[cmake-developers] [CMake] libc++ usage in CMake with Clang?

Craig Scott craig.scott at crascit.com
Wed Aug 22 05:05:53 EDT 2018


On Wed, Aug 22, 2018 at 1:39 PM, Ian Henriksen <
insertinterestingnamehere at gmail.com> wrote:

>
>
> On Tue, Aug 21, 2018 at 6:40 PM Craig Scott <craig.scott at crascit.com>
> wrote:
>
>>
>> On Wed, Aug 22, 2018 at 7:18 AM, Robert Dailey <rcdailey.lists at gmail.com>
>> wrote:
>>
>>> On Tue, Aug 21, 2018 at 3:47 PM Craig Scott <craig.scott at crascit.com>
>>> wrote:
>>> > Excuse the brevity, but it sounds like you might be looking for the
>>> CXX_EXTENSIONS target property (sorry if I've misunderstood your problem,
>>> let me know why it isn't appropriate if so). See the following article for
>>> a more complete overview of this and related properties:
>>> >
>>> > https://crascit.com/2015/03/28/enabling-cxx11-in-cmake/
>>>
>>> Unfortunately that's not the same. Extensions manage C++ language
>>> features and STL capabilities, but -stdlib is for selecting an STL
>>> implementation, AFAIK. Such as GNU STL and LLVM STL (which is libc++
>>> to clang).
>>>
>>
>> Sorry, yes I misunderstood your problem. After a little digging, it seems
>> like you probably shouldn't be using the -stdlib option on Linux
>> <https://stackoverflow.com/a/50407611/1938798> anyway. FWIW, for
>> Android, the roadmap
>> <https://android.googlesource.com/platform/ndk/+/master/docs/Roadmap.md>
>> is converging on a single STL implementation too.
>>
>
> All that first link says is that -stdlib is a flag that is specific to
> clang and that it shouldn't be used with gcc. You can use clang on Linux
> with either libstdc++ or libc++. I often use libc++ on Linux by setting
> CMAKE_CXX_FLAGS on the command line, though I'll admit that for me it's
> usually just to check if problems that come up are OS dependent, compiler
> dependent, or standard library dependent. You have to be careful since
> libstdc++ and libc++ have incompatible ABIs, but it's a useful feature.
> That said, I have no idea if specifying the standard library implementation
> merits handling at the CMake level since only clang supports switching
> anyway.
>


Good clarification, thanks. I was only thinking GCC on Linux and wasn't
considering clang (which was a bit dumb on my part - blame the lack of
coffee early in the morning ;) ).

Getting back to Robert's original query, the only part of the CMake code
base that I can see attempting to account for a -stdlib option is for
detection of gcc include paths, and this is only for Eclipse and CodeBlocks
generators (according to the comments in
Modules/CMakeExtraGeneratorDetermineCompilerMacrosAndIncludeDirs.cmake). It
doesn't seem to be related to providing any support for manipulating it in
a project. The only other place -stdlib seems to be mentioned is in the
setup of the macOS release build of CMake itself, which isn't relevant to
the discussion here.

If CMake were to offer direct support for -stdlib, it sounds like it would
be a clang-specific feature, so a clang-specific target property and/or
variable may be a way forward, analogous to the way it is done for Android.
Alternatively, maybe it could be done with generator expressions, but it
would be a bit verbose and harder to ensure the same -stdlib was used
consistently throughout if many targets were involved. But maybe just a
fairly simple check is good enough here, something like this (using
directory properties instead of variables):

if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT ANDROID)
    add_compile_options(-stdlib=libc++)
    # Presumably need the above for linking too, maybe other options
missing as well
    add_link_options(-stdlib=libc++)   # New command on CMake master, not
in 3.12 release
endif()

The linking comments above are in response to discussions in a recent issue
<https://gitlab.kitware.com/cmake/cmake/issues/18275> also related to the
-stdlib option. One down side of the above is no transitive dependency
details, something that a target property could achieve. I'll pause here
and see what others think.




>
> Just my two cents though.
>
> Best,
>
> Ian
>
>
>> Regarding your earlier comment:
>>
>> I'll explain a bit why I'm asking. I noticed that for code bases that
>>> work on Android plus other UNIX platforms, they unconditionally
>>> specify `-stdlib=libc++`, however this doesn't work on Ubuntu by
>>> default, which uses gnu stl + gcc/clang. So  you get compiler errors.
>>> There's no way for me to "search" a platform to see if it is eligible
>>> for the libc++ flag, I simply have to either disable it completely or
>>> conditionally include it based on target platform and/or toolchain.
>>> None of these really address the root cause.
>>
>> If you are trying to control which STL to use for Android builds, CMake variables like CMAKE_ANDROID_STL_TYPE are probably the more appropriate way to do that rather than hard-coding compiler flags. This would also mean that non-Android builds won't be affected since they would simply ignore that variable (and target properties it may affect) and should then pick up the right STL implementation automatically.The Android-specific variable would ideally be set in a toolchain file rather than in the project itself.
>>
>>
>> --
>>
>> Craig Scott
>> Melbourne, Australia
>> https://crascit.com
>>
>> New book released: Professional CMake: A Practical Guide
>> <https://crascit.com/professional-cmake/>
>> --
>>
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>


-- 
Craig Scott
Melbourne, Australia
https://crascit.com

New book released: Professional CMake: A Practical Guide
<https://crascit.com/professional-cmake/>
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