How can I make CMake use GCC instead of Clang on Mac OS X?

I can't find any info on it, but only the other way around (e.g., how to set CMake to use clang).

I've installed gcc-4.8 using brew, setup all dependencies, headers, etc, and now CMake refuses to use gcc.

I've set my bash profile with both aliases and actual entries:

export CC=/usr/bin/gcc
export CXX=/usr/bin/g++
alias gcc='gcc-4.8'
alias cc='gcc-4.8'
alias g++='g++-4.8'
alias c++='c++-4.8'

Yet CMake stubbornly refuses to use gcc and instead reverts back to clang:

air:build alex$ cmake -DCMAKE_BUILD_TYPE=DEBUG ..
-- The C compiler identification is Clang 5.1.0
-- The CXX compiler identification is Clang 5.1.0
-- Check for working C compiler: /usr/bin/gcc
-- Check for working C compiler: /usr/bin/gcc -- works

CMake doesn't (always) listen to CC and CXX. Instead use CMAKE_C_COMPILER and CMAKE_CXX_COMPILER:

cmake -DCMAKE_C_COMPILER=/usr/bin/gcc -DCMAKE_CXX_COMPILER=/usr/bin/g++ ...

See also the documentation.

Alternatively, you can provide a toolchain file, but that might be overkill in this case.

Current versions of CMake do not respect the CC and CXX environment variables like one would expect. Specifically if they are absolute paths to the compiler binaries they seem to be ignored. On my system with a freshly compiled cmake 3.7.1 I have to do cmake -H. -Bbuild -DCMAKE_C_COMPILER=$CC -DCMAKE_CXX_COMPILER=$CXX.

As others have stated it is not a great idea to force a compiler choice within your CMakeLists.txt, however if this is required for your use case here's how you do it:

cmake_minimum_required(VERSION 3.5) # Or whatever version you use

# THIS HAS TO COME BEFORE THE PROJECT LINE
set(CMAKE_C_COMPILER "gcc")
set(CMAKE_CXX_COMPILER "g++")
# THIS HAS TO COME BEFORE THE PROJECT LINE

project(my_project VERSION 0.0.0 LANGUAGES C CXX)

In this case cmake will fail if the indicated compiler is not found. Note that you must set these variables before the project line as this command is what finds and configures the compilers.

I guess the case in OP was run on a macOS, and the real problem is /usr/bin/gcc and /usr/bin/g++ by default are clang and clang++.

% ls -al /usr/bin/clang
-rwxr-xr-x  1 root  wheel  167088  8 Dec 07:39 /usr/bin/clang
% ls -al /usr/bin/gcc
-rwxr-xr-x  1 root  wheel  167088  8 Dec 07:39 /usr/bin/gcc
% /usr/bin/gcc --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 13.0.0 (clang-1300.0.29.30)
Target: x86_64-apple-darwin21.2.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

One way avoid that is to install gcc/g++ and then use the versioned binary names.

% which gcc-11 g++-11
/usr/local/bin/gcc-11
/usr/local/bin/g++-11
% gcc-11 --version
gcc-11 (Homebrew GCC 11.2.0_3) 11.2.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

So, instead of

cmake .. -DCMAKE_C_COMPILER=/usr/bin/gcc -DCMAKE_CXX_COMPILER=/usr/bin/g++

one would use

cmake .. -DCMAKE_C_COMPILER=/usr/local/bin/gcc-11 -DCMAKE_CXX_COMPILER=/usr/local/bin/g++-11

Just to add that, there is also a CMake variable CMAKE_Fortran_COMPILER to pick up GNU FORTRAN rather than clang FORTRAN. But it seems to be missing in the documentation

cmake -DCMAKE_Fortran_COMPILER=/usr/.../bin/gfortran-6.x.0

it should be enough to use CC and CXX environment variables, but in macOS are not "working as expected".

but it works in this way instead eg:

CXX="gcc-8" CC="gcc-8" cmake ..