[CMake] Get linker flags / include directories for a library target

[frodak17]

On Fri, Dec 28, 2018 at 6:04 PM Unknown <ax487 at gmx.de> wrote:

> I would like to thank all of you for your suggestions. I have gathered
> the following (correct me if I am wrong):
>
> - The LINK_FLAGS target property is used in cmake 2.x, the
>   INTERFACE_LINK_LIBRARIES in cmake 3.x, the latter contains a ;-list
>   of libraries.
> - The list may contain generator expressions, it is however
>   possible to to use file(GENERATE ..) to obtain evaluate those.
> - In any case, the library list contains libraries having different
>   formats (see [1]), full paths, (imported) targets, and plain names.
> - When Makefiles (or files for other build systems) are generated,
>   the list is turned into -L/-l flags used by ld. This happens
>   on the C++ side of things, the functionality is not exposed
>   to cmake scripts.
>
> As for automatic generation of a pkg-config .pc file, there have
> been some inquiries ([2], [3], and [4]), the last one being rather
> recent.
>
> The answers point out that pkg-config files can be generated using
> configure_file(...), that cmake has its own (imported target) method
> for handling dependencies.
>
> I don't mean to be disrespectful or unappreciative of the work put
> into cmake (in fact I think it is a vast improvement over automake),
> but since there is no way to obtain the required information
> programatically, all users of my library have to either use
> cmake themselves, or use non-portable workarounds which are prone
> to break sooner than later.
>
> If I want to use an external tool (think setup.py) to build
> extensions (in-place or not), the same problem occurs.
>
> This is rather disappointing to be honest...
>
> ax487
>
>
>
The answer to [2] was that the information required to automatically
generate a .pc file was not available.
https://linux.die.net/man/1/pkg-config
https://people.freedesktop.org/~dbn/pkg-config-guide.html#writing
I don't see how CMake could know which packages your library conflicts with
or which versions of which libraries are required.
For example a library target can link against an imported target but it
won't know that only imported target version 1.0.0 is compatible as opposed
to versions >= 1.5.

It seems that you are trying to provide more than how to link against your
library but also against everything your library wants to be linked
against.
For example the .pc file you want generated contains "Libs: -L${libdirbar}
-L${libdirfoo}  -lbar -lfoo".
But that isn't the proper way of a .pc file should reference separate
libraries it should use the Requires field.

Also if your library is linking against an imported library
`libbaz::libbaz` how is this library being provided to the people using
your library?
Are you trying to generate a .pc file for the imported library because it
didn't provide one and incorporate the flags into library you are creating?

I think that no one has volunteered to write a CMake package to create .pc
files is because generating a .pc file is pretty simple. It's just a
template and a configure_file() command.
https://cmake.org/pipermail/cmake/2018-March/067293.html
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