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

[Jeremy Vignelles]

> Le 17 mai 2019 à 11:23, Romain Vimont <rom1v at videolabs.io> a écrit :
> 
> 
> On Fri, May 17, 2019 at 10:04:59AM +0200, Jérémy VIGNELLES wrote:
> > Hi,
> > 
> > In libvlc, there is currently a `libvlc_log_set` function, that can register
> > log callback in a vprintf style, that is, with a `va_list` argument.
> 
> I agree that we should avoid va_list in callbacks :)
> > This is nice when you are trying to call from C, but when using other languages
> > like C# or Java, va_list is impossible to get right, and even if it did,
> > those languages do not provide ways to format that kind of stuff.
> 
> (not counter-arguments, just comments)
> 
> I don't know C#, but in Java you will need a JNI layer in C, so it can
> be handled here.

I don't really know java, but that JNI layer will still need to be built for the target platform you're building for. Wouldn't that make a lot of combinations too? I think it would be similar to what I did in libvlcLogInterop

> BTW, in pure Java, you can format using String.format():
> 
>  String s = String.format("%s %d", "hello", 42).

C# has a 
string.Format("{0} {1}", "hello", 42)

But that doesn't make it easier to consume, because a va_list doesn't map well to the C# version (object[] params)

> > The best attempt so far seems to write an external native project to do that,
> > like this one : https://github.com/jeremyVignelles/libvlcLogInterop, but it has
> > a major drawback: it needs to be compiled for every host triplet that vlc supports,
> > which is a lot of work (if even possible), and requires the correct v*printf to be
> > available on the standard library.
> > 
> > I'm attempting here another approach : Modify libvlc v4 to include a preformatted
> > version of libvlc_log_set. The caller is now able to get log message as a single string,
> > and can filter by log level before formatting.
> > 
> > The new API requires a context to be created and destroyed, in order to keep compatibility with
> > libvlc_log_set.
> > 
> > I thought about modifying the libvlc_instance_t.log structure and put my context there,
> > but I was unsure about how that would interact with the existing things
> > 
> > Comments welcome :)
> > 
> > Jérémy VIGNELLES (2):
> >  libvlc: added a formatted log callback
> >  test: libvlc: Added a formatted log callback test
> > 
> >  include/vlc/libvlc.h | 64 ++++++++++++++++++++++++++++++++++++++++++++
> >  lib/libvlc.sym | 3 +++
> >  lib/log.c | 55 +++++++++++++++++++++++++++++++++++++
> >  test/libvlc/core.c | 27 +++++++++++++++++++
> >  4 files changed, 149 insertions(+)
> > 
> > -- 
> > 2.21.0.windows.1
> > 
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