cmake: How do I correctly set variables to precede make in an external project?

Question

I am working on setting up a cross compile project for libraries I need in a pi project. I want to cross the latest mosquitto library and I have figured out what I need to pass to make for it to build correctly. Unfortunately when I define my BUILD_COMMAND I cannot seem to get the variables set correctly to preceed the call to make.

Here is the external project defined in my CMakeLists.txt:

ExternalProject_Add(mosquitto
    URL ${SRC_URL} URL_MD5 ${SRC_MD5}
    BUILD_IN_SOURCE 1
    CONFIGURE_COMMAND echo "No configuration necessary."
    BUILD_COMMAND cd <SOURCE_DIR>/lib && export CC=gcc && export CXX=g++ && export CROSS_COMPILE=${CROSS_COMPILE_TRIPLE} && export CFLAGS=--sysroot=${CMAKE_SYSROOT} && export LDFLAGS="--sysroot=${CMAKE_SYSROOT} -Wl,-rpath-link,${CMAKE_SYSROOT}/lib/arm-linux-gnueabihf -Wl,-rpath-link,${CMAKE_SYSROOT}/usr/lib/arm-linux-gnueabihf -L${CMAKE_SYSROOT}/usr/lib/arm-linux-gnueabihf" && make --trace

Here is the make output of the step that fails:

arm-linux-gnueabihf-gcc -shared "--sysroot=/home/heardg/pi/system/devroot -Wl,-rpath-link,/home/heardg/pi/system/devroot/lib/arm-linux-gnueabihf -Wl,-rpath-link,/home/heardg/pi/system/devroot/usr/lib/arm-linux-gnueabihf -L/home/heardg/pi/system/devroot/usr/lib/arm-linux-gnueabihf" -Wl,--version-script=linker.version -Wl,-soname,libmosquitto.so.1 mosquitto.o logging_mosq.o memory_mosq.o messages_mosq.o net_mosq.o read_handle.o read_handle_client.o read_handle_shared.o send_mosq.o send_client_mosq.o socks_mosq.o srv_mosq.o thread_mosq.o time_mosq.o tls_mosq.o util_mosq.o will_mosq.o -o libmosquitto.so.1  -lrt -lssl -lcrypto -lpthread -lcares
/home/heardg/pi/linaro/gcc-linaro-4.9-2016.02-x86_64_arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.4/../../../../arm-linux-gnueabihf/bin/ld: cannot find crti.o: No such file or directory
/home/heardg/pi/linaro/gcc-linaro-4.9-2016.02-x86_64_arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.4/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lrt
/home/heardg/pi/linaro/gcc-linaro-4.9-2016.02-x86_64_arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.4/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lssl
/home/heardg/pi/linaro/gcc-linaro-4.9-2016.02-x86_64_arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.4/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lcrypto
/home/heardg/pi/linaro/gcc-linaro-4.9-2016.02-x86_64_arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.4/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lpthread
/home/heardg/pi/linaro/gcc-linaro-4.9-2016.02-x86_64_arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.4/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lcares
collect2: error: ld returned 1 exit status
Makefile:46: recipe for target 'libmosquitto.so.1' failed`

I have found that it is the quotes around the LDFLAGS that are causing the problem.

"--sysroot=/home/heardg/pi/system/devroot -Wl,-rpath-link,/home/heardg/pi/system/devroot/lib/arm-linux-gnueabihf -Wl,-rpath-link,/home/heardg/pi/system/devroot/usr/lib/arm-linux-gnueabihf -L/home/heardg/pi/system/devroot/usr/lib/arm-linux-gnueabihf"

If I run the link command by hand and remove the double quotes above it succeeds.

How can I better form the arguments in the BUILD_COMMAND to get rid of the quotes?

Answer 1

When there are a lot of commands for ExternalProject_Add, it could be wise to use script, which executes them. As for passing CMake variables to the script, them could be passed via script's arguments, or via configuration of the script itself:

build_mosquitto.sh.in:

# The only argument to the script is mosquitto's source directory.
source_dir=$1

exports CC=gcc
export CXX=g++
export CROSS_COMPILE=@CROSS_COMPILE_TRIPLE@
# ... other exports

cd ${source_dir}/lib && make

CMakeLists.txt:

configure_file("build_mosquitto.sh.in" "build_mosquitto.sh" @ONLY)

ExternalProject_Add(mosquitto
    URL ${SRC_URL} URL_MD5 ${SRC_MD5}
    BUILD_IN_SOURCE 1
    CONFIGURE_COMMAND echo "No configuration necessary."
    BUILD_COMMAND sh "${CMAKE_CURRENT_BINARY_DIR}/build_mosquitto.sh" <SOURCE_DIR>
)