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

[Lukasz Marek]

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
so this can be replaced by something similar and (maybe) less misleading

intptr_t *tab = *(void**)tab_ptr;

This line is copied from Nicolas implementation/original implementation.

Still don't know what strict aliasing have anything to do here.

Best Regards,
Lukasz Marek