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:

https://github.com/jevinskie/cmake/blob/master/Modules/Compiler/IAR.cmake

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?

Answer 1

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.

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.

What to do

Never set a compiler in CMakeLists.txt.

If you want, e.g., to use clang instead of defaulted gcc, then either:

1. Pass -DCMAKE_C_COMPILER=<compiler> to cmake when configure the project. That way CMake will use this compiler instead of default one and on the project() call it will adjust all flags for the specified compiler.

2. Set CC environment variable (CXX for C++ compiler). CMake checks this variable when selects a default compiler.

3. (Only in rare cases) Set CMAKE_C_COMPILER variable before the project() call. This approach is similar to the first one, but makes the project less flexible.

If the ways above do not work

If on setting CMAKE_C_COMPILER in the command line CMake errors that a compiler cannot "compile a simple project", then something wrong in your environment.. or you specify a compiler incompatible for chosen generator or platform.

Examples:

• Visual Studio generators work with cl compiler but cannot work with gcc.
• A MinGW compiler usually requires MinGW Makefiles generator.

Incompatible generator cannot be fixed in CMakeLists.txt. One need to pass the proper -G option to the cmake executable (or select the proper generator in CMake GUI).

Cross-compiling

Cross-compiling usually requires setting CMAKE_SYSTEM_NAME variable, and this setting should normally be done in the toolchain file. That toolchain file is also responsible for set a compiler.

Setting CMAKE_SYSTEM_NAME in the CMakeLists.txt is almost always an error.

Answer 3

You need to create a toolchain file, and use the CmakeForceCompiler module.

Here is an example toolchain file for bare-metal ARM development with IAR:

include(CMakeForceCompiler)

set(CMAKE_SYSTEM_NAME Generic) # Or name of your OS if you have one
set(CMAKE_SYSTEM_PROCESSOR arm) # Or whatever
set(CMAKE_CROSSCOMPILING 1)

set(CMAKE_C_COMPILER iccarm) # Change the arm suffix if appropriate
set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY) # Required to make the previous line work for a target that requires a custom linker file

The last line is necessary because CMake will try to compile a test program with the compiler to make sure it works and to get some version information from preprocessor defines. Without this line, CMake will use add_executable() for the test program, and you will get the error "The C compiler "XXX" is not able to compile a simple test program." This is because the test program fails to link, as it doesn't have your custom linker file (I'm assuming bare-metal development since this is what IAR is usually used for). This line tells CMake to use add_library() instead, which makes the test succeed without the linker file. Source of this workaround: this CMake mailing list post.

Then, assuming that your toolchain file is named iar-toolchain.cmake, invoke CMake like this:

cmake -DCMAKE_TOOLCHAIN_FILE=iar-toolchain.cmake .

Answer 4

You can call cmake like this:

cmake -DCMAKE_C_COMPILER=iccarm ...

or

cmake -DCMAKE_CXX_COMPILER=...

Answer 5

If you don't want to use your PC's standard compiler, you have to give CMake the path to the compiler. You do this via environment variables, a toolchain file or direct definitions in the CMake command line (see e.g. CMake Error at CMakeLists.txt:30 (project): No CMAKE_C_COMPILER could be found).

Putting the compiler's name/path into your CMakeLists.txt would stop your project from being cross-platform.

CMake does check for the compiler ids by compiling special C/C++ files. So no need to manually include from Module/Compiler or Module/Platform.

This will be automatically done by CMake based on its compiler and platform checks.

References

• CMake: In which Order are Files parsed (Cache, Toolchain, …)?
• CMake GitLab Commit: Add support files for C, C++ and ASM for the IAR toolchain.