[CMake] XCode Generation Issue

[David Morsberger]

Eric,

I went down a rabbit hole and pulled myself back out. I narrowed it down to the following.

We are using cmake to check if the linker supports -Wl,—as-needed. The test compile (and link?) command cmake builds using ‘cmake -G Xcode’  does not fail.

The command from ‘cmake -G’ has been narrowed down to the following:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang \
 -Qunused-arguments \
 -DWS_LD_FLAG_VALID0 \
 -Wl,--as-needed \
 -c src.c \
 -o src.o 
$

The following modified command generates an error:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang \
 -Qunused-arguments \
 -DWS_LD_FLAG_VALID0 \
 -Wl,--as-needed \
 src.c

ld: unknown option: --as-needed
clang: error: linker command failed with exit code 1 (use -v to see invocation)
$

It appears the linker isn’t called when the -c and the -o options are present. The following warning is generated when the ‘-Qunused-arguments’ is removed.

clang: warning: -Wl,--as-needed: 'linker' input unused

The cmake logic in CMakeLists.txt is basically:

   SET(CMAKE_REQUIRED_FLAGS "${_FLAG}")
   CHECK_C_SOURCE_RUNS(“int main() { return 0;}" ${_RESULT})

where _FLAG is set to ‘-DWS_LD_FLAG_VALID0 -Wl,—-as-needed’


So, how/where is the clang compile/link command generated and why isn’t it being setup for clang to link?



> On Jan 21, 2016, at 1:51 AM, Eric Wing <ewmailing at gmail.com> wrote:
> 
> On 1/20/16, David Morsberger <dave at morsberger.com> wrote:
>> 
>>> On Jan 19, 2016, at 10:11 PM, Eric Wing <ewmailing at gmail.com> wrote:
>>> 
>>> On 1/19/16, David Morsberger <dave at morsberger.com> wrote:
>>>> Any help would be appreciated.
>>>> 
>>>> In particular I’d like to know how the default compiler arguments are
>>>> set
>>>> when ‘cmake -G Xcode’ is executed. The attached file contains the cmake
>>>> test
>>>> compile extracted and reformatted from CMakeOutput.log that should have
>>>> failed. Again, what is setting all the clang compiler options when this
>>>> is
>>>> executed?
>>>> 
>>>> 
>>>> 
>>>> All help is appreciated
>>>> 
>>>>> On Jan 18, 2016, at 2:58 PM, David Morsberger <dave at morsberger.com>
>>>>> wrote:
>>>>> 
>>>>> I’m having an issue generating and using a XCode project using cmake
>>>>> -G.
>>>>> 
>>>>> cmake —version
>>>>>   cmake version 3.4.1
>>>>> 
>>>>> Xcode Version 7.2 (7C68)
>>>>> 
>>>>> Mac OS 10.11.
>>>>> 
>>>>> 
>>>>> My CMakeLists.txt has a clang option that fails during the -G Xcode
>>>>> pass
>>>>> but fails when linking within XCode.
>>>>> 
>>>>> The compiler / linker option is ‘-Wl,—as-needed’
>>>>> 
>>>>> The test compile/link step in ‘cmake -G Xcode' creates a long clang
>>>>> command with arguments ‘-Wl,—as-needed’ plus ‘—serialize-diagnostics
>>>>> <path
>>>>> to .dia file>’
>>>>> 
>>>>> The link step within Xcode after the project created has the
>>>>> ‘-Wl,—as-needed’ argument and does not have the ‘—serialize-diagnostics
>>>>> <path to .dia file>’ argument. The link fails because ‘-Wl,—as-needed’
>>>>> isn’t supported.
>>>>> 
>>>>> If copy and execute the long clang command from the ‘cmake -G Xcode’
>>>>> command and remove the ‘—serialize-diagnostics <path to .dia file>’
>>>>> then
>>>>> it successfully fails with ‘-Wl, —as-needed’ isn’t supported.
>>>>> 
>>>>> Finally, when I use cmake to generate unix makefiles it works meaning
>>>>> cmake determines ‘-Wl—as-needed’ isn’t supported and doesn’t add it to
>>>>> the
>>>>> argument list.
>>>>> 
>>>>> Questions:
>>>>> - Where is the ‘—serialized-diagnostics’ argument coming from when
>>>>> executing ‘cmake -G Xcode’?
>>>>> 
>>>>> - What is a .dia file and how do I read it?
>>>>> 
>>>>> - What’s the preferred / recommended way of detecting the non-supported
>>>>> ‘-Wl,—as-needed’ argument during the ‘cmake -G Xcode’ phase? One option
>>>>> might to be to remove the ‘—serialize-diagnostics’ argument during the
>>>>> generation phase.
>>>>> 
>>>>> I can provide more details on request.
>>>>> 
>>>>> David
>>>>> 
>>>>> 
>>>>> 
>>> 
>>> 
>>> I just spot checked one of my Xcode compile logs. I see the
>>> —serialize-diagnostics flag, but I don't have —as-needed anywhere.
>>> Maybe one of your CMake scripts is adding that in, or you have set
>>> some kind of environment variable that clang or Xcode is picking up?
>>> (This recently happened to me on Linux.)
>> 
>> Ed,
>> 
>> In this case cmake is attempting to determine if ‘—as-needed' is a supported
>> option and the build should fail.
>> 
>> I have searched (grep’d) everything I can think of for the string
>> ’serialize-diagnostics’ with zero hits.
>> 
>> I can easily add an ‘if( Xcode )’ around the setting in CMakeLists.txt
>> however I’m trying to figure out what is exactly going on. The best
>> solutions are chosen when you know what is going on under the hood.
>> 
>> 
>> 
> 
> I believe the serialize-diagnostics flag is added by Xcode itself. I
> see it (along with many other flags) in a native (non-CMake generated)
> Xcode project.
> 
> -Eric

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