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
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