# CMake 3.15.5 bootstrap fail with g++ 9.3.0

**URL:** https://discourse.cmake.org/t/cmake-3-15-5-bootstrap-fail-with-g-9-3-0/2115
**Category:** Community
**Tags:** os:linux, comp:gcc
**Created:** [November 4, 2020, 8:57am UTC](https://discourse.cmake.org/t/cmake-3-15-5-bootstrap-fail-with-g-9-3-0/2115 "2020-11-04T08:57:23Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![Massimiliano\_Cialdi](https://discourse.cmake.org/user_avatar/discourse.cmake.org/massimiliano_cialdi/32/924_2.png) [@Massimiliano\_Cialdi](https://discourse.cmake.org/u/Massimiliano_Cialdi)
#### Post date: [November 4, 2020, 8:57am UTC](https://discourse.cmake.org/t/cmake-3-15-5-bootstrap-fail-with-g-9-3-0/2115/1 "2020-11-04T08:57:23Z")

</div>

I’m trying to build CMake 3.15.5 on a 20.04 xubuntu machine (but I also tried 18.04 and 20.10 with the same results).  
(actually I want to build host-cmake in buildroot 2020.02.1)

The error is as follows:

```
loading initial cache file /home/max/Lavori/nova/src/repos/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Bootstrap.cmk/InitialCacheFlags.cmake
-- The C compiler identification is GNU 9.3.0
-- The CXX compiler identification is GNU 9.3.0
-- Check for working C compiler: /usr/bin/gcc
-- Check for working C compiler: /usr/bin/gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/g++
-- Check for working CXX compiler: /usr/bin/g++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Checking if compiler supports C11 _Thread_local
-- Checking if compiler supports C11 _Thread_local - yes
-- Checking if compiler supports needed C++17 constructs
-- Checking if compiler supports needed C++17 constructs - yes
-- Checking if compiler supports C++ make_unique
-- Checking if compiler supports C++ make_unique - no
-- Checking if compiler supports C++ unique_ptr
-- Checking if compiler supports C++ unique_ptr - no
CMake Error at CMakeLists.txt:92 (message):
  The C++ compiler does not support C++11 (e.g. std::unique_ptr).

-- Configuring incomplete, errors occurred!
See also "/home/max/Lavori/nova/src/repos/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeOutput.log".
See also "/home/max/Lavori/nova/src/repos/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeError.log".
---------------------------------------------
Error when bootstrapping CMake:
Problem while running initial CMake

```

I am absolutely sure that g++ 9.3.0 supports C++11, so where could the problem be?

Here is the `CMakeError.log` file:

```
Determining if compiler supports C++ make_unique failed with the following output:
Change Dir: /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_f5a77/fast && make[2]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: Warning: File 'Makefile' has modification time 0,27 s in the future
make -f CMakeFiles/cmTC_f5a77.dir/build.make CMakeFiles/cmTC_f5a77.dir/build
make[3]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[3]: Warning: File 'CMakeFiles/cmTC_f5a77.dir/flags.make' has modification time 0,26 s in the future
Building CXX object CMakeFiles/cmTC_f5a77.dir/cm_cxx_make_unique.cxx.o
/usr/bin/g++ -O2 -std=gnu++17 -o CMakeFiles/cmTC_f5a77.dir/cm_cxx_make_unique.cxx.o -c /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Source/Checks/cm_cxx_make_unique.cxx
Linking CXX executable cmTC_f5a77
/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Bootstrap.cmk/cmake -E cmake_link_script CMakeFiles/cmTC_f5a77.dir/link.txt --verbose=1
/usr/bin/g++ -O2 -L/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib -Wl,-rpath,/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib CMakeFiles/cmTC_f5a77.dir/cm_cxx_make_unique.cxx.o -o cmTC_f5a77 
make[3]: warning: Clock skew detected. Your build may be incomplete.
make[3]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: warning: Clock skew detected. Your build may be incomplete.
make[2]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'

Determining if compiler supports C++ unique_ptr failed with the following output:
Change Dir: /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_02c6e/fast && make[2]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: Warning: File 'Makefile' has modification time 0,27 s in the future
make -f CMakeFiles/cmTC_02c6e.dir/build.make CMakeFiles/cmTC_02c6e.dir/build
make[3]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[3]: Warning: File 'CMakeFiles/cmTC_02c6e.dir/flags.make' has modification time 0,26 s in the future
Building CXX object CMakeFiles/cmTC_02c6e.dir/cm_cxx_unique_ptr.cxx.o
/usr/bin/g++ -O2 -std=gnu++17 -o CMakeFiles/cmTC_02c6e.dir/cm_cxx_unique_ptr.cxx.o -c /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Source/Checks/cm_cxx_unique_ptr.cxx
Linking CXX executable cmTC_02c6e
/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Bootstrap.cmk/cmake -E cmake_link_script CMakeFiles/cmTC_02c6e.dir/link.txt --verbose=1
/usr/bin/g++ -O2 -L/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib -Wl,-rpath,/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib CMakeFiles/cmTC_02c6e.dir/cm_cxx_unique_ptr.cxx.o -o cmTC_02c6e 
make[3]: warning: Clock skew detected. Your build may be incomplete.
make[3]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: warning: Clock skew detected. Your build may be incomplete.
make[2]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'

```

I cannot figure out wath is wrong.  
I thought it could be a clock skew problem but I tried also outside a virtual machine with identical results.

I also tried to compile and link by hand (using the command lines above) without any errors or warnings.

```
/usr/bin/g++ -O2 -std=gnu++17 -o cm_cxx_make_unique.cxx.o -c /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Source/Checks/cm_cxx_make_unique.cxx

/usr/bin/g++ -O2 -L/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib -Wl,-rpath,/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib cm_cxx_make_unique.cxx.o -o cmTC_f5a77 

```

I also tried to run the binary without errors.  
So even more 'I can not figure out what is wrong

best regards  
Max

---

<div class="post-metadata">

### Author: ![brad.king](https://discourse.cmake.org/user_avatar/discourse.cmake.org/brad.king/32/11_2.png) [@brad.king](https://discourse.cmake.org/u/brad.king)
#### Post date: [November 4, 2020, 12:53pm UTC](https://discourse.cmake.org/t/cmake-3-15-5-bootstrap-fail-with-g-9-3-0/2115/2 "2020-11-04T12:53:11Z")

</div>

The clock skew warning is probably causing [this code](https://gitlab.kitware.com/cmake/cmake/-/blob/v3.15.5/Source/Checks/cm_cxx_features.cmake#L29) to assume that the feature is unavailable.