Am 01.11.19 um 17:24 schrieb Stephen Morris:
On 1 November at 10:02, Stephen Morris wrote:
My approach is basically to set up a custom command thus:
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)
My earlier question still stands, but at the time I wrote it I had only tested
it for Debug builds where ${CMAKE_CXX_FLAGS_DEBUG} consisted of a single item,
'-g'.
I've since tried doing the same thing for a Release build, and this failed because
${CMAKE_CXX_FLAGS_RELEASE} has two items, '-O3 -DNDEBUG'. When the custom command is
executed, this somehow becomes "-O3\ -DNDEBUG" on the command line, and the
presence of the stray backslash causes the compilation to fail with the message,
"cc1plus: error: argument to '-O' should be a non-negative integer. 'g', 's' or
'fast'"
So why isn't the cmake variable placed properly onto the command line, and what
can I do to prevent this behaviour?
Welcome to the wonderful world of lists and their interaction with
quoting in CMake ! Since space is used as a list item separator outside
of double quoted strings, CMake is escaping the space within CXX_FLAGS
to keep it as a single element within the list of arguments passed to
add_custom_command. To avoid this, you need to turn CXX_FLAGS into a
list prior to using it in add_custom_command:
string(REGEX REPLACE " " ";" CXX_FLAGS "${CXX_FLAGS}")
Of course, this will not work if you want to pass a double quoted string
as an actual command line argument ...
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