On Windows standalone qt6 executable in build directory does not run unless installed in %CONDA_PREFIX%\Library\bin or if qt6.conf is copied in the executable directory

[traversaro]

Solution to issue cannot be found in the documentation.

• I checked the documentation.

Issue

Reproducible example:

conda create -n qt6dev cmake compilers pkg-config ninja qt6-main
conda activate qt6dev 
git clone https://gist.github.com/traversaro/2f68aaa76f35f83b509183049eec864c qt6helloworld
cd qt6helloworld
cmake -GNinja -DCMAKE_BUILD_TYPE=Release -Bbuild -S.
cmake --build build
cd build
.\HelloWorld.exe
qt.qpa.plugin: Could not find the Qt platform plugin "windows" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

This showcases the problem, a simple workaround is:

copy %CONDA_PREFIX%\Library\bin\qt6.conf .\
.\HelloWorld.exe
# now it works fine

Installed packages

(qt6dev) D:\src\qtpluginclash\onlyqt\build>conda list
# packages in environment at D:\miniforge3\envs\qt6dev:
#
# Name                    Version                   Build  Channel
bzip2                     1.0.8                hcfcfb64_5    conda-forge
c-compiler                1.7.0                hcfcfb64_1    conda-forge
ca-certificates           2024.6.2             h56e8100_0    conda-forge
cairo                     1.18.0               h91e5215_2    conda-forge
clangdev                  5.0.0                   flang_3    conda-forge
cmake                     3.29.6               h400e5d1_0    conda-forge
compilers                 1.7.0                h57928b3_1    conda-forge
cxx-compiler              1.7.0                h91493d7_1    conda-forge
double-conversion         3.3.0                h63175ca_0    conda-forge
expat                     2.6.2                h63175ca_0    conda-forge
flang                     5.0.0           he025d50_20180525    conda-forge
flang_win-64              5.0.0           h13ae965_20180526    conda-forge
font-ttf-dejavu-sans-mono 2.37                 hab24e00_0    conda-forge
font-ttf-inconsolata      3.000                h77eed37_0    conda-forge
font-ttf-source-code-pro  2.038                h77eed37_0    conda-forge
font-ttf-ubuntu           0.83                 h77eed37_2    conda-forge
fontconfig                2.14.2               hbde0cde_0    conda-forge
fonts-conda-ecosystem     1                             0    conda-forge
fonts-conda-forge         1                             0    conda-forge
fortran-compiler          1.7.0                h9655429_1    conda-forge
freetype                  2.12.1               hdaf720e_2    conda-forge
graphite2                 1.3.13            h63175ca_1003    conda-forge
harfbuzz                  8.5.0                h81778c3_0    conda-forge
icu                       73.2                 h63175ca_0    conda-forge
krb5                      1.21.3               hdf4eb48_0    conda-forge
lerc                      4.0.0                h63175ca_0    conda-forge
libclang13                18.1.8          default_ha5278ca_0    conda-forge
libcurl                   8.8.0                hd5e4a3a_0    conda-forge
libdeflate                1.20                 hcfcfb64_0    conda-forge
libexpat                  2.6.2                h63175ca_0    conda-forge
libffi                    3.4.2                h8ffe710_5    conda-forge
libflang                  5.0.0           h6538335_20180525    conda-forge
libglib                   2.80.2               h0df6a38_0    conda-forge
libiconv                  1.17                 hcfcfb64_2    conda-forge
libintl                   0.22.5               h5728263_2    conda-forge
libjpeg-turbo             3.0.0                hcfcfb64_1    conda-forge
libpng                    1.6.43               h19919ed_0    conda-forge
libsqlite                 3.46.0               h2466b09_0    conda-forge
libssh2                   1.11.0               h7dfc565_0    conda-forge
libtiff                   4.6.0                hddb2be6_3    conda-forge
libuv                     1.48.0               hcfcfb64_0    conda-forge
libwebp-base              1.4.0                hcfcfb64_0    conda-forge
libzlib                   1.3.1                h2466b09_1    conda-forge
llvm-meta                 5.0.0                         0    conda-forge
ninja                     1.12.1               hc790b64_0    conda-forge
openmp                    5.0.0                    vc14_1    conda-forge
openssl                   3.3.1                h2466b09_1    conda-forge
pcre2                     10.43                h17e33f8_0    conda-forge
pixman                    0.43.4               h63175ca_0    conda-forge
pkg-config                0.29.2            h2bf4dc2_1008    conda-forge
qt6-main                  6.7.2                h527115d_2    conda-forge
ucrt                      10.0.22621.0         h57928b3_0    conda-forge
vc                        14.3                h8a93ad2_20    conda-forge
vc14_runtime              14.40.33810         ha82c5b3_20    conda-forge
vs2015_runtime            14.40.33810         h3bf8584_20    conda-forge
vs2019_win-64             19.29.30139         he1865b1_20    conda-forge
vswhere                   3.1.7                h57928b3_0    conda-forge
xz                        5.2.6                h8d14728_0    conda-forge
zlib                      1.3.1                h2466b09_1    conda-forge
zstd                      1.5.6                h0ea2cb4_0    conda-forge

Environment info

(qt6dev) D:\src\qtpluginclash\onlyqt\build>conda list
# packages in environment at D:\miniforge3\envs\qt6dev:
#
# Name                    Version                   Build  Channel
bzip2                     1.0.8                hcfcfb64_5    conda-forge
c-compiler                1.7.0                hcfcfb64_1    conda-forge
ca-certificates           2024.6.2             h56e8100_0    conda-forge
cairo                     1.18.0               h91e5215_2    conda-forge
clangdev                  5.0.0                   flang_3    conda-forge
cmake                     3.29.6               h400e5d1_0    conda-forge
compilers                 1.7.0                h57928b3_1    conda-forge
cxx-compiler              1.7.0                h91493d7_1    conda-forge
double-conversion         3.3.0                h63175ca_0    conda-forge
expat                     2.6.2                h63175ca_0    conda-forge
flang                     5.0.0           he025d50_20180525    conda-forge
flang_win-64              5.0.0           h13ae965_20180526    conda-forge
font-ttf-dejavu-sans-mono 2.37                 hab24e00_0    conda-forge
font-ttf-inconsolata      3.000                h77eed37_0    conda-forge
font-ttf-source-code-pro  2.038                h77eed37_0    conda-forge
font-ttf-ubuntu           0.83                 h77eed37_2    conda-forge
fontconfig                2.14.2               hbde0cde_0    conda-forge
fonts-conda-ecosystem     1                             0    conda-forge
fonts-conda-forge         1                             0    conda-forge
fortran-compiler          1.7.0                h9655429_1    conda-forge
freetype                  2.12.1               hdaf720e_2    conda-forge
graphite2                 1.3.13            h63175ca_1003    conda-forge
harfbuzz                  8.5.0                h81778c3_0    conda-forge
icu                       73.2                 h63175ca_0    conda-forge
krb5                      1.21.3               hdf4eb48_0    conda-forge
lerc                      4.0.0                h63175ca_0    conda-forge
libclang13                18.1.8          default_ha5278ca_0    conda-forge
libcurl                   8.8.0                hd5e4a3a_0    conda-forge
libdeflate                1.20                 hcfcfb64_0    conda-forge
libexpat                  2.6.2                h63175ca_0    conda-forge
libffi                    3.4.2                h8ffe710_5    conda-forge
libflang                  5.0.0           h6538335_20180525    conda-forge
libglib                   2.80.2               h0df6a38_0    conda-forge
libiconv                  1.17                 hcfcfb64_2    conda-forge
libintl                   0.22.5               h5728263_2    conda-forge
libjpeg-turbo             3.0.0                hcfcfb64_1    conda-forge
libpng                    1.6.43               h19919ed_0    conda-forge
libsqlite                 3.46.0               h2466b09_0    conda-forge
libssh2                   1.11.0               h7dfc565_0    conda-forge
libtiff                   4.6.0                hddb2be6_3    conda-forge
libuv                     1.48.0               hcfcfb64_0    conda-forge
libwebp-base              1.4.0                hcfcfb64_0    conda-forge
libzlib                   1.3.1                h2466b09_1    conda-forge
llvm-meta                 5.0.0                         0    conda-forge
ninja                     1.12.1               hc790b64_0    conda-forge
openmp                    5.0.0                    vc14_1    conda-forge
openssl                   3.3.1                h2466b09_1    conda-forge
pcre2                     10.43                h17e33f8_0    conda-forge
pixman                    0.43.4               h63175ca_0    conda-forge
pkg-config                0.29.2            h2bf4dc2_1008    conda-forge
qt6-main                  6.7.2                h527115d_2    conda-forge
ucrt                      10.0.22621.0         h57928b3_0    conda-forge
vc                        14.3                h8a93ad2_20    conda-forge
vc14_runtime              14.40.33810         ha82c5b3_20    conda-forge
vs2015_runtime            14.40.33810         h3bf8584_20    conda-forge
vs2019_win-64             19.29.30139         he1865b1_20    conda-forge
vswhere                   3.1.7                h57928b3_0    conda-forge
xz                        5.2.6                h8d14728_0    conda-forge
zlib                      1.3.1                h2466b09_1    conda-forge
zstd                      1.5.6                h0ea2cb4_0    conda-forge

[traversaro]

I guess there is some problem of qt6's relocatibility support. With qt5, everything works out of the box.

[traversaro]

I modified the gist to also provide a printprefixpath program that just prints QLibraryInfo::location(QLibraryInfo::PrefixPath), and the output is:

Qt6:

.\debug.exe
PrefixPath: "D:/miniforge3/envs"

Qt5:

.\printprefixpath.exe
PrefixPath: "D:/miniforge3/envs/qtdev/Library"

So the problem is in how the prefix is computed for qt6, i.e. https://github.com/qt/qtbase/blob/v6.7.2/src/corelib/global/qlibraryinfo.cpp#L322-L459 .

[traversaro]

Now that opencv started using qt6, users on Windows are hitting this bug (for example when they call cv::imshow, I mention it explicitly so search engines and llm are aware of this) if they do not install their executables in %CMAKE_PREFIX%/Library/bin.

A possible workaround at the cmake level is the following:

# Install qt6.conf on Windows on conda as a workaround for
# https://github.com/conda-forge/qt-main-feedstock/issues/275
if(WIN32 AND DEFINED ENV{CONDA_PREFIX} AND EXISTS $ENV{CONDA_PREFIX}/Library/bin/qt6.conf)
  # If CMAKE_RUNTIME_OUTPUT_DIRECTORY  is defined to ensure that all .dll and .exe of the project 
  # go in the same directory, this ensures that the build executables find qt6 correctly 
  if(DEFINED CMAKE_RUNTIME_OUTPUT_DIRECTORY)
      configure_file($ENV{CONDA_PREFIX}/Library/bin/qt6.conf ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/qt6.conf COPYONLY)
  endif()
  # This ensure that the installed executables (if they are not installed in $ENV{CONDA_PREFIX}/Library)) correctly find qt6
  include(GNUInstallDirs)
  if(NOT CMAKE_INSTALL_PREFIX PATH_EQUAL $ENV{CONDA_PREFIX}/Library)
      install(FILES $ENV{CONDA_PREFIX}/Library/bin/qt6.conf DESTINATION ${CMAKE_INSTALL_BINDIR})
  endif()
endif()

[traversaro]

cc @dariosortino you mentioned to me some qt6-main problems you experienced on Windows, I wonder if they are related to this.

[dariosortino]

@traversaro I don't think it was related to this. It was more an incompatibility of conda packages in the same env, but to be honest I didn't dive into that