Strange behaviour of ExternalProject_Add with a command containing a space - c++

Question

I need to compile boost libraries with ExternalProject_Add, and the build command needs
c++11 flags on MacOS platform with Clang.
The command should look like this:
./bin/b2 debug release cxxflags="-std=c++11 -stdlib=libc++" linkflags=-stdlib=libc++
But I have a problem with the quotes and space.
set(BOOST_CXX_FLAGS cxxflags="-std=c++11 -stdlib=libc++")
set(BOOST_TOOL_SET toolset=clang ${BOOST_CXX_FLAGS}
linkflags=-stdlib=libc++)
ExternalProject_Add(boost
....
BUILD_COMMAND ./bin/b2 debug release
${BOOST_TOOL_SET}
....
)
The ${BOOST_TOOL_SET} value is a list, and cxxflags="-std=c++11
-stdlib=libc++" is one item in it. The generated command line becomes strange:
./bin/b2 debug release "cxxflags=\"-std=c++11 -stdlib=libc++\""
linkflags=-stdlib=libc++
It seems the flag is translated by CMake when it detected the space inside the argument and wrapped it with quote marks, but it's not what I want.
I searched on the Internet, but haven't found any help. Is there any tip about this issue?

Answer 1

This should work:
set(BOOST_CXX_FLAGS "cxxflags=-std=c++11 -stdlib=libc++")
This should produce
./bin/b2 debug release "cxxflags=-std=c++11 -stdlib=libc++"
Under normal shell parsing rules, this is equivalent to what works for you:
./bin/b2 debug release cxxflags="-std=c++11 -stdlib=libc++"

Answer 2

This doesn't answer your question exactly, as in I can't see a way to get your requested command line to the b2 exe with the cxxflags="-std=c++11 -stdlib=libc++" portion correctly formatted.
However, I believe you can accomplish the desired effect by calling cxxflags= twice. Each argument is appended to the compiler flags eventually invoked by b2.
So you should be able to do:
set(BOOST_CXX_FLAGS cxxflags=-std=c++11 cxxflags=-stdlib=libc++)
and the eventual command invoked by b2 will be something like
"clang++" ... -std=c++11 -stdlib=libc++ ...
To verify this, you can add -d+2 to your command:
BUILD_COMMAND ./bin/b2 debug release ${BOOST_TOOL_SET} -d+2
This causes the full commands to be written to the boost-build-out.log file in your boost-stamp directory.