[MuJoCo-3.5.0] Build fails on Ubuntu 20.04 / GCC 9.4: constinit not recognized in user_threadpool.h

[HoangGiang93]

Intro

Hi!

I am a researcher at AICOR, I use MuJoCo for my research.

Build log excerpt

Key part:

 40%] Building CXX object CMakeFiles/mujoco.dir/src/user/user_api.cc.o
[ 40%] Building CXX object CMakeFiles/mujoco.dir/src/user/user_cache.cc.o
[ 42%] Building CXX object CMakeFiles/mujoco.dir/src/user/user_composite.cc.o
[ 42%] Building CXX object CMakeFiles/mujoco.dir/src/user/user_flexcomp.cc.o
[ 42%] Building C object CMakeFiles/mujoco.dir/src/user/user_init.c.o
[ 43%] Building CXX object CMakeFiles/mujoco.dir/src/user/user_mesh.cc.o
[ 43%] Building CXX object CMakeFiles/mujoco.dir/src/user/user_model.cc.o
In file included from /media/giangnguyen/Storage/Ubuntu2004/Multiverse-MuJoCo-Connector/src/mujoco-3.5.0/src/user/user_model.cc:57:
/media/giangnguyen/Storage/Ubuntu2004/Multiverse-MuJoCo-Connector/src/mujoco-3.5.0/src/user/user_threadpool.h:72:3: error: ‘constinit’ does not name a type
   72 |   constinit static thread_local int worker_id_;
      |   ^~~~~~~~~
/media/giangnguyen/Storage/Ubuntu2004/Multiverse-MuJoCo-Connector/src/mujoco-3.5.0/src/user/user_threadpool.h: In static member function ‘static int mujoco::user::ThreadPool::WorkerId()’:
/media/giangnguyen/Storage/Ubuntu2004/Multiverse-MuJoCo-Connector/src/mujoco-3.5.0/src/user/user_threadpool.h:42:34: error: ‘worker_id_’ was not declared in this scope; did you mean ‘WorkerId’?
   42 |   static int WorkerId() { return worker_id_; }
      |                                  ^~~~~~~~~~
      |                                  WorkerId
make[2]: *** [CMakeFiles/mujoco.dir/build.make:695: CMakeFiles/mujoco.dir/src/user/user_model.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:642: CMakeFiles/mujoco.dir/all] Error 2
make: *** [Makefile:136: all] Error 2
-- Install configuration: "Release"
CMake Error at build/mujoco-3.5.0/simulate/cmake_install.cmake:46 (file):
  file INSTALL cannot find
  "/media/giangnguyen/Storage/Ubuntu2004/Multiverse-MuJoCo-Connector/build/mujoco-3.5.0/bin/simulate":
  No such file or directory.
Call Stack (most recent call first):
  build/mujoco-3.5.0/cmake_install.cmake:102 (include)

My setup

• OS: Ubuntu 20.04
• Compiler:
  • cc: GNU 9.4.0
  • c++: GNU 9.4.0
• CMake: 4.0.0
• MuJoCo version: 3.5.0
• Build type: Release

What's happening? What did you expect?

I was building MuJoCo 3.5.0 from source, it fails during compilation with GCC 9.4.0. The error is:

src/user/user_threadpool.h:72:3: error: ‘constinit’ does not name a type
   72 |   constinit static thread_local int worker_id_;
      |   ^~~~~~~~~
...
src/user/user_threadpool.h:42:34: error: ‘worker_id_’ was not declared in this scope
   42 |   static int WorkerId() { return worker_id_; }
      |                                  ^~~~~~~~~~

This blocks building the mujoco target, and the subsequent install step also fails because simulate was not produced.

Expected behavior

MuJoCo should compile successfully (or fail early with a clear message) on supported toolchains, or CMake should enforce a minimum C++ standard/compiler that supports constinit.

Actual behavior

Compilation fails with ‘constinit’ does not name a type in src/user/user_threadpool.h, causing the entire build to fail.

After the compile failure, cmake --install additionally errors:

file INSTALL cannot find ".../bin/simulate": No such file or directory.

Notes / suspicion

constinit is a C++20 feature. This looks like either:

• MuJoCo now requires C++20 (or a newer GCC), but the minimum compiler/C++ standard is not enforced at configure time, or
• This should be guarded / replaced for older compilers (e.g., GCC 9.x).

If MuJoCo requires C++20, it would be helpful if CMake produced a clear error (e.g., “C++20 required; GCC >= X or Clang >= Y”) instead of failing mid-build.

Additional info

• Building with -DMUJOCO_BUILD_SIMULATE=ON
• Dependencies were fetched via FetchContent (qhull, tinyxml2, tinyobjloader, ccd, glfw3) and those built successfully before the failure.

Steps for reproduction

Steps to reproduce

1. Configure and build with:

cmake -S "$MUJOCO_SRC_DIR" -B "$BUILD_DIR" \
  -DCMAKE_INSTALL_PREFIX="$INSTALL_DIR" \
  -DMUJOCO_BUILD_EXAMPLES=OFF \
  -DMUJOCO_BUILD_TESTS=OFF \
  -DMUJOCO_BUILD_SIMULATE=ON \
  -DMUJOCO_TEST_PYTHON_UTIL=OFF \
  -DCMAKE_POLICY_VERSION_MINIMUM="3.5" \
  -Wno-deprecated -Wno-dev

cmake --build "$BUILD_DIR"
cmake --install "$BUILD_DIR"

2. Build fails around ~43% when compiling src/user/user_model.cc.

Minimal model for reproduction

No response

Code required for reproduction

No response

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[yuvaltassa]

@HoangGiang93 any chance you could upgrade your setup to newer GCC / Ubuntu? Your versions are pretty old...

Since GCC 9.4 is no longer supported (it's 5 years old!) I'm marking this as won't fix. Sorry!