[CMake] Proper behaviour and use of CMAKE_INSTALL_* variables

[Mojca Miklavec]

On Mon, Dec 9, 2013 at 4:52 PM, Andreas Pakulat wrote:
> Hi,
>
> On Mon, Dec 9, 2013 at 3:50 PM, Mojca Miklavec wrote:
>>
>> Dear list members,
>>
>> I often like or need to install two versions of the same software.
>> Ideally the software should put its files by default to
>>     $prefix/include/$NAME/*.h
>>     $prefix/lib/$NAME/*.dylib
>>     ...
>> ($prefix => $CMAKE_INSTALL_PREFIX)
>>
>> and in order to be able to install multiple versions side-by-side I
>> would like to be able to specify something like
>>     -DCMAKE_INSTALL_INCLUDEDIR=include/$NAME/$VERSION
>>     -DCMAKE_INSTALL_LIBDIR=lib/$NAME/$VERSION
>> to end up with
>>     $prefix/include/$NAME/$VERSION/*.h
>>     $prefix/lib/$NAME/$VERSION/*.dylib
>> instead of default paths.
>
>
> Do you have any reasons that speak against using separate prefixes for each
> version of that software?

First, both versions of the software *have to* to be installed under
$prefix (in my case that is usually /opt/local) because that is the
location where all the packages installed by a specific package
manager (MacPorts) need to end up. What I could do is to install the
two versions under
    CMAKE_INSTALL_PREFIX=$prefix/Library/Frameworks/$NAME.framework/Versions/$VERSION/
even if the software doesn't really satisfy the layout of a framework,
but that layout doesn't really simplify that much other than no need
to specify 5 separate variables to change include, lib, man, doc, ...

> In particular since such a setup will make your
> life harder when you want to use the installed software in some cmake-based
> project.

That's not a really good argument.

Life is already hard. For FindX11 I need to manually specify about 20
different variables to be able to get rid of searching in /usr/X11R6
(Mac OS X) for example and use the libraries from /opt/local instead.
For almost any given library I usually need to specify 2-5 variables
(one for includes and several for each single dylib) and a lot of
FindWhatever.cmake are simply too "stupid" to allow easy
configuration. The pkg-config approach works a whole lot better in a
lot of cases. I would really like to avoid searching in some prefixes
(for example I would like to avoid searching for libraries in
/usr/local and only search in /opt/local), but I don't know any
elegant way to do so.

And in case of some specific software I have in mind, the software
itself provides its own CMake configuration file, so all dependent
packages/software need to do is to find that configuration file,
nothing else.

> In that case you'd have to manually specify include-dir and lib-dir
> when configuring the project since many find-modules simply expect a
> 'standard' unix-like layout below the prefix under which they should search
> for a given software. So they don't look into include/<version> or
> lib/<version> so you'd need to specify that manually. Compared with the ease
> of just specifying CMAKE_PREFIX_PATH to the installation directory for the
> version you want thats a lot more effort each time you setup a build
> directory.

I'm still interested in the answer to my original question and would
like to avoid discussion about why not specifying a different $prefix.
There are surely many pros and contras to each approach and it makes
no sense to get off-topic with a different discussion. (I could surely
write an essay about pros of my approach, but that wouldn't help me
answer my question. I can open a new thread about that if needed, but
let's stick to the original question here.)

Mojca