Cmake Complains "The Cxx Compiler Identification Is Unknown"

CMake complains The CXX compiler identification is unknown

Your /home/gnu/bin/c++ seem to require additional flag to link things properly and CMake doesn't know about that.

To use /usr/bin/c++ as your compiler run cmake with -DCMAKE_CXX_COMPILER=/usr/bin/c++.

Also, CMAKE_PREFIX_PATH variable sets destination dir where your project' files should be installed. It has nothing to do with CMake installation prefix and CMake itself already know this.

CMake does not find Visual C++ compiler

I have found the solution. While the Visual Studio IDE installed successfully it did not install any build tools and therefore did not install the C++ compiler.

By attempting to manually create a C++ project in the Visual Studio 2015 GUI I was able to prompt it to download the C++ packages. CMake was then able to find the compiler without any difficulty.

CMake error at CMakeLists.txt:30 (project): No CMAKE_C_COMPILER could be found

Those error messages

CMake Error at ... (project):
    No CMAKE_C_COMPILER could be found.
-- Configuring incomplete, errors occurred!
See also ".../CMakeFiles/CMakeOutput.log".
See also ".../CMakeFiles/CMakeError.log".

or

CMake Error: your CXX compiler: "CMAKE_CXX_COMPILER-NOTFOUND" was not found.
Please set CMAKE_CXX_COMPILER to a valid compiler path or name.
...
-- Configuring incomplete, errors occurred!

just mean that CMake was unable to find your C/CXX compiler to compile a simple test program (one of the first things CMake tries while detecting your build environment).

The steps to find your problem are dependent on the build environment you want to generate. The following tutorials are a collection of answers here on Stack Overflow and some of my own experiences with CMake on Microsoft Windows 7/8/10 and Ubuntu 14.04.

Preconditions

• You have installed the compiler/IDE and it was able to once compile any other program (directly without CMake)

  • You e.g. may have the IDE, but may not have installed the compiler or supporting framework itself like described in Problems generating solution for VS 2017 with CMake or How do I tell CMake to use Clang on Windows?

• You have the latest CMake version

• You have access rights on the drive you want CMake to generate your build environment

• You have a clean build directory (because CMake does cache things from the last try) e.g. as sub-directory of your source tree

  Windows cmd.exe

    > rmdir /s /q VS2015
    > mkdir VS2015
    > cd VS2015
  

  Bash shell

    $ rm -rf MSYS
    $ mkdir MSYS
    $ cd MSYS
  

  and make sure your command shell points to your newly created binary output directory.

General things you can/should try

1. Is CMake able find and run with any/your default compiler? Run without giving a generator

    > cmake ..
    -- Building for: Visual Studio 14 2015
    ...
   

   Perfect if it correctly determined the generator to use - like here Visual Studio 14 2015

2. What was it that actually failed?

   In the previous build output directory look at CMakeFiles\CMakeError.log for any error message that make sense to you or try to open/compile the test project generated at CMakeFiles\[Version]\CompilerIdC|CompilerIdCXX directly from the command line (as found in the error log).

CMake can't find Visual Studio

1. Try to select the correct generator version:

    > cmake --help
    > cmake -G "Visual Studio 14 2015" ..
   

2. If that doesn't help, try to set the Visual Studio environment variables first (the path could vary):

    > "c:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat"
    > cmake ..
   

   or use the Developer Command Prompt for VS2015 short-cut in your Windows Start Menu under All Programs/Visual Studio 2015/Visual Studio Tools (thanks at @Antwane for the hint).

Background: CMake does support all Visual Studio releases and flavors (Express, Community, Professional, Premium, Test, Team, Enterprise, Ultimate, etc.). To determine the location of the compiler it uses a combination of searching the registry (e.g. at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\[Version];InstallDir), system environment variables and - if none of the others did come up with something - plainly try to call the compiler.

CMake can't find GCC (MinGW/MSys)

1. You start the MSys bash shell with msys.bat and just try to directly call gcc

    $ gcc
    gcc.exe: fatal error: no input files
    compilation terminated.
   

   Here it did find gcc and is complaining that I didn't gave it any parameters to work with.

   So the following should work:

    $ cmake -G "MSYS Makefiles" ..
    -- The CXX compiler identification is GNU 4.8.1
    ...
    $ make
   

   If GCC was not found call export PATH=... to add your compilers path (see How to set PATH environment variable in CMake script?) and try again.

2. If it's still not working, try to set the CXX compiler path directly by exporting it (path may vary)

    $ export CC=/c/MinGW/bin/gcc.exe
    $ export CXX=/c/MinGW/bin/g++.exe
    $ cmake -G "MinGW Makefiles" ..
    -- The CXX compiler identification is GNU 4.8.1
    ...
    $ mingw32-make
   

   For more details see How to specify new GCC path for CMake

   Note: When using the "MinGW Makefiles" generator you have to use the mingw32-make program distributed with MinGW

3. Still not working? That's weird. Please make sure that the compiler is there and it has executable rights (see also preconditions chapter above).

   Otherwise the last resort of CMake is to not try any compiler search itself and set CMake's internal variables directly by

    $ cmake -DCMAKE_C_COMPILER=/c/MinGW/bin/gcc.exe -DCMAKE_CXX_COMPILER=/c/MinGW/bin/g++.exe ..
   

   For more details see Cmake doesn't honour -D CMAKE_CXX_COMPILER=g++ and Cmake error setting compiler

   Alternatively those variables can also be set via cmake-gui.exe on Windows. See Cmake cannot find compiler

Background: Much the same as with Visual Studio. CMake supports all sorts of GCC flavors. It searches the environment variables (CC, CXX, etc.) or simply tries to call the compiler. In addition it will detect any prefixes (when cross-compiling) and tries to add it to all binutils of the GNU compiler toolchain (ar, ranlib, strip, ld, nm, objdump, and objcopy).

Why is CMake ignoring the compiler settings via command line?

I'm guessing you're running a version of CMake > 3.0? CMake doesn't necessarily honor the environment paths correctly as it is doing its lookups.

I tried installing 2.8.12.2 (the version recommended by the author) and it seems to build fine.

I also tried with the latest 3.x version (3.22.x) and I see the same error as you do. Rather than try to fix up the CMake file, it might be quicker to just downgrade CMake 2.8.12.x.

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