[vlc-devel] [PATCH 0/2] Libvlc formatted log callback

[Steve Lhomme]

On 2019-05-22 19:47, Romain Vimont wrote:
> 
> 
> Le 21 mai 2019 22:34:31 GMT+02:00, Jeremy Vignelles <jeremy.vignelles at dev3i.fr> a écrit :
>> After reading through the comments, I think it is a good idea to sum
>> things up:
>>
>> == The issue ==
>> `libvlc_log_set` is a good API that can register log callbacks.
>>
>> An application using libvlc is able to get log messages, format them,
>> and either show them to the user, or write them to a file.
>>
>> This is great when the application is written in C, or uses an
>> intermediate layer written in C.
>>
>> However, the current shape of the API makes it hard to call from
>> languages calling libvlc through FFI, because of the va_list argument.
>> For a language like C#, there is no easily-accessible vsnprintf
>> equivalent on all platforms that supports all formatted string markers.
>>
>> LibVLCSharp, the official libvlc binding for .net, is impacted by this
>> issue because it can't offer a binding for libvlc_log_set to C# users.
>>
>> == What can we do about it? ==
>>
>> = Leave things as-is =
>>
>> + Keeps compatibility with existing apps
>> + conforms to what is done in other libraries, like libavutil
>> + does not add allocation overhead (No allocation means no malloc
>> failure too)
>> - Requires higher-level languages to have a native glue code, built for
>> each platform libvlc will be used on, which is a hard work for bindings
>> like LibVLCSharp.
>>
>> = Keep libvlc_log_set, but add a formatted log callback next to it =
>>
>> + Keeps compatibility
>> + Adds ability to consume from other languages
>> - Might create confusion about which API to call
>> - libvlc_instance_t.log will need to keep track of the two forms of log
>> callback, which I don't find really clean.
>>
>> = Use the v2 of the patch : Remove libvlc_log_set and use a
>> preformatted log callback instead =
>>
>> + Solves the issue, and makes the API easier to consume from any
>> language using libvlc from FFI.
>> + Keeps only one logging API
>> + Allows to filter by log level before formatting the log.
>> - Breaks the libvlc logging API
>>
>> = Introduce a libvlc_vsnprintf that can format log message the way
>> libvlc expects? =
>>
>> + Cross-platfrom solution
>> + Higher-level languages could probably pass the args as a pointer (not
>> sure about this one, this will need to be checked)
>> + Doesn't touch the libvlc_log_set API and let the user decide how
>> things are formatted.
>> - Adds marshalling overhead
>> - The API has nothing to do with libvlc's main purpose.
> 
> I like the last one, the current usage is not affected at all.

+1

>> Did I miss something? Has anyone other ideas/objections?
>> What's your preference everyone?
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