[CMake] What is the correct way to manage a possibly user-defined directory?

[Petr Kmoch]

Hi Cedric.

if(EXISTS) does not automatically dereference its argument. So your current
code is testing for the existence of a directory literally named "FOO_DIR".
You want to dereference the variable:

    if(NOT EXISTS ${FOO_DIR})

Second, option() is intended for on/off options only (a checkbox). If you
want to provide a path selector, use set(... CACHE) instead:

    set(FOO_DIR "" CACHE PATH "Provide path to FOO library here")


This will create a path selector widget in CMake UI, with "Provide path to
FOO library here" serving as its tooltip/documentation.

Petr

On Mon, May 18, 2015 at 11:48 AM, Cedric Doucet <cedric.doucet at inria.fr>
wrote:

>
> Hello,
>
> I would like to let users choose between providing third party libraries
> or let CMake download them.
> To do that, I try to write a simple code like this to manage a third party
> library FOO of an executable MY_EXEC:
> =================================================
> IF(NOT EXISTS FOO_DIR)
>       MESSAGE(STATUS "FOO LIBRARY WILL BE DOWNLOADED AND INSTALLED")
>       # download and install FOO, define FOO_DIR
> ENDIF()
> INCLUDE_DIRECTORIES(MY_EXEC ${FOO_DIR}/include)
> TARGET_LINK_LIBRARIES(MY_EXEC ${FOO_DIR}/include)
> =================================================
> To do that, I think the user should have the possibility to define FOO_DIR
> with an option.
> So, I put the line
> OPTION(FOO_DIR)
> before the code below.
>
> However, it doesn't seem to work because FOO_DIR does not seem to be
> defined.
> The message "FOO LIBRARY WILL BE DOWNLOADED AND INSTALLED" always appears,
> - even if I type "cmake -D FOO_DIR=path", where path is a user-defined
> path to FOO library,
> - even if I put a default value to FOO_DIR with OPTION(FOO_DIR path),
> where path is a hard-coded path to FOO.
>
> Where does I make a mistake here?
> What is the correct way to manage FOO_DIR when it can be defined by a user?
>
> Note: the next step is to define FOO_DIR in a FindFoo.cmake file and use
> find_package(Foo), but it's not my goal yet.
>
> Thank you very much for your help!
>
> Cédric
>
>
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