[FFmpeg-devel] [PATCH 1/3] lavformat: Prepare to make avio_enum_protocols const correct

[Andreas Rheinhardt]

Andreas Rheinhardt:
> Andreas Rheinhardt:
>> Using avio_enum_protocols works as follows: One initializes a pointer to
>> void and gives avio_enum_protocols the address of said pointer as
>> argument; the pointer will be updated to point to a member of the
>> url_protocols array. Now the address of the pointer can be reused for
>> another call to avio_enum_protocols.
>> Said array consists of constant pointers (to constant URLProtocols),
>> but the user now has a pointer to non-const to it; of course it was always
>> intended that the user is not allowed to modify what the pointer points
>> to and this has been enforced by hiding the real type of the underlying
>> object. But it is better to use a const void ** as parameter to enforce
>> this. This way avio_enum_protocols can be implemented without resorting
>> to casting a const away or ignoring constness as is done currently.
>>
>> Given that this amounts to an ABI and API break, this can only be done
>> at the next major version bump; as usual, the break is currently hidden
>> behind an appropriate #if.
>>
>> It was also necessary to change the type of a pointer used in
>> avio_enum_protocols. This makes the line that is not const correct move
>> as long as the old function signature is used. With the new signature,
>> avio_enum_protocols will be const correct.
>>
>> This change will eventually force changes in their callers, e.g. to
>> show_protocols in fftools/cmdutils. (This function contains all currently
>> existing calls to avio_enum_protocols in FFmpeg's codebase. It hasn't
>> been touched in this commit.)
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>>  libavformat/avio.h      | 4 ++++
>>  libavformat/protocols.c | 6 +++++-
>>  libavformat/version.h   | 3 +++
>>  3 files changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/libavformat/avio.h b/libavformat/avio.h
>> index 9141642e75..e067ea8985 100644
>> --- a/libavformat/avio.h
>> +++ b/libavformat/avio.h
>> @@ -805,7 +805,11 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer);
>>   *
>>   * @return A static string containing the name of current protocol or NULL
>>   */
>> +#if FF_API_NONCONST_ENUM_PROTOCOLS
>>  const char *avio_enum_protocols(void **opaque, int output);
>> +#else
>> +const char *avio_enum_protocols(const void **opaque, int output);
>> +#endif
>>  
>>  /**
>>   * Pause and resume playing - only meaningful if using a network streaming
>> diff --git a/libavformat/protocols.c b/libavformat/protocols.c
>> index ad95659795..c722f9a897 100644
>> --- a/libavformat/protocols.c
>> +++ b/libavformat/protocols.c
>> @@ -91,9 +91,13 @@ const AVClass *ff_urlcontext_child_class_next(const AVClass *prev)
>>  }
>>  
>>  
>> +#if FF_API_NONCONST_ENUM_PROTOCOLS
>>  const char *avio_enum_protocols(void **opaque, int output)
>> +#else
>> +const char *avio_enum_protocols(const void **opaque, int output)
>> +#endif
>>  {
>> -    const URLProtocol **p = *opaque;
>> +    const URLProtocol * const *p = *opaque;
>>  
>>      p = p ? p + 1 : url_protocols;
>>      *opaque = p;
>> diff --git a/libavformat/version.h b/libavformat/version.h
>> index 9814db8633..b0b9264382 100644
>> --- a/libavformat/version.h
>> +++ b/libavformat/version.h
>> @@ -106,6 +106,9 @@
>>  #ifndef FF_API_AVIOFORMAT
>>  #define FF_API_AVIOFORMAT               (LIBAVFORMAT_VERSION_MAJOR < 59)
>>  #endif
>> +#ifndef FF_API_NONCONST_ENUM_PROTOCOLS
>> +#define FF_API_NONCONST_ENUM_PROTOCOLS  (LIBAVFORMAT_VERSION_MAJOR < 59)
>> +#endif
>>  
>>  
>>  #ifndef FF_API_R_FRAME_RATE
>> I am unsure what to do next given the feedback I received: If ABI
> compability is no problem, then should I simply add the const and add
> an entry to doc/APIchanges? Or is a deprecation period necessary?
> 
> - Andreas
> 
Ping for all three patches.

- Andreas