_generic_write(), it can be a struct that may contain a pointer
for example that needs updating.

To summarize this
av_fifo_generic_write MUST NOT have a const src
av_fifo_write SHOULD have a const src
gcc prints a warning if you by whatever means change a const to a non
const with the current flags.

solutions are
* close your eyes
* change the flags
* change gcc
* filter the gcc output

please dont change correct code due to incorrect warnings (or at least
idiotic warnings)

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Thouse who are best at talking, realize last or never when they are wrong.
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