[cmake-developers] automatically download library

[Ruslan Baratov]

On 08-Oct-15 18:06, Marsel Galimullin wrote:
> The idea is that CMake has possibility to contact with package 
> managers for download necessary libraries.
> If “find_package” can’t find a library, it need to call apt-get install.
> Example:
>     auto_downland(apt-get)
>     find_package (Boost COMPONENTS system thread REQUIRED) 
re:{possibility to contact with package managers} *if* you have a 
suitable package manager. And even if you have one result may differs 
for different platforms, e.g.:

* OSX, brew install boost => 1.58
* Ubuntu, apt-get install boost-dev => 1.54

Note that 'find_package(Boost)' can find any user-installed libraries or 
non-host packages for cross-compile, like building Android on Linux, or 
iOS on OSX (both issues can be solved by using ExternalProject + 
superbuild). If you and your users are okay with that then it's should 
be not so hard to implement this CMake module.

On 06-Oct-15 15:22, Marsel Galimullin wrote:
>
> Hello!
> I'm student of the University LETI and as a masrer'sthesis I want to 
> developan extension to CMake. It is expected that this extension will 
> automatically download missinglibrary ifinstruction such as 
> «find_package (Boost COMPONENTS system thread REQUIRED)» can not find 
> the package. Could you give me some advices where to begin and which 
> the best direction to design, developand whether to do it?
>
So with all this peculiarities I've described above this will be not 
such an easy task as you might expect. You can start one from scratch 
for education or fun (or both) but if you want to do some really helpful 
stuff my advice to you is to feed "cmake package manager" request to 
your web search engine and investigate existing solutions.

On 08-Oct-15 18:41, Ryan Schmidt wrote:
> On Oct 8, 2015, at 10:06 AM, Marsel Galimullin wrote:
>
>> The idea is that CMake has possibility to contact with package managers for download necessary libraries.
>> If “find_package” can’t find a library, it need to call apt-get install.
>> Example:
>>     auto_downland(apt-get)
>>     find_package (Boost COMPONENTS system thread REQUIRED)
> I understand the idea, and I'm telling you that if you need this for your own personal project -- one that will never be included in a package management system -- then do whatever you like. But if you are proposing this as something that should be included in cmake, and you are suggesting that it could be used by any project that wants to -- even in projects that might someday be included in a package management system -- then I can tell you that this would not be welcomed by the people who maintain those package management systems. I am such a person. If I were tasked with the request to add to my package management system a project that wanted to control the installation of its own dependencies, or that in some other way tried to be in charge of downloading its dependencies, I would either have to rip all that code out, or more likely I would reject the request to add that project as being too labor-intensive. It is the package management system's job to download and install dependencies, not the job of the project's build system.
>
re:{I would either have to rip all that code out, or more likely I would 
reject the request to add that project as being too labor-intensive} 
this can be solved easily by adding option which is set to OFF by default:

option(USE_SUPERTOOL "Download host packages automatically" OFF)
function(auto_download ...)
   if(NOT USE_SUPERTOOL)
     # do nothing
     return()
   endif()
   # do install
endfunction()
auto_download(...) # by default do nothing
find_package(Boost COMPONENT system thread REQUIED) # find the packages 
you are expecting

On 08-Oct-15 18:23, Brad King wrote:
> On 10/08/2015 11:06 AM, Marsel Galimullin wrote:
>> The idea is that CMake has possibility to contact with package managers
>> for download necessary libraries.
>> If “find_package” can’t find a library, it need to call apt-get install.
>> Example:
>>       auto_downland(apt-get)
>>       find_package (Boost COMPONENTS system thread REQUIRED)
> This does not provide enough information to know what package to install.
> This problem is not in scope for a build system.  Another tool should
> be used to install dependencies ahead of time before running CMake.
> The source tree could come with some kind of dependency spec file for
> such a tool to use but this would be independent of the build system.
>
> -Brad
>
re:{The source tree could come with some kind of dependency spec file 
for such a tool to use but this would be independent of the build 
system} As the one who is using such approach in my product I disagree 
with that completely :) CMake code and dependencies are in fact coupled 
strongly. For example:

    option(USE_GUI "Use some GUI for this project" OFF)
    if(USE_GUI)
       if(WIN32)
         option(USE_QT "Use Qt GUI" ON)
       else()
         option(USE_WXWIDGETS "Use WxWidgets GUI" ON)
       endif()

       # check at least one and not both

       if(USE_QT)
         find_package(Qt5Widgets REQUIED)
         target_compile_definitions(... USE_QT_GUI)
       else()
         find_package(WxWidgets REQUIRED)
         target_compile_definitions(... USE_WXWIDGETS_GUI)
       endif()
    endif()

    if(USE_SECURITY)
       find_package(OpenSSL REQUIRED)
       target_compile_definitions(... USE_OPENSSL)
    endif()

    if(BUILD_TESTS)
       find_package(GTest REQUIRED)
       ...
    endif()

    if(GENERATE_DOCUMENTATION)
       find_package(Doxygen REQUIRED)
       ...
    endif()

So instead of violating DRY and duplicating this information in some 
*newly* created format for spec file, like:

       option(USE_GUI) && WINDOWS && option(USE_QT): Qt
       option(USE_GUI) && !WINDOWS && option(USE_QT): WxWidgets
       option(USE_GUI) && WINDOWS && option(USE_WXWIDGETS): WxWidgets
       ...
       option(USE_SECURITY): OpenSSL
       option(BUILD_TESTS): GTest
       option(GENERATE_DOCUMENTATION): Doxygen

We can use CMake code:

       if(USE_QT)
         install_package(Qt)
         find_package(Qt5Widgets REQUIED)
         target_compile_definitions(... USE_QT_GUI)
       else()
         install_package(WxWidgets)
         find_package(WxWidgets REQUIRED)
         target_compile_definitions(... USE_WXWIDGETS_GUI)
       endif()


I think spec files used in many package managers because of obstructions 
that are not applying to CMake or which CMake is able to solve, some 
examples:
* python requirements.txt - no need to have a built system at all
* cocoapods - it's hard to customize/extend Xcode project to support it 
(one of the reason why such tools like CMake exist)
* brew/apt-get/emerge - (don't have any spec files actually, set 
dependencies in README file) because widely used tools like Makefile is 
not designed to do even a simple task like finding libraries, other 
tools like autotools are too complex to extend/use (again, that's why 
CMake exist)
* ... your examples here ...

Cheers, Ruslo
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