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fix: When cross-compiling, CMake miss the correct library files. #27978

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fix: When cross-compiling, CMake miss the correct library files. #27978

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@MambaWong MambaWong commented Nov 7, 2025 •
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When cross-compiling, CMake miss the correct library files.

  1. CMAKE_FIND_ROOT_PATH can interfere with OpenBLAS detection, causing CMake to miss the correct library include files.
  2. When using find_package(OpenBLAS QUIET), OpenBLAS_LIBRARIES does not contain OpenBLAS::OpenBLAS. 
     aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x278): undefined reference to `sgels_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x414): undefined reference to `sgeqrf_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x498): undefined reference to `sgeqrf_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x4fc): undefined reference to `dgels_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x52c): undefined reference to `dgeqrf_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x6c4): undefined reference to `sgels_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x75c): undefined reference to `sgels_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x834): undefined reference to `dgels_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x8bc): undefined reference to `dgeqrf_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x918): undefined reference to `dgels_'
 aarch64-linux-gnu/bin/ld: hal_internal.cpp:(.text._Z12lapack_QR64fPdmiiiS_mS_Pi+0x950): undefined reference to `dgels_'

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

  • I agree to contribute to the project under Apache 2 License.
  • To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
  • The PR is proposed to the proper branch
  • There is a reference to the original bug report and related work
  • There is accuracy test, performance test and test data in opencv_extra repository, if applicable
    Patch to opencv_extra has the same branch name.
  • The feature is well documented and sample code can be built with the project CMake

@MambaWong
fix: during cross-compilation, CMAKE_FIND_ROOT_PATH can interfere wit…
ee30baf 
…h OpenBLAS detection, causing CMake to miss the correct library files.
@asmorkalov asmorkalov requested a review from mshabunin November 8, 2025 08:33
@MambaWong MambaWong changed the title fix: during cross-compilation, CMAKE_FIND_ROOT_PATH can interfere with OpenBLAS detection, causing CMake to miss the correct library files. fix: during cross-compilation, CMake miss the correct library files. Nov 9, 2025
@MambaWong MambaWong changed the title fix: during cross-compilation, CMake miss the correct library files. fix: When cross-compiling, CMake miss the correct library files. Nov 9, 2025
@asmorkalov asmorkalov added this to the 4.13.0 milestone Nov 10, 2025
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asmorkalov commented Nov 10, 2025

The solution looks a bit wired. Could you describe your use-case: compiler, toolchain file, the way how the BLAS is searched, etc?

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MambaWong commented Nov 10, 2025 •
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I first cross-compiled OpenBLAS, and then used this previously cross-compiled OpenBLAS when cross-compiling OpenCV.

  1. Host machine (a Docker container running on WSL2)

NAME="Ubuntu"
VERSION="20.04.6 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.6 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

  1. Cross compiler
aarch64-mix210-linux-gcc -v

Using built-in specs.
COLLECT_GCC=aarch64-mix210-linux-gcc
COLLECT_LTO_WRAPPER=/opt/linux/x86-arm/aarch64-mix210-linux/host_bin/../libexec/gcc/aarch64-linux-gnu/7.3.0/lto-wrapper
Target: aarch64-linux-gnu
Configured with: /home/pub/workspace/toolchaines/build/aarch64-mix210-linux/../../open_source/aarch64-mix210-linux_src/gcc-7.3.0/configure --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=aarch64-linux-gnu --with-arch=armv8-a --prefix=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/aarch64_mix210 --enable-threads --disable-libmudflap -enable-libssp --enable-gold=yes --disable-libstdcxx-pch --disable-multilib --enable-multiarch --with-gnu-as --with-gnu-ld --enable-libgomp --enable-gnu-indirect-function --enable-checking=yes --enable-lto --enable-c99 --enable-long-long --disable-nls --enable-fix-cortex-a53-835769 --enable-fix-cortex-a53-843419 --enable-shared --with-pkgversion='HC&C V1R3C00SPC200B042_20221123' --enable-languages=c,c++ --with-headers=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/aarch64_mix210/target/usr/include --with-sysroot=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/aarch64_mix210/target --with-build-sysroot=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/aarch64_mix210/target --with-gmp=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/obj/host-libs/usr --with-mpfr=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/obj/host-libs/usr --with-mpc=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/obj/host-libs/usr --with-build-time-tools=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/aarch64_mix210/aarch64-linux-gnu/bin --libdir=/home/pub/workspace/toolchaines/build/aarch64-mix210-linux/aarch64_mix210_build_dir/aarch64_mix210/lib --disable-bootstrap --with-system-zlib
Thread model: posix
gcc version 7.3.0 (HC&C V1R3C00SPC200B042_20221123)

  1. toolchain file

set(CMAKE_SYSTEM_PROCESSOR aarch64)
set(GCC_COMPILER_VERSION "" CACHE STRING "GCC Compiler version")
set(GNU_MACHINE "aarch64-mix210-linux" CACHE STRING "GNU compiler triple")
include("${CMAKE_CURRENT_LIST_DIR}/arm.toolchain.cmake")

  1. find OpenBLAS

  • case a

    -DOpenBLAS_DIR=${INSTALL_PREFIX}/lib/cmake/OpenBLAS

    The method used in OpenCV during cross-compilation.

    opencv/cmake/OpenCVFindOpenBLAS.cmake

    Line 4 in 83026bf

    find_package(OpenBLAS QUIET)

    find_package(OpenBLAS QUIET)

    In this case, OpenBLAS_LIBRARIES is similar to m;OpenMP::OpenMP_C and does not contain OpenBLAS::OpenBLAS. This will cause functions from the OpenBLAS library to be missing at link time.

  • case b

    export OpenBLAS_HOME=${INSTALL_PREFIX}

    opencv/cmake/OpenCVFindOpenBLAS.cmake

    Lines 11 to 13 in 83026bf

    find_library(OpenBLAS_LIBRARIES NAMES openblas PATHS ENV "OpenBLAS" ENV "OpenBLAS_HOME" PATH_SUFFIXES "lib" NO_DEFAULT_PATH)
    find_path(OpenBLAS_INCLUDE_DIRS NAMES cblas.h PATHS ENV "OpenBLAS" ENV "OpenBLAS_HOME" PATH_SUFFIXES "include" NO_DEFAULT_PATH)
    find_path(OpenBLAS_LAPACKE_DIR NAMES lapacke.h PATHS "${OpenBLAS_INCLUDE_DIRS}" ENV "OpenBLAS" ENV "OpenBLAS_HOME" PATH_SUFFIXES "include" NO_DEFAULT_PATH)

    In this case, these three lines are unable to locate the corresponding header and library files, even though I am absolutely certain that they exist.
    However, if I add NO_CMAKE_FIND_ROOT_PATH, the files can be found correctly.

In both cases, the line below cannot find the files unless I add NO_CMAKE_FIND_ROOT_PATH.

opencv/cmake/OpenCVFindLAPACK.cmake

Line 35 in 2dd0ac3

find_path(${VAR} "${NAME}" ${ARGN} NO_DEFAULT_PATH)

opencv/platforms/linux/arm.toolchain.cmake

Line 76 in 83026bf

set(CMAKE_FIND_ROOT_PATH ${CMAKE_FIND_ROOT_PATH} ${ARM_LINUX_SYSROOT})

When cross-compiling, the toolchain defines CMAKE_FIND_ROOT_PATH. In the CMake documentation, it is described as:

The CMake variable CMAKE_FIND_ROOT_PATH specifies one or more directories to be prepended to all other search directories.

It might be more appropriate to modify it by adding a check for CMAKE_CROSSCOMPILING.

if(CMAKE_CROSSCOMPILING)
  find_path(${VAR} "${NAME}" ${ARGN} NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
else()
  find_path(${VAR} "${NAME}" ${ARGN} NO_DEFAULT_PATH)
endif()

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