abstraction library and linkage

[Timo Weingärtner <timo@tiwe.de>]

Hi,

consider a library libabstr implementing an interface and a library libimplX
that is currently used by libabstr to actually do stuff. The public headers
are called abstr.h and implX.h

libabstr could be designed in basically two different ways:

1:  * abstr.h contains just prototypes, perhaps also inline functions that
      only call libabstr functions
    * libabstr is linked with -limplX
    * applications are linked with -labstr
2:  * abstr.h contains only inline functions that call libimplX functions
    * libabstr is neither built nor linked, libabstr-dev ships a pkg-config
      file or similar
    * application build process uses pkg-config to determine what to link with

In each case no changes to the source of applications are needed if libabstr
decides to use libimplY instead, in case 1 it may even work without a
rebuild.

Here comes the tricky part:

What shall maintainers of applications using libabstr do if:
* libabstr used to fit way 1
* an inline function in abstr.h now contains:
  __asm ("" : : "r" (&libimplX_foo));
  so no real calls to libimplX functions are done from the header, but the
  application is forced to link with -limplX
* no pkgconfig file or similar is provided
* the change in libabstr was introduced in a debian release
* the breakage from that change is not documented in debian/NEWS


Greetings
Timo

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