Building Qt6 WebAssembly GUI with Emscripten fails because of missing GLESv2/EGL

Question

I am trying to build a Qt6 GUI project for WebAssembly using Emscripten.

I managed to build the project successfully with nlohmann_json and spdlog. Now I also want to add the cpr library to use an HTTP client but since then the whole project no longer builds.

I use:

Qt 6.8.3 (wasm_singlethread)
Emscripten SDK (latest, emcc/em++)
CMake 3.31 (via CLion 2025.1.3)
Using Qt’s qt.toolchain.cmake and chaining Conan’s toolchain for dependencies (spdlog, cpr, nlohmann_json, …)

My CMakeLists.txt adds dummy GLESv2/EGL targets for WebAssembly:

cmake_minimum_required(VERSION 3.21)
project(qt3d_wasm_project VERSION 0.1 LANGUAGES CXX)
message(STATUS "CMAKE_SYSTEM_NAME = ${CMAKE_SYSTEM_NAME}")
if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
    if(NOT TARGET emscripten_flags)
        add_library(emscripten_flags INTERFACE)
        target_link_options(emscripten_flags INTERFACE
                "SHELL:-sUSE_ZLIB=1"
                # "SHELL:-sFETCH=1"
        )
    endif()

    if(NOT TARGET GLESv2::GLESv2)
        add_library(GLESv2::GLESv2 INTERFACE IMPORTED GLOBAL)
        target_link_options(GLESv2::GLESv2 INTERFACE "SHELL:-sUSE_WEBGL2=1")
    endif()
    if(NOT TARGET EGL::EGL)
        add_library(EGL::EGL INTERFACE IMPORTED GLOBAL)
        target_link_options(EGL::EGL INTERFACE "SHELL:-sUSE_WEBGL2=1")
    endif()
endif()

add_subdirectory(libraries/material_data_lib)

if(ENABLE_GUI)
    find_package(Qt6 REQUIRED COMPONENTS Core Gui Quick Quick3D Qml Quick3DHelpers Network)

    qt_add_executable(appqt3d
            resources.qrc
            main.cpp
            libraries/material_data_lib/include/IMaterialDataFetcher.h
            libraries/material_data_lib/src/MaterialDataFactory.cpp
    )

    if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
        target_link_libraries(appqt3d PRIVATE emscripten_flags  GLESv2::GLESv2 EGL::EGL)
    endif()

    target_link_libraries(appqt3d PRIVATE
            Qt6::Core Qt6::Gui Qt6::Quick Qt6::Quick3D Qt6::Quick3DHelpers Qt6::Network
    )
endif()

I use the following CMake Options:

-DCMAKE_TOOLCHAIN_FILE=/home/emmynoether/Qt/6.8.3/wasm_singlethread/lib/cmake/Qt6/qt.toolchain.cmake
-DQT_CHAINLOAD_TOOLCHAIN_FILE=/home/emmynoether/Dokumente/Webseite/MaterialStack/build_wasm/build/Release/generators/conan_toolchain.cmake
-DENABLE_GUI=ON
-DBUILD_TESTING=OFF
-DQt6_DIR=/home/emmynoether/Qt/6.8.3/wasm_singlethread/lib/cmake/Qt6
-DQT_HOST_PATH=/home/emmynoether/Qt/6.8.3/gcc_64
-DQT_DEBUG_FIND_PACKAGE=ON

The output is always:


-- Could NOT find EGL (missing: HAVE_EGL) (found version "1.5")
-- Could NOT find GLESv2 (missing: GLESv2_INCLUDE_DIR GLESv2_LIBRARY HAVE_GLESv2 HAVE_GLESv2) 
CMake Warning at /home/emmynoether/Qt/6.8.3/wasm_singlethread/lib/cmake/Qt6/Qt6Config.cmake:196 (find_package):
  Found package configuration file:

    /home/emmynoether/Qt/6.8.3/wasm_singlethread/lib/cmake/Qt6Gui/Qt6GuiConfig.cmake

  but it set Qt6Gui_FOUND to FALSE so package "Qt6Gui" is considered to be
  NOT FOUND.  Reason given by package:

  Qt6Gui could not be found because dependency GLESv2 could not be found.

  Configuring with --debug-find-pkg=GLESv2 might reveal details why the
  package was not found.

Why does Qt6GuiConfig.cmake for WebAssembly still insist on real GLESv2/EGL libraries, even though WebAssembly uses WebGL via Emscripten?

Suslik · Accepted Answer · 2025-10-06 14:54:26Z

0

I had to run:

#prepare the emscription environment  
source ~/emsdk/emsdk_env.sh
#then i had to build the required GL ports:
embuilder build libGL
#embuilder:INFO: building libGL
#embuilder:INFO: ...success. Took (0.00s)

I learnt that embuilder compiles the WebGL/EGL stubs into static archives located in
$EMSDK/upstream/emscripten/system/lib. Without them the CMake find_package(Qt6Gui) call fails, because the Qt 6 Gui module depends on the OpenGL ES 2 symbols.

then i could go to my build folder and perform the conan install to resolve 3-party dependencies and configure the project with cmake .

answered Oct 6 at 14:54

Suslik

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