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Re: saving a URL



Gecse Roland writes:

> On Mon, 20 May 1996, James Gallagher wrote:
> 
> Sacha wrote:
> >   /* before I added this next line it wouldn't write */
> >   HTRequest_setOutputFormat(request, WWW_SOURCE);
> 
> Of course it wouldn't write, because you have to set the output format
> according to the libwww interface you'll use.

> I hope I was right -- Henrik?,

Yep - you are on the right track!

Just to summarize - libwww has a small set of "internal" media types 
(content types) which do not correspond to any "real" type. The WWW_SOURCE 
and WWW_PRESENT are both examples of this. WWW_SOURCE is a define for "*/*" 
which is a way of saying any 'media type of type "*" and subtype "*"' or in 
orther words: we ask libwww to pass the exact same format to the application 
as it gets in from the network.

WWW_PRESENT is a type representing the output format that the end user sees 
on the screen, for example. We could also call it WWW_CANVAS. Only "200 OK" 
responses are passed to this output. That is, WWW_PRESENT will never have to 
deal with error messages etc.

However, in some cases, HTTP servers can send back "error documents" for 
example if there are multiple choices of a document. Libwww defined 
WWW_DEBUG to handle these error documents. WWW_DEBUG is very similar to 
WWW_PRESENT in that it may present something to the end user - it is just 
the content of the document that differs.

The default output format is set to WWW_PRESENT but it is a common problem 
to confuse the default output format - would it be easier if we set the 
default output format to WWW_SOURCE instead?

> The stream stack will be initialized using that format type as its tail,
> and then it tries to figure out the appropriate converters.
> The WWW_SOURCE uses an unstructured stream, so the converters you've 
> initialized are enough to do the work. But when you would choose 
> WWW_PRESENT which belongs to the HText structured stream, you'd have to
> make the HText_* functions!

One thing that is importan to note is that the

*** The application defines the binding between a media type and a stream to 
handle the data***

Hence, libwww does _not_ know what WWW_PRESENT or WWW_DEBUG points to. The 
library comes with a default set of streams that can be used - a default 
HTML parser etc. but these are not part of libwww core as such. They are in 
fact on the realm of the application. If WWW_PRESENT does not point to 
anything at all, for example, then libwww decides to dump the data to local 
file.


-- 
Henrik Frystyk Nielsen, <frystyk@w3.org>
World-Wide Web Consortium, MIT/LCS NE43-356
545 Technology Square, Cambridge MA 02139, USA



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