Using non-default compilers

[bforsbe]

Originally reported by: Jon Diprose (Bitbucket: well-jon, GitHub: Unknown)

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cmake appears to determinedly ignore both PATH and CC environment variables when deciding what compiler to use. It might be useful to others if you could add a section to your installation documentation that suggests using something like:

cmake -DCMAKE_C_COMPILER=which gcc -DCMAKE_CXX_COMPILER=which g++ ...

to get the behaviour that I was naively expecting.

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• Bitbucket: https://bitbucket.org/tcblab/relion2-beta/issue/13

[bforsbe]

Original comment by Jon Diprose (Bitbucket: well-jon, GitHub: Unknown):

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OK, how do you make this not eat backticks? `? No. `? Oh, it's markdown, so with a backslash. The above should show:

cmake -DCMAKE_C_COMPILER=`which gcc` -DCMAKE_CXX_COMPILER=`which g++` ...

So I actually do:

/apps/well/cmake/2.8.12.2/bin/cmake -DCMAKE_C_COMPILER=`which gcc` -DCMAKE_CXX_COMPILER=`which g++` -DFORCE_OWN_FFTW=ON -DFORCE_OWN_FLTK=ON ../

[bforsbe]

Original comment by Bjoern Forsberg (Bitbucket: bforsbe, GitHub: bforsbe):

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That's strange. I just tried on my system and it appears to work for me;

#!bash

$ gcc --version
gcc (Ubuntu 5.2.1-22ubuntu2) 5.2.1 20151010
Copyright (C) 2015 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.
$ cmake ..
-- The C compiler identification is GNU 5.2.1
-- The CXX compiler identification is GNU 5.2.1
{...}
$ export CC=gcc-4.8 CXX=g++-4.8 OMPI_CC=gcc-4.8 OMPI_CXX=g++-4.8
$ cmake ..
-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler: /usr/bin/gcc-4.8
-- Check for working C compiler: /usr/bin/gcc-4.8 -- works

What system are you on?

[bforsbe]

Original comment by Stefan Fleischmann (Bitbucket: sfle, GitHub: Unknown):

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Hi Jon,

regarding "eating backticks", just put command lines and program output in between code tags:

$ cmake -DCMAKE_C_COMPILER=`which gcc` -DCMAKE_CXX_COMPILER=`which g++`

What is the output of "which gcc" on your system? I'm guessing it is /usr/bin/gcc which is in turn a link to something like gcc-5 or gcc-4.8.
In that case specifying the real name of the binary works just fine:

$ CC=gcc-4.8 CXX=g++-4.8 cmake .. -DGUI=OFF
-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler: /usr/bin/gcc-4.8
-- Check for working C compiler: /usr/bin/gcc-4.8 -- works
...

vs.

$ CC=gcc-5 CXX=g++-5 cmake .. -DGUI=OFF
-- The C compiler identification is GNU 5.3.1
-- The CXX compiler identification is GNU 5.3.1
-- Check for working C compiler: /usr/bin/gcc-5
-- Check for working C compiler: /usr/bin/gcc-5 -- works
...

[bforsbe]

Original comment by Jon Diprose (Bitbucket: well-jon, GitHub: Unknown):

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Understood - it was more that I always forget cmake doesn't do what I expect and adding it to the docs might save others a few minutes googling. As building c++11 code (or, in this case, getting avx instructions out of fftw) requires a non-standard compiler on EL6-based distros, I'm regularly using a non-standard compiler.

For this build I'm using a local build of gcc4.9.3 installed to a non-standard (but visible cluster-wide) location. We use the environment-modules package to set up the correct environment for that build. I also use the scl-provided devtoolset-1.1 (gcc4.7.2, /opt/centos/devtoolset-1.1/root/usr/bin/gcc) and devtoolset-2 (gcc4.8.2, /opt/rh/devtoolset-2/root/usr/bin/gcc) versions. It's just easier for me to set my environment up right outside the build script and use `which gcc` inside the script.

[bforsbe]

Original comment by Jon Diprose (Bitbucket: well-jon, GitHub: Unknown):

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Ah. From cmake's wiki: "Any other way (like writing make CC=gcc-3.3 CXX=g++-3.3) will not work". Every day is an education...