[FFmpeg-devel] [PATCH] Avoid running malloc/free for 0-sized allocs and return a pointer to const memory so we will still detect invalid writes to malloc(0) areas. This also fixes av_realloc to treat size == 0 specially.

[Michael Niedermayer]

On Sat, May 07, 2011 at 02:17:14PM +0200, Reimar Döffinger wrote:
> On Sat, May 07, 2011 at 01:35:54PM +0200, Michael Niedermayer wrote:
> > On Sat, May 07, 2011 at 09:56:31AM +0200, Reimar Döffinger wrote:
> > > ---
> > >  libavutil/mem.c |   10 +++++++++-
> > >  1 files changed, 9 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/libavutil/mem.c b/libavutil/mem.c
> > > index f0f18d1..134fcba 100644
> > > --- a/libavutil/mem.c
> > > +++ b/libavutil/mem.c
> > > @@ -57,6 +57,8 @@ void  free(void *ptr);
> > >  
> > >  #endif /* MALLOC_PREFIX */
> > >  
> > > +static const int dummy_alloc;
> > > +
> > >  /* You can redefine av_malloc and av_free in your project to use your
> > >     memory allocator. You do not need to suppress this file because the
> > >     linker will do it automatically. */
> > > @@ -72,7 +74,7 @@ void *av_malloc(size_t size)
> > >      if(size > (INT_MAX-32) )
> > >          return NULL;
> > >      else if(!size)
> > > -        size= 1;
> > > +        return (void *)&dummy_alloc;
> > 
> > This violates ISO C malloc() semantics, as well as attribute(malloc)
> > semantics and its also not correctly aligned.
> > We might ignore ISO C as this isnt malloc(), the rest looks like a
> > possinble issue though
> > ignoring the attribute semantics means we have to remove
> > attribute(malloc) or risk undefined behavior on av_malloc(0)
> > the align should be easy to fix
> 
> I have some doubts any of these really matter -

maybe it doesnt, iam not saying it does ...


> the align
> certainly doesn't since using the returned pointer in any way
> would be a programming error.

> But we can just leave it, but in that case: could you please
> fix av_realloc to behave consistently?

will do

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

Republics decline into democracies and democracies degenerate into
despotisms. -- Aristotle
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