[FFmpeg-devel] [PATCH 1/4] lavu: add simple array implementation

[Michael Niedermayer]

On Fri, Mar 21, 2014 at 01:44:18AM +0100, Michael Niedermayer wrote:
> On Fri, Mar 21, 2014 at 01:26:28AM +0100, Lukasz Marek wrote:
> > On 21.03.2014 00:03, Michael Niedermayer wrote:
> > >On Thu, Mar 20, 2014 at 10:44:41PM +0100, Lukasz Marek wrote:
> > >>>>  mem.c |   13 +++++++++++++
> > >>>>  mem.h |   19 +++++++++++++++++--
> > >>>>  2 files changed, 30 insertions(+), 2 deletions(-)
> > >>>>7fad1be9083fcaef4435fa0273f79bceca98821b  0001-lavu-mem-add-av_dynarray_add_nofree-function.patch
> > >>>> From 4cadb3328ba018b37c5dfe05a637b50a262151c6 Mon Sep 17 00:00:00 2001
> > >>>>From: Lukasz Marek <lukasz.m.luki at gmail.com>
> > >>>>Date: Tue, 25 Feb 2014 01:06:06 +0100
> > >>>>Subject: [PATCH] lavu/mem: add av_dynarray_add_nofree function
> > >>>>
> > >>>>av_dynarray_add_nofree function have similar functionality
> > >>>>as existing av_dynarray_add, but it doesn't deallocate memory
> > >>>>on fails.
> > >>>>
> > >>>>TODO: minor bump and update doc/APIChanges
> > >>>>
> > >>>>Signed-off-by: Lukasz Marek <lukasz.m.luki at gmail.com>
> > >>>>---
> > >>>>  libavutil/mem.c |   13 +++++++++++++
> > >>>>  libavutil/mem.h |   19 +++++++++++++++++--
> > >>>>  2 files changed, 30 insertions(+), 2 deletions(-)
> > >>>>
> > >>>>diff --git a/libavutil/mem.c b/libavutil/mem.c
> > >>>>index 7206ddc..b9b5742 100644
> > >>>>--- a/libavutil/mem.c
> > >>>>+++ b/libavutil/mem.c
> > >>>>@@ -278,6 +278,19 @@ void *av_memdup(const void *p, size_t size)
> > >>>>      return ptr;
> > >>>>  }
> > >>>>
> > >>>>+int av_dynarray_add_nofree(void *tab_ptr, int *nb_ptr, void *elem)
> > >>>>+{
> > >>>
> > >>>>+    intptr_t *tab = *(intptr_t**)tab_ptr;
> > >>>
> > >>>undefined behavior (strict aliasing violation)
> > >>
> > >>
> > >>Can you tell how it is wrong?
> > >
> > >yes sure
> > >
> > >quoting the spec:
> > >
> > >7 An object shall have its stored value accessed only by an lvalue expression that has one of
> > >   the following types:76)
> > >   - a type compatible with the effective type of the object,
> > >   - a qualified version of a type compatible with the effective type of the object,
> > >   - a type that is the signed or unsigned type corresponding to the effective type of the
> > >       object,
> > >   - a type that is the signed or unsigned type corresponding to a qualified version of the
> > >       effective type of the object,
> > >   - an aggregate or union type that includes one of the aforementioned types among its
> > >       members (including, recursively, a member of a subaggregate or contained union), or
> > >   - a character type.
> > >
> > >
> > >i dont think intptr_t and some pointer are compatible types
> > >but maybe iam missing something
> > 
> > tab_ptr is pointer to pointer to array data
> 
> "array data" ?
> 
> 
> > so this can be replaced by something similar and (maybe) less misleading
> > 
> > intptr_t *tab = *(void**)tab_ptr;
> 
> void **tab = *(void***)tab_ptr;

i think i was a bit too terse here

above is just meant to be "less misleading" its not solving the
aliasing issue (completely)

solving it completely is a bit tricky and maybe could be done with
a union ...

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

it is not once nor twice but times without number that the same ideas make
their appearance in the world. -- Aristotle
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