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<div class="moz-cite-prefix">Am 01.11.19 um 11:02 schrieb Stephen
Morris:<br>
</div>
<blockquote type="cite"
cite="mid:VE1PR06MB6350867023D11505C22C0CBFC9620@VE1PR06MB6350.eurprd06.prod.outlook.com">
<pre class="moz-quote-pre" wrap="">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?</pre>
</blockquote>
<p>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):</p>
<p><tt>...</tt><tt><br>
</tt><tt>get_target_property(_tmp mytarget
POSITION_INDEPENDENT_CODE)</tt><tt><br>
</tt><tt>if(_tmp AND CMAKE_CXX_COMPILE_OPTIONS_PIC)</tt><tt><br>
</tt><tt> string(APPEND CXX_FLAGS "
${CMAKE_CXX_COMPILE_OPTIONS_PIC}")</tt><tt><br>
</tt><tt>endif()</tt><tt><br>
</tt><tt>...</tt><br>
</p>
<p>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.</p>
<p>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.</p>
<p>
</p>
<blockquote type="cite"
cite="mid:VE1PR06MB6350867023D11505C22C0CBFC9620@VE1PR06MB6350.eurprd06.prod.outlook.com"></blockquote>
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