Cross compilation qt6 for raspberry pi build failure, can't find .so files

matio · wrote on 15 May 2022, 16:24

Hello,

I try to build Qt 6.1.3 for Raspberry Pi 3 with my x64 PC.

I had no big issue while configuring for cross compilation. I also succeed to build Qt for the host following the page:

https://wiki.qt.io/Building_Qt_6_from_Git

I encoutered some issue at build. It seems that at build, the compiler/linker doesn't find the ".so" files he just generated.

The output message is :

$MY_QT_TEST_DIR_PATH/toolchain/qt-cross/qtbase/libexec/uic: error while loading shared libraries: libQt6Core.so.6: cannot open shared object file: No such file or directory

I followed the informations in this page:

https://doc-snapshots.qt.io/qt6-dev/configure-linux-device.html

my work dir is set like this:

$MY_QT_TEST_DIR_PATH/qt6-build: build dir for host's Qt
$MY_QT_TEST_DIR_PATH/qt-host: install dir for host's Qt
$MY_QT_TEST_DIR_PATH/rpi-sdk/sysroot: sysroot creatred with rsync from rpi
$MY_QT_TEST_DIR_PATH/qt-cross: build dir for target's Qt
$MY_QT_TEST_DIR_PATH/qt-sources: Qt sources configured

My toolchain.cmake file:

set(TARGET_NAME "arm-rpi")
	
# project template options
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR armv)
#set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)


# crosschain compiler
set(CROSSCHAIN_SYSROOT "$MY_QT_TEST_DIR_PATH/toolchain/rpi-sdk/sysroot")
set(CROSSCHAIN_PATH "/usr/bin")      # not sure
set(CMAKE_C_COMPILER arm-linux-gnueabihf-gcc)
set(CMAKE_CXX_COMPILER arm-linux-gnueabihf-g++)
set(CMAKE_SYSROOT ${SYSROOTS})
	
set(CMAKE_C_FLAGS "${CMAKE_CXX_FLAGS} --sysroot=\"${CROSSCHAIN_SYSROOT}\"")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --sysroot=\"${CROSSCHAIN_SYSROOT}\"")

set(LD_LIBRARY_PATH "$MY_QT_TEST_DIR_PATH/toolchain/qt-cross/qtbase/lib/") # just tried this because of current problem but doesn't changed anything, set even if out of this file as env variable

set(QT_COMPILER_FLAGS "-march=armv7-a -mfpu=neon -mfloat-abi=hard")
set(QT_COMPILER_FLAGS_RELEASE "-O2 -pipe")
set(QT_LINKER_FLAGS "-Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed")

message("CMAKE_C_FLAGS=${CMAKE_C_FLAGS}")
message("CMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}")	

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

include(CMakeInitializeConfigs)

function(cmake_initialize_per_config_variable _PREFIX _DOCSTRING)
  if (_PREFIX MATCHES "CMAKE_(C|CXX|ASM)_FLAGS")
    set(CMAKE_${CMAKE_MATCH_1}_FLAGS_INIT "${QT_COMPILER_FLAGS}")

    foreach (config DEBUG RELEASE MINSIZEREL RELWITHDEBINFO)
      if (DEFINED QT_COMPILER_FLAGS_${config})
        set(CMAKE_${CMAKE_MATCH_1}_FLAGS_${config}_INIT "${QT_COMPILER_FLAGS_${config}}")
      endif()
    endforeach()
  endif()

  if (_PREFIX MATCHES "CMAKE_(SHARED|MODULE|EXE)_LINKER_FLAGS")
    foreach (config SHARED MODULE EXE)
      set(CMAKE_${config}_LINKER_FLAGS_INIT "${QT_LINKER_FLAGS}")
    endforeach()
  endif()

  _cmake_initialize_per_config_variable(${ARGV})
endfunction()

My configure command:

$MY_QT_TEST_DIR_PATH/toolchain/qt-sources/configure -release -opengl es2 -nomake examples -nomake tests -qt-host-path $MY_QT_TEST_DIR_PATH/toolchain/qt-host -extprefix $MY_QT_TEST_DIR_PATH/toolchain/qt-cross -prefix $MY_QT_TEST_DIR_PATH/toolchain/rpi-sdk/toolchain -DCMAKE_TOOLCHAIN_FILE=$MY_QT_TEST_DIR_PATH/toolchain/qt-cross/toolchain.cmake

I have some doubts about things:

For the toolchain, for Qt5 I used a cross compilation toolchain I found here:

https://github.com/raspberrypi/tools

But it seems to be obsolete. So I followed the instructions by installing "gcc-arm-linux-gnueabihf" and "g++-arm-linux-gnueabihf" with apt. So I presume that I should consider that the toolchain is installed in "/usr/bin" since "gcc-arm-linux-gnueabihf" is installed there? So I should set "CROSSCHAIN_PATH" as "/usr/bin"? I'd prefer downloading an up to date toolchain like I did for qt5 but I haven't found one. Does this exists?

Another point: I suppose that the "so" files have to be the one generated by the cross compiler in "$MY_QT_TEST_DIR_PATH/qt-cross/qtbase" ?

I'm not an expert on build tools, I try to understand but I think some knowledges are missing to me!

Can you help me? Thank you for any help!

Tatiana Borisova · wrote on 14 Jun 2022, 16:27

Hi matio,

I see I am a little bit late. You asked your question almost month ago, but..

For cross-compiling you need:

Build Qt 6.1.3 for HOST and install it (run "cmake install . " )
create sysroot
create toolchain.cmake file
configure
cmake build .
cmake install .

I cross-built for Rasp Pi device recently and maybe following configure command can be helpful for you cross-building:

cmake ../qt5/ -GNinja -DCMAKE_BUILD_TYPE=Release -DINPUT_opengl=es2 -DQT_BUILD_EXAMPLES=OFF -DQT_BUILD_TESTS=OFF -DQT_HOST_PATH=$MY_QT_TEST_DIR_PATH/toolchain/qt-cross -DCMAKE_INSTALL_PREFIX=$MY_QT_TEST_DIR_PATH/toolchain/rpi-sdk/toolchain -DCMAKE_TOOLCHAIN_FILE=$MY_QT_TEST_DIR_PATH/toolchain/qt-cross/toolchain.cmake  -DQT_FEATURE_brotli=OFF -DQT_FEATURE_libudev=OFF  -DQT_QMAKE_TARGET_MKSPEC=devices/linux-rasp-pi3-g++

Also regarding CROSSCHAIN_PATH - it is optional.
You can install compilers on sustem and do directly following inside toolchain file:

set(CMAKE_C_COMPILER /usr/bin/arm-linux-gnueabihf-gcc)
set(CMAKE_CXX_COMPILER /usr/bin/arm-linux-gnueabihf-g++)

I think you can exchange:

set(TARGET_NAME "arm-rpi")

# project template options
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR armv)
#set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)

# crosschain compiler
set(CROSSCHAIN_SYSROOT "$MY_QT_TEST_DIR_PATH/toolchain/rpi-sdk/sysroot")
set(CROSSCHAIN_PATH "/usr/bin")      # not sure
set(CMAKE_C_COMPILER arm-linux-gnueabihf-gcc)
set(CMAKE_CXX_COMPILER arm-linux-gnueabihf-g++)
set(CMAKE_SYSROOT ${SYSROOTS})
	
set(CMAKE_C_FLAGS "${CMAKE_CXX_FLAGS} --sysroot=\"${CROSSCHAIN_SYSROOT}\"")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --sysroot=\"${CROSSCHAIN_SYSROOT}\"")

With following:

cmake_minimum_required(VERSION 3.18)
include_guard(GLOBAL)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

set(TARGET_SYSROOT "$MY_QT_TEST_DIR_PATH/toolchain/rpi-sdk/sysroot") 
set(CMAKE_SYSROOT ${TARGET_SYSROOT})

set(ENV{PKG_CONFIG_PATH} "")
set(ENV{PKG_CONFIG_LIBDIR} ${CMAKE_SYSROOT}/usr/lib/pkgconfig:${CMAKE_SYSROOT}/usr/share/pkgconfig:${CMAKE_SYSROOT}/${TARGET_SYSROOT}/pkgconfig)
set(ENV{PKG_CONFIG_SYSROOT_DIR} ${CMAKE_SYSROOT})

set(CMAKE_C_COMPILER /usr/bin/arm-linux-gnueabihf-gcc)
set(CMAKE_CXX_COMPILER /usr/bin/arm-linux-gnueabihf-g++)

About the "so" files have to be the one generated by the cross compiler:
Please, if problem is still valid share full compilation error log.

NOTE: Also speaking about Raspberry Pi 3, are you sure that you HW is march=armv7-a based?*
(you can check like pi@raspberrypi:~ $ uname -a)

I see from qt configs for Raspbery pi3: QMAKE_CFLAGS = -march=armv8-a -mtune=cortex-a53 -mfpu=crypto-neon-fp-armv8

In case it is arm8 you should use another compilers (for example aarch64-linux-gnu-gcc/aarch64-linux-gnu-g++) + flags

set(QT_COMPILER_FLAGS "-march=armv8-a " .....)

Tatiana Borisova · wrote on 23 Jun 2022, 07:33

Also there is a new instruction for Rasp + Qt6:
https://wiki.qt.io/Cross-Compile_Qt_6_for_Raspberry_Pi