Re: [Mxflib-developers] Overriding file IO in mxflib

[Mathew S. <mat...@eu...>]

Matt Beard wrote:

> At 09:50 05/02/2004, Mathew Spencer wrote:
>
>> There is one very good reason for this (in the GStreamer example) as 
>> far as I can see.  I want to abstract the MXF file IO to allow for 
>> the GStreamer pipeline to control it (In this case, the file io is 
>> controlled by a GstBytestream structure), but the dictionary and 
>> types file loading still needs to be done by the generic access 
>> methods.  I have already written the code that allows this to be 
>> achieved and will supply a patch (hopefully today, I'll see how my 
>> time goes).
>
>
> At the moment the sax parser will still read from a file as it is 
> within klvlib, soon this will not be the case. There are a number of 
> ways to get around this - such as the client code switching its 
> handlers from file mode to stream mode after loading the dictionaries.
>
>> One other point, the MXFLIB_NO_FILE_IO system requires the library to 
>> be recompiled for each purpose, with this method you simply need to 
>> supply the library with an alternate set of access methods at run time.
>
>
> It seems to be a very bad idea to add an extra layer between the file 
> i/o and the library as this will kill many optimizations.  Consider 
> the case of checking for EOF - normally this will optimize into a read 
> of a single value from a memory structure. If non-file handling were 
> to be required in a significant proportion of applications it may be 
> worth building this layer in, but at the moment it would simply reduce 
> the efficiency for everyone.  A simple answer might be to "optionally" 
> add this layer - so you either have file only (with full 
> optimizations) or switchable (with a little extra overhead).

Agreed.
I have attached a tarball with the modifications I made to mxffile and 
system.h to enable runtime selection of io methods.  Its not very 
elegant at the moment, but it works and could form a starting point for 
getting this functionality into mxflib.

For what its worth, I now have a gstreamer plugin that will play some 
formats of MXF file (namely XDCam formats at the moment, others to come 
soon).

By the way Matt, have you had a chance to look at the Autotools version 
of mxflib that I sent you?

> Matt

Regards,
Mat


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