[PATCH weston sdk v2 1/8] weston.pc: Add dependencies for pixman-1 and xkbcommon

[Thiago Macieira]

On quinta-feira, 4 de julho de 2013 18.35.16, sardemff7+wayland at sardemff7.net 
wrote:
> > Suppose that our headers #include headers from a project whose headers
> > aren't usually in /usr/include (that is, they require -I). In order for
> > our users' applications to build, they need to add those -I as well. The
> > Requires is what tells pkg-config to add them.
> 
> Requires.private too.

See below.

> > What's more, even if the include paths are not an issue, the linking could
> > be. Since we included some third party API in our API, it's conceivable
> > that users might need to call functions in those libraries. If that
> > happens, then user applications must also link directly to the libraries,
> > not indirectly only.
>
> And they must link directly! That is the proper way to do that.

I partially agree. If I know in my API that I am using both libs X and Y, I 
should tell my buildsystem so.

But there are cases when I don't know of the indirect dependency. I'm using X, 
and I'm not using any of Y's API. Yet to compile my application, X requires 
Y's headers. That means X's .pc file should declare it depends non-privately on 
Y.

This is especially the case in C++, when base classes, operators, constructors 
and other things can hide from you the fact that you're using some other 
library.

> A plugin is using libxkbcommon for a feature, but provides several more.
> Weston is build with libxkbcommon support. All is fine.

Yes.

> Now, the same plugin is build against a non-libxkbcommon Weston (this is
> the now-possible case): it will break badly.
> Explicit usage is the way. Period.

Agreed: the plugin should declare explicitly. If a project uses intentionally 
a library, it should declare explicitly the dependency.

But I also disagree: if libweston's public API headers include unconditionally 
some other library's headers, libweston's .pc file should declare the public 
dependency. Otherwise, no plugin will compile.

Of course, there's also a grey area: an optional feature. Suppose that there's 
exactly one Weston header that requires xkbcommon and that header is not 
included by any other: weston-xkbcommon.h. In that case, anyone who uses that 
header is intentionally depending on xkbcommon. So in that case, weston.pc 
doesn't declare the public Requires.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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