How to Specify a Compiler in Cmake?

Question

I would like to use the IAR compiler. I noticed CMake has already have a bunch of files about this compiler:

From what I read the common solution is to specify manually ALL the toolchain in my CMakeLists.txt:

set(CMAKE_C_COMPILER iccarm)
set(CMAKE_CPP_COMPILER iccarm)

How CMake can link these definitions with `Modules/Compiler/IAR.cmake"?

I thought I would just have to do

include("Modules/Compiler/IAR.cmake")

What is the correct way to specify my IAR compiler?

When I do

cmake .

It still tries to use gcc instead of my IAR compiler. Why?

Accepted Answer

To select a specific compiler, you have several solutions, as exaplained in CMake wiki:

Method 1: use environment variables

For C and C++, set the CC and CXX environment variables. This method is not guaranteed to work for all generators. (Specifically, if you are trying to set Xcode's GCC_VERSION, this method confuses Xcode.) For example:

CC=gcc-4.2 CXX=/usr/bin/g++-4.2 cmake -G "Your Generator" path/to/your/source

Method 2: use cmake -D

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

Method 3 (avoid): use set()

Set the appropriate CMAKE_FOO_COMPILER variable(s) to a valid compiler name or full path in a list file using set(). This must be done before any language is set (ie: before any project() or enable_language() command). For example:

set(CMAKE_C_COMPILER "gcc-4.2")
set(CMAKE_CXX_COMPILER "/usr/bin/g++-4.2")

project("YourProjectName")

The wiki doesn't provide reason why 3rd method should be avoided...

Answer 2

I see more and more people who set CMAKE_C_COMPILER and other compiler-related variables in the CMakeLists.txt after the project call and wonder why this approach breaks sometimes.

Must Read

What happens actually

When CMake executes the project() call, it looks for a default compiler executable and determines the way for use it: default compiler flags, default linker flags, compile features, etc.

And CMake stores path to that default compiler executable in the CMAKE_C_COMPILER variable.

When one sets CMAKE_C_COMPILER variable after the project() call, this only changes the compiler executable: default flags, features all remains set for the default compiler.

AS RESULT: When the project is built, a build system calls the project-specified compiler executable but with parameters suitable for the default compiler.

As one could guess, this approach would work only when one replaces a default compiler with a highly compatible one. E.g. replacement of gcc with clang could work sometimes.

This approach will never work for replacement of cl compiler (used in Visual Studio) with gcc one. Nor this will work when replacing a native compiler with a cross-compiler.

How to Specify a Compiler in Cmake?

6 Answers

Must Read

What happens actually

Sarah Jenkins