[cmake-developers] CMAKE_TOOLCHAIN_FILE and ExternalProject_Add

[Andrey Pokrovskiy]

Because it will be an ad-hoc solution. Also each time I will start
using a new toolchain file (for another device/platform) I will need
to modify that wrapper script. And each time I will override some
toolchain variable I will need to look into that wrapper to verify
that this variable will be forwarded. Wouldn't it be nice if there was
a simple way to tell "build this external project the same way as a
main one"?

I'm not saying that the problem is unsolvable. I'm saying that right
now CMake is not very convenient for projects with a lot of external
dependencies. And specifying toolchain and it's options is a one
aspect of it (not the most important, though).


On Sun, Apr 26, 2015 at 5:36 PM, David Cole <DLRdave at aol.com> wrote:
> Why wouldn't you just write your own "toolchain wrapper" file which
> has all the variables you speak of, but also includes the "real"
> toolchain file, and then use the wrapper as your CMAKE_TOOLCHAIN_FILE
> value ... ?
>
>
> D
>
>
>
> On Sun, Apr 26, 2015 at 7:32 PM, Andrey Pokrovskiy
> <wonder.mice at gmail.com> wrote:
>> Hi,
>>
>> I'm using cmake and CMAKE_TOOLCHAIN_FILE to build the project and it's
>> dependencies. Some toolchain files have additional options (like API
>> version, target architecture, linker type, etc.) which you need to
>> pass to cmake command with "-D" in addition to the
>> "-DCMAKE_TOOLCHAIN_FILE=" itself.
>>
>> So when you want to build some dependency with ExternalProject_Add()
>> it's not enough to pass the same CMAKE_TOOLCHAIN_FILE. You also need
>> to explicitly specify all other toolchain variables that you care
>> about. That means that each time you start to override another
>> toolchain variables or add a new toolchain, you need to carefully add
>> proper variable forwardings to each ExternalProject_Add() call. This
>> is not convenient and error prone.
>>
>> What do you think about formalizing toolchains a bit further? For
>> example, it would be nice to have a way to get all variables (with
>> their values) that are relevant for the current toolchain. Consider:
>>
>>     ExternalProject_Add(websockets_ep
>>         CMAKE_ARGS
>>             -DCMAKE_TOOLCHAIN_FILE:string=${CMAKE_TOOLCHAIN_FILE}
>>             -DCMAKE_TOOLCHAIN_ARGS:string=${CMAKE_TOOLCHAIN_ARGS})
>>
>> Where ${CMAKE_TOOLCHAIN_ARGS} expands to "VAR1=VAL1;VAR2=VAL2;" (I
>> realize that will not work like that, but conceptually) which defines
>> toolchain configuration.
>>
>> For example, well behaved toolchain file could be asked to mark each
>> configuration variable:
>>
>>     toolchain_option(ANDROID_NATIVE_API_LEVEL)
>>
>> so CMake after processing of a toolchain file can have a list of all
>> relevant variables. Or maybe CMake can track all variables "touched"
>> by the toolchain file.
>>
>> What do you think about that?
>> --
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