[Cmake] Automated library dependencies

[John Biddiscombe]

Target A uses some library X
Target B links against target A, but has no knowledge of X
Cmake puts X in the link line for B but doesn't supply the path.

Builds do not work. The unsuspecting user has to put paths in manually.

B should not link against X.

" You might call this a "feature". I'll try to explain how it works, and
we can then discuss whether the behaviour should change or not."

I vote the "Feature" be corrected.

JB



Keep in mind that CMake normally keeps full paths to everything. So,
when you say FIND_LIBRARY( FOO_LIB foo /usr/local/lib ), it will set
FOO_LIB=/usr/local/lib/libfoo.so. Notice how the path is embedded in
the library name. Now, this whole path gets propagated to library A,
to use your example. Therefore, library A will have the directory in
which to find foo. Good.

Now suppose you do
  LINK_DIRECTORIES( "/some/system/path" )
  TARGET_LINK_LIBRARIES( B "-ljpeg" )
all that will go forward is "-ljpeg". Therefore, A will not have the
path to find jpeg. However, since it is an external library,
presumably the system is able to find it, and so you *do not* need to
know where it is. (Rather, where it was on the system that compiled A;
it may not be in the same place in some other system.)

If you do
  TARGET_LINK_LIBRARIES( B /some/system/path/libjpeg.so )
then all will be well again. In general, if you use FIND_LIBRARY, this
problem should not arise.

My feeling is that explicitly providing "-ljpeg" on the link line is
specifying a "system" library, and the user is responsible for making
sure it is found. I mean in the sense of a deliverable: you don't know
where on the user's system to find this external library. But I admit
the distinction is rather arbitrary and inconsistent.

>  (I assume this is only a problem when library A is not in a
> subdirectory of library B).

Not really. If A is a subdirectory of B, then A will link fine, but if
some executable outside of these two directories tries to link to A,
you'll probably have the same error.

> Should the LINK_DIRECTORIES also be propagated from library B to
> library A.  It is basically the same argument for why the link
> libraries are propagated from library B to library A.

I think this *can* be done. I'll leave the policy decisions--should it
be, and into which version--to others.

Amitha.
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