[FFmpeg-devel] [PATCH 2/3] avutil/dict: Improve appending values

[Paul B Mahol]

On 9/13/22, Andreas Rheinhardt <andreas.rheinhardt at outlook.com> wrote:
> When appending two values (due to AV_DICT_APPEND), the earlier code
> would first zero-allocate a buffer of the required size and then
> copy both parts into it via av_strlcat(). This is problematic,
> as it leads to quadratic performance in case of frequent enlargements.
> Fix this by using av_realloc() (which is hopefully designed to handle
> such cases in a better way than simply throwing the buffer we already
> have away) and by copying the string via memcpy() (after all, we already
> calculated the strlen of both strings).
>
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
> ---
>  libavutil/dict.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/libavutil/dict.c b/libavutil/dict.c
> index c01165634e..1968063b0b 100644
> --- a/libavutil/dict.c
> +++ b/libavutil/dict.c
> @@ -98,16 +98,17 @@ int av_dict_set(AVDictionary **pm, const char *key,
> const char *value,
>              return 0;
>          }
>          if (copy_value && flags & AV_DICT_APPEND) {
> -            size_t len = strlen(tag->value) + strlen(copy_value) + 1;
> -            char *newval = av_mallocz(len);
> +            size_t oldlen = strlen(tag->value);
> +            size_t new_part_len = strlen(copy_value);
> +            size_t len = oldlen + new_part_len + 1;
> +            char *newval = av_realloc(tag->value, len);
>              if (!newval)
>                  goto err_out;
> -            av_strlcat(newval, tag->value, len);
> -            av_strlcat(newval, copy_value, len);
> +            memcpy(newval + oldlen, copy_value, new_part_len + 1);
>              av_freep(&copy_value);
>              copy_value = newval;
> -        }
> -        av_free(tag->value);
> +        } else
> +            av_free(tag->value);
>          av_free(tag->key);
>          *tag = m->elems[--m->count];
>      } else if (copy_value) {
> --
> 2.34.1
>


LGTM

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