[FFmpeg-devel] [PATCH 1/2] avutil/dict: Add av_dict_iterate

[Marvin Scholz]

On 22 Sep 2022, at 13:33, Andreas Rheinhardt wrote:

> Marvin Scholz:
>> This is a more explicit iteration API rather than using the "magic"
>> av_dict_get(d, "", t, AV_DICT_IGNORE_SUFFIX) which is not really
>> trivial to grasp what it does when casually reading through code.
>> ---
>>  libavutil/dict.c    | 19 +++++++++++++++++++
>>  libavutil/dict.h    | 27 +++++++++++++++++++++++++++
>>  libavutil/version.h |  2 +-
>>  3 files changed, 47 insertions(+), 1 deletion(-)
>>
>> diff --git a/libavutil/dict.c b/libavutil/dict.c
>> index 14ad780a79..2f690a5b8e 100644
>> --- a/libavutil/dict.c
>> +++ b/libavutil/dict.c
>> @@ -20,6 +20,7 @@
>>
>>  #include <string.h>
>>
>> +#include "avassert.h"
>>  #include "avstring.h"
>>  #include "dict.h"
>>  #include "dict_internal.h"
>> @@ -38,6 +39,24 @@ int av_dict_count(const AVDictionary *m)
>>      return m ? m->count : 0;
>>  }
>>
>> +AVDictionaryEntry *av_dict_iterate(const AVDictionary *m,
>> +                                   const AVDictionaryEntry *prev)
>> +{
>> +    int i = 0;
>> +
>> +    if (!m)
>> +        return NULL;
>> +
>> +    if (prev)
>> +        i = prev - m->elems + 1;
>> +
>> +    av_assert2(i >= 0);
>> +    if (i >= m->count)
>> +        return NULL;
>> +
>> +    return &m->elems[i];
>> +}
>> +
>>  AVDictionaryEntry *av_dict_get(const AVDictionary *m, const char *key,
>>                                 const AVDictionaryEntry *prev, int flags)
>>  {
>> diff --git a/libavutil/dict.h b/libavutil/dict.h
>> index 0d1afc6c64..b42b3f07fd 100644
>> --- a/libavutil/dict.h
>> +++ b/libavutil/dict.h
>> @@ -32,6 +32,8 @@
>>
>>  #include <stdint.h>
>>
>> +#include "attributes.h"
>> +
>>  /**
>>   * @addtogroup lavu_dict AVDictionary
>>   * @ingroup lavu_data
>> @@ -101,6 +103,31 @@ typedef struct AVDictionary AVDictionary;
>>  AVDictionaryEntry *av_dict_get(const AVDictionary *m, const char *key,
>>                                 const AVDictionaryEntry *prev, int flags);
>>
>> +/**
>> + * Iterate over a dictionary
>> + *
>> + * Iterates through all entries in the dictionary.
>> + *
>> + * @warning The returned AVDictionaryEntry key/value must not be changed.
>> + *
>> + * @param m     The dictionary to iterate over
>> + * @param prev  Pointer to the previous AVDictionaryEntry, NULL initially
>> + *
>> + * @retval AVDictionaryEntry* The next element in the dictionary
>> + * @retval NULL               No more elements in the dictionary
>> + *
>> + * Typical usage:
>> + * @code
>> + * AVDictionaryEntry *e = NULL;
>> + * while (e = av_dict_iterate(m, e)) {
>> + *     // ...
>> + * }
>> + * @endcode
>> + */
>> +av_warn_unused_result
>> +AVDictionaryEntry *av_dict_iterate(const AVDictionary *m,
>> +                                   const AVDictionaryEntry *prev);
>
> The user is not allowed to modify the returned AVDictionaryEntries, so
> you should return a const AVDictionaryEntry here.

Ok.

Shouldn't _get return const as well then? If so, can that be changed or would
it break ABI?

> And there is no reason
> to use av_warn_unused_result at all; nothing bad happens if you ignore
> the result (except that you called av_dict_iterate unnecessarily).

Yeah nothing bad happens but it is probably still wrong use of the API
that seemed beneficial to highlight or is there any valid use-case
to call it without using the return? I could not think of any…

>
> Furthermore, av_dict_set's documentation contains "Adding a new entry to
> a dictionary invalidates all existing entries previously returned with
> av_dict_get." This needs to be updated, too.

Sure, I will update the docs more, I just wanted to have some general
feedback if this is acceptable addition before I invest more time to
polish it.

>
>> +
>>  /**
>>   * Get number of entries in dictionary.
>>   *
>> diff --git a/libavutil/version.h b/libavutil/version.h
>> index 0585fa7b80..6fd07ed2a4 100644
>> --- a/libavutil/version.h
>> +++ b/libavutil/version.h
>> @@ -80,7 +80,7 @@
>>
>>  #define LIBAVUTIL_VERSION_MAJOR  57
>>  #define LIBAVUTIL_VERSION_MINOR  36
>> -#define LIBAVUTIL_VERSION_MICRO 102
>> +#define LIBAVUTIL_VERSION_MICRO 103
>
> New API additions need a minor bump (and need to reset micro).

Oh indeed, sorry, will fix that in the next version.

>
>>
>>  #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
>>                                                 LIBAVUTIL_VERSION_MINOR, \
>
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