[FFmpeg-user] How to correctly free a double-linked list

[Paul B Mahol]

On 7/7/19, Ulf Zibis <Ulf.Zibis at gmx.de> wrote:
>
> Am 07.07.19 um 13:51 schrieb Paul B Mahol:
>> There is av_calloc, it calls memset for you after allocation
> Thanks for the hint. I now have:
>     if (s->report >= R_SHIFTS) {
>         s->shifts_sums = av_malloc((s->lines + 1) *
> sizeof(*s->shifts_sums));

This above needs to be calloc.

>         for (int l = s->lines; l >= 0; l--)
>             s->shifts_sums[l] = av_calloc(s->span_r - s->span_l + 1,
> sizeof(**s->shifts_sums));
>     }
>
>> It looks like you access unallocated array in uninit because your
>> allocation is under condition.
> You mean this? :
>     if (s->report >= R_SHIFTS)
> It's always the same, so it should always hold.
>
>> Before freeing lines you need to check if array is not NULL.
> OK, now I have:
>         av_log(ctx, AV_LOG_WARNING, "now start freeing shifts_sums ...\n");
>         for (int l = 0; l <= s->lines; l++) {
>             av_log(ctx, AV_LOG_WARNING, "freeing line %d ...\n", l);
>             if (s->shifts_sums[l])
>                 av_freep(&(s->shifts_sums[l]));
>         }
>         av_log(ctx, AV_LOG_WARNING, "lines of shifts_sums freed!\n");
>         av_freep(&(s->shifts_sums));
>
>         av_log(ctx, AV_LOG_WARNING, "shifts_sums freed!\n");
>
> Unfortunately it doesn't help :-(
>
>
>> You need to memset/av_calloc array after allocation that hold pointers to
>> lines.
>> Otherwise you use uninitialized memory.
> I'm not sure if I understand correctly. Isn't this enough initialization? :
>         for (int l = s->lines; l >= 0; l--)
>             s->shifts_sums[l] = av_calloc(s->span_r - s->span_l + 1,
> sizeof(**s->shifts_sums));
> I don't get, why I first should write NULLs in there.
>
>> If you can, install and use valgrind to help you debug such problems.
>
> Good idea, I will do that ... guess it needs some time to understand how
> it works.
>
> -Ulf
>
>
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