How to specify new GCC path for CMake

问题:

My OS is centos which has a default gcc in path /usr/bin/gcc. But it is old, I need a new version of gcc. So I install a new version in a new path /usr/local/bin/gcc.

But when I run cmake, it still uses the old version gcc path(/usr/bin/gcc) . How can I specify the gcc to new path(/usr/local/bin/gcc).

I have tried to overwrite /usr/bin/gcc with /usr/local/bin/gcc, but it not work.

回答1:

Do not overwrite CMAKE_C_COMPILER, but export CC (and CXX) before calling cmake:

export CC=/usr/local/bin/gcc
export CXX=/usr/local/bin/g++
cmake /path/to/your/project
make

The export only needs to be done once, the first time you configure the project, then those values will be read from the CMake cache.

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UPDATE: longer explanation on why not overriding CMAKE_C(XX)_COMPILER after Jake's comment

I recommend against overriding the CMAKE_C(XX)_COMPILER value for two main reasons: because it won't play well with CMake's cache and because it breaks compiler checks and tooling detection.

When using the set command, you have three options:

• without cache, to create a normal variable
• with cache, to create a cached variable
• force cache, to always force the cache value when configuring

Let's see what happens for the three possible calls to set:

Without cache

set(CMAKE_C_COMPILER /usr/bin/clang)
set(CMAKE_CXX_COMPILER /usr/bin/clang++)

When doing this, you create a "normal" variable CMAKE_C(XX)_COMPILER that hides the cache variable of the same name. That means your compiler is now hard-coded in your build script and you cannot give it a custom value. This will be a problem if you have multiple build environments with different compilers. You could just update your script each time you want to use a different compiler, but that removes the value of using CMake in the first place.

Ok, then, let's update the cache...

With cache

set(CMAKE_C_COMPILER /usr/bin/clang CACHE PATH "")
set(CMAKE_CXX_COMPILER /usr/bin/clang++ CACHE PATH "")

This version will just "not work". The CMAKE_C(XX)_COMPILER variable is already in the cache, so it won't get updated unless you force it.

Ah... let's use the force, then...

Force cache

set(CMAKE_C_COMPILER /usr/bin/clang CACHE PATH "" FORCE)
set(CMAKE_CXX_COMPILER /usr/bin/clang++ CACHE PATH "" FORCE)

This is almost the same as the "normal" variable version, the only difference is your value will be set in the cache, so users can see it. But any change will be overwritten by the set command.

Breaking compiler checks and tooling

Early in the configuration process, CMake performs checks on the compiler: Does it work? Is it able to produce executables? etc. It also uses the compiler to detect related tools, like ar and ranlib. When you override the compiler value in a script, it's "too late", all checks and detections are already done.

For instance, on my machine with gcc as default compiler, when using the set command to /usr/bin/clang, ar is set to /usr/bin/gcc-ar-7. When using an export before running CMake it is set to /usr/lib/llvm-3.8/bin/llvm-ar.

回答2:

This question is quite old but still turns up on Google Search. The accepted question wasn't working for me anymore and seems to be aged. The latest information about cmake is written in the cmake FAQ.

There are various ways to change the path of your compiler. One way would be

Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path on the command-line using cmake -D. For example:

cmake -G "Your Generator" -D CMAKE_C_COMPILER=gcc-4.2 -D CMAKE_CXX_COMPILER=g++-4.2 path/to/your/source

instead of gcc-4.2 you can write the path/to/your/compiler like this

 cmake -D CMAKE_C_COMPILER=/path/to/gcc/bin/gcc -D CMAKE_CXX_COMPILER=/path/to/gcc/bin/g++ .

回答3:

Set CMAKE_C_COMPILER to your new path.

See here: http://www.cmake.org/Wiki/CMake_Useful_Variables

回答4:

Export should be specific about which version of GCC/G++ to use, because if user had multiple compiler version, it would not compile successfully.

 export CC=path_of_gcc/gcc-version
 export CXX=path_of_g++/g++-version
 cmake  path_of_project_contain_CMakeList.txt
 make 

In case project use C++11 this can be handled by using -std=C++-11 flag in CMakeList.txt

回答5:

An alternative solution is to configure your project through cmake-gui, starting from a clean build directory. Among the options you have available at the beginning, there's the possibility to choose the exact path to the compilers

回答6:

This not only works with cmake, but also with ./configure and make:

./configure CC=/usr/local/bin/gcc CXX=/usr/local/bin/g++

Which is resulting in:

checking for gcc... /usr/local/bin/gcc
checking whether the C compiler works... yes