Understanding CheckLanguage in CMake: Ensuring Language Compatibility

Purpose

This is crucial for projects that might require support for multiple languages like C, C++, Fortran, CUDA, etc.
The CheckLanguage module in CMake helps you determine if a specific programming language compiler is available on your system.

Usage

The module is used with the following syntax:

check_language(<language>)

Replace <language> with the actual language you want to check for (e.g., C, CXX, Fortran, CUDA).

Functionality

CMake performs the following steps when you call check_language:
1. Cache Check
  It first checks if a variable named CMAKE_<language>_COMPILER is already defined in the CMake cache. This variable is typically set by a previous check_language call or by explicitly setting it.
2. If Cached
  If the variable is found, the check is skipped, assuming the cached value is still valid.
3. If Not Cached
  If the variable is not found:
  - CMake creates a temporary test project.
  - It attempts to compile a simple source code snippet using the compiler for the specified language.
  - If compilation succeeds, the name of the compiler found is stored in the CMAKE_<language>_COMPILER variable.
  - If compilation fails or the compiler is not found, the variable is set to NOTFOUND.

Output

After the check is complete, you can access the result using the CMAKE_<language>_COMPILER variable.
- If the compiler was found: The variable will hold the path to the compiler.
- If the compiler was not found: The variable will be set to NOTFOUND.

Example

check_language(Fortran)

if(CMAKE_Fortran_COMPILER)
  message(STATUS "Fortran compiler found: ${CMAKE_Fortran_COMPILER}")
  enable_language(Fortran)  # Enable Fortran support (if necessary)
else()
  message(STATUS "No Fortran compiler found")
endif()

This example checks for a Fortran compiler. If found, it enables Fortran support in your project and prints a message. Otherwise, it prints a message indicating the absence of a Fortran compiler.

This module helps you write portable CMake code that can adapt to different development environments.
For languages like CUDA that might require a separate host compiler, the CMAKE_<language>_HOST_COMPILER variable might also be set if a host compiler is needed.
The CheckLanguage module is usually included by default in CMake, so you don't need to explicitly load it.

Checking for C++11 Support

check_language(CXX)

if(CMAKE_CXX_COMPILER)
  # Check for C++11 support using compiler flags
  try_compile(CXX11_FLAG
               SOURCE "${CMAKE_CURRENT_SOURCE_DIR}/TestC++11.cpp"
               OUTPUT_VARIABLE CXX11_FLAG
               FLAGS "-std=c++11")

  if(CXX11_FLAG)
    message(STATUS "C++11 support detected")
    set(CMAKE_CXX_STANDARD 11)  # Enable C++11 standard
  else()
    message(STATUS "C++11 support not detected")
  endif()
else()
  message(STATUS "No C++ compiler found")
endif()

This example first checks for a C++ compiler using check_language(CXX). Then, it tries to compile a test source file (TestC++11.cpp) with the -std=c++11 flag. If compilation succeeds, it indicates C++11 support, and the code sets the CMAKE_CXX_STANDARD variable to 11.

Handling Multiple Language Support (C and CUDA)

# Check for C compiler
check_language(C)

if(CMAKE_C_COMPILER)
  enable_language(C)
  message(STATUS "C compiler found: ${CMAKE_C_COMPILER}")
endif()

# Check for CUDA compiler (optional)
check_language(CUDA)

if(CMAKE_CUDA_COMPILER)
  enable_language(CUDA)
  message(STATUS "CUDA compiler found: ${CMAKE_CUDA_COMPILER}")
else()
  message(STATUS "No CUDA compiler found (CUDA support disabled)")
endif()

This example demonstrates checking for both C and CUDA compilers. It enables C support if found and optionally checks for CUDA. If CUDA is not found, a message is printed, and CUDA support might be disabled in your project.

Using find_package with CheckLanguage

check_language(Python)

if(CMAKE_Python_COMPILER)
  find_package(PythonLibs REQUIRED)  # Look for Python libraries
  if(NOT PythonLibs_FOUND)
    message(FATAL_ERROR "Python libraries not found")
  endif()
else()
  message(STATUS "No Python compiler found. Python support disabled")
endif()

This example checks for a Python compiler and then uses find_package to locate the necessary Python libraries. It throws a fatal error if Python libraries are not found. This is useful when your project relies on external libraries that require specific languages.

You can directly attempt to compile a simple source file using the compiler command from the CMakeLists.txt file.
This method offers more fine-grained control over compiler flags and arguments.
However, it can be less portable and require more manual work on different systems.

set(CMAKE_C_COMPILER_FLAGS "-Wall")  # Set desired compiler flags
try_compile(COMPILER_FOUND
            SOURCE "${CMAKE_CURRENT_SOURCE_DIR}/TestC.c"
            OUTPUT_VARIABLE OUTFILE
            COMMAND ${CMAKE_C_COMPILER} ${CMAKE_C_COMPILER_FLAGS} TestC.c)

if(COMPILER_FOUND)
  message(STATUS "C compiler found")
else()
  message(STATUS "No C compiler found")
endif()

External Scripts
- You could use external scripts (e.g., shell scripts) that check for specific compilers and set CMake variables accordingly.
- This offers flexibility but can increase complexity and introduce external dependencies.
- Ensure proper integration and error handling when using external scripts.
CMake Find Modules (Limited Applicability)
- CMake offers find_package modules for specific libraries like Python, Boost, etc.
- These modules often include checks for the underlying language compilers along with library discovery.
- This might be suitable if your project depends on specific libraries, but it doesn't provide a general-purpose language check.

Choosing the Right Approach

External scripts and find modules should be considered cautiously due to their potential drawbacks.
If you need more control over compiler flags or have specific requirements, manual invocation might be an option.
For most cases, CheckLanguage is the recommended approach due to its simplicity and built-in nature.