Re: [Xen-devel] recent major -unstable changes cause ia64 build to be broken

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On May 10, 2005, at 6:05 PM, Magenheimer, Dan (HP Labs Fort Collins) 
wrote:
> > it. Here I will repeat my point about relying on implicit includes... 
> > :)
> Though I agree in principle with not using implicit includes,
> it doesn't really work in reality.
Sorry, I strongly disagree. In this case, if every user of 
alloc_xenheap_page() had included xen/mm.h, you would not have had any 
problem.
> For example, many
> header files declare structs that use typedefs for elements
> of the struct, which creates a direct dependency on a header
> file.  The only way to avoid including the header file directly
> in the "parent" header file is by "instructing" all the users
> of the "parent" header to first include the subsidiary ones.
> The C language doesn't deal with this very well, resulting
> in very obscure error messages if something is missing.
> And since typedefs cannot be duplicately declared even if
> identical (a bug in C I think), there's a catch-22.
If a.h uses something defined in b.h, a.h must include b.h. It is a 
dependency. It's that simple.
I'm not saying recursive includes don't happen; they do. But once 
you've got that working and you explicitly include your dependencies, 
you won't see problems like this mm.h one, where changing one header 
file causes 10 loosely related files to break. Coincidentally, it also 
greatly simplifies writing a new set of header files for a new 
architecture. ;)
I suspect the source of many of your problems is due to the strange 
combination of two unrelated code bases, which makes your headers even 
more brittle than Xen/x86...
--
Hollis Blanchard
IBM Linux Technology Center


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