[CMake] Accessing -fPIC and -std settings within CMake definitions

[Eric Doenges]

Am 01.11.19 um 11:02 schrieb Stephen Morris:
> I'm setting up a custom build step to build a precompiled header in gcc (yes I know that native support is coming with CMake 3.16, but I need to get something working on an older version).
>
> My approach is basically to set up a custom command thus:
>
> set_target_properties(mytarget PRIVATE CXX_STANDARD 17)
> set_target_properties(mytarget PRIVATE  POSITION_INDEPENDENT CODE_1)
> set(CMAKE_VERBOSE_MAKEFILE TRUE)
> ...
> ...
> set(CXX_FLAGS ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_DEBUG})   .. or whatever, depending on the current configuration..
> get_target_property(compile_options, mytarget, COMPILE_OPTIONS)
> add_custom_command(OUTPUT myheader.h.gch
>                                             COMMAND ${CMAKE_CXX_COMPILER} ${CXX_FLAGS} ${compile_options} -fPIC -std=gnu++17 -c myheader.h -o myheader.h.gch
>                                             DEPENDS myheader.h)
> add_custom_target(BuildMyPCH
>                                       DEPENDS myheader.h.gch)
> add_dependencies(mytarget, BuildMyPCH)
>
> You'll note that in my custom command I've had to specify -fPIC and -std-gnu++17 explicitly, because they aren't included among either CXX_FLAGS or COMPILE_OPTIONS. I've looked for them among the  COMPILE_DEFINITIONS, COMPILE_FEATURES and COMPILE_FLAGS properties of my target as well, but there's no sign of them. And yet if I look at the verbose output when gcc builds other files using the CMake-generated makefile, I see that they are being included automatically.
>
> So, my question: where is CMake storing these settings, and how can I apply them in my custom command without having to do so manually?

As far as I am aware, there is no way of querying the entirety of the 
command line arguments applied by CMake - you will need to recreate 
CMake's behavior manually. In case of -fPIC, this could look like this 
(not tested, so there may be syntax errors, but the principle should work):

...
get_target_property(_tmp mytarget POSITION_INDEPENDENT_CODE)
if(_tmp AND CMAKE_CXX_COMPILE_OPTIONS_PIC)
   string(APPEND CXX_FLAGS " ${CMAKE_CXX_COMPILE_OPTIONS_PIC}")
endif()
...

If you have multiple targets that may have different compile options you 
will probably want to put this in a function that takes a target as 
input and returns the compiler flags as output.

To figure out the names of the variables CMake uses to hold flags to 
pass to the compiler, search the <...>/share/cmake-3.xx/Modules/Compiler 
directory.

-- 

*Dr. Eric Dönges*
Senior Software Engineer

MVTec Software GmbH | Arnulfstr. 205 | 80634 Munich | Germany
doenges at mvtec.com <mailto:mustermann at mvtec.com> | Tel: +49 89 457 695-0 
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