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Thread: Cross compiling qt6 for RPI 64 bit (aarch64)

  1. #1
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    Default Cross compiling qt6 for RPI 64 bit (aarch64)

    I'm trying to cross compile qt6 for Bullseye on an Ubuntu 22.04.

    Everything compiles, but I seeing unresolved externals that (I think) should be in clib:
    Qt Code:
    1. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_dlopen_mode@GLIBC_PRIVATE'
    2. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_current_sigrtmin_private@GLIBC_PRIVATE'
    3. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_dlclose@GLIBC_PRIVATE'
    4. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libdl.so.2: undefined reference to `_dl_sym@GLIBC_PRIVATE'
    5. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_longjmp@GLIBC_PRIVATE'
    6. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_allocate_rtsig_private@GLIBC_PRIVATE'
    7. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_thread_freeres@GLIBC_PRIVATE'
    8. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_dlsym@GLIBC_PRIVATE'
    9. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_current_sigrtmax_private@GLIBC_PRIVATE'
    10. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libdl.so.2: undefined reference to `_dl_addr@GLIBC_PRIVATE'
    11. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libdl.so.2: undefined reference to `_dl_vsym@GLIBC_PRIVATE'
    12. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `__libc_pthread_init@GLIBC_PRIVATE'
    13. /usr/lib/gcc-cross/aarch64-linux-gnu/10/../../../../aarch64-linux-gnu/bin/ld: /home/dave/rpi-sysroot/lib/aarch64-linux-gnu/libpthread.so.0: undefined reference to `_dl_make_stack_executable@GLIBC_PRIVATE'
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    Another forum has a post saying that this is probably due to a mismatch between the cross-compiler on Ubuntu and the rpi.

    Here's what's on the pi:

    Qt Code:
    1. pi@raspberrypi:~ $ g++ --version
    2. g++ (Debian 10.2.1-6) 10.2.1 20210110
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    Here's what I have in the "toolchain.cmake":

    Qt Code:
    1. set(CMAKE_C_COMPILER /usr/bin/aarch64-linux-gnu-gcc-10)
    2. set(CMAKE_CXX_COMPILER /usr/bin/aarch64-linux-gnu-g++-10)
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    I've also tried the -9 and -11 versions of gcc and g++, but get the same errors.

    The poster said he solved the error by downgrading the Ubuntu host to 20.x. I'd do that, if I thought it would work, but seems like I should be able to use the same cross-compiler.

    Three questions:

    1) Are the unresolved externals indeed (likely) due to an incompatible C run time used by the cross-compiler?
    2) What cross compiler toolchain should I use?
    3) What cross compiler is available for cross-compiling on Ubuntu 20 (if any) that's not on Ubuntu 22?

    Here's the guide I've been using:

    https://wiki.qt.io/Cross-Compile_Qt_6_for_Raspberry_Pi
    Thanks!

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    Default Re: Cross compiling qt6 for RPI 64 bit (aarch64)

    I do not know if this is the cause, but unlike in Windows with Microsoft compilers, the order of modules specified in the linker command matters in linux / gcc. If any module depends on references defined in another module, the module that resolves the dependency must come after the module that needs it on the command line.

    I pounded my head on the wall for some time trying to get something to link before I learned this nugget.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

  3. #3
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    Default Re: Cross compiling qt6 for RPI 64 bit (aarch64)

    Quote Originally Posted by d_stranz View Post
    I do not know if this is the cause, but unlike in Windows with Microsoft compilers, the order of modules specified in the linker command matters in linux / gcc. If any module depends on references defined in another module, the module that resolves the dependency must come after the module that needs it on the command line.

    I pounded my head on the wall for some time trying to get something to link before I learned this nugget.
    I don't understand why, but starting over with host Ubuntu 20.04 works fine.

    Someone posted that on another forum, but that didn't make sense to me. And, since it takes a LONG time to recompile everything that's needed to get to the link stage, I was hesitant to even try. Seemed like all I should need was the right toolchain.

    But, spent a couple of days trying various gcc and g++ versions, including the version on the target (Bullseye aarch rpi) and the version on Ubuntu 20.04 on the Ubuntu 22 host.

    I followed the guide exactly--on Ubuntu 22 there were several issues I was able to work-around just to get to the "show-stopper" at link time.

    On Ubuntu 20.04, worked first time, zero issues!

    Go figure.

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