Beyond CheckCSourceCompiles: Alternative Approaches for C Compilation Checks in CMake

Purpose

• Commonly used during the configuration stage (CMakeLists.txt) to ensure your project's C code can compile in the current environment.
• Verifies if a provided C source code snippet can be successfully compiled and linked into an executable.

Location

• You can directly use it within your CMakeLists.txt.
• This macro is part of CMake's core functionality and is not typically located in a separate module file.

Usage

CHECK_C_SOURCE_COMPILES(<code> <variable> [FAIL_REGEX <fail_regex>])

• [FAIL_REGEX <fail_regex>]: (Optional) A regular expression to match against the compiler output to consider the compilation a failure even if the code compiles. This can be useful for catching specific error messages.
• <variable>: A variable to store the result (TRUE if compilation succeeds, FALSE otherwise).
• <code>: The C source code to be compiled. It must define a main function.**

Example

set(C_CODE "
int main() {
  printf(\"Hello, world!\\n\");
  return 0;
}
")

CHECK_C_SOURCE_COMPILES("${C_CODE}" HAS_C_COMPILER)

if(HAS_C_COMPILER)
  message(STATUS "C compiler found and code compiles successfully.")
else()
  message(STATUS "C compiler not found or code compilation failed.")
endif()

Behavior

1. CMake attempts to compile the provided <code> using the available C compiler.
2. If compilation succeeds, the HAS_C_COMPILER variable is set to TRUE.
3. If compilation fails or the optional FAIL_REGEX is matched in the output, HAS_C_COMPILER is set to FALSE.

Customization (Optional)

• You can influence the compilation process by setting variables before calling CHECK_C_SOURCE_COMPILES:
  • CMAKE_REQUIRED_FLAGS: Additional compiler flags (e.g., -Wall -Wextra).
  • CMAKE_REQUIRED_DEFINITIONS: Preprocessor macros to define (e.g., -DFOO=bar).
  • CMAKE_REQUIRED_INCLUDES: Include directories to search for header files.
  • CMAKE_REQUIRED_LIBRARIES: Libraries to link against.
  • CMAKE_REQUIRED_QUIET: Suppress compiler output (useful for internal checks).

• While CheckCSourceCompiles is a convenient way to verify basic C compilation capabilities, for more complex checks or building actual executables, consider using other CMake features like add_executable and target_compile_definitions.

---

Checking for C++11 Support

This example checks if the C++ compiler supports the C++11 standard:

set(C_CODE "
#include <iostream>

int main() {
  std::cout << "Hello, C++11!" << std::endl;
  return 0;
}
")

CHECK_C_SOURCE_COMPILES("${C_CODE}" HAS_CXX11_COMPILER
  FAIL_REGEX "error:.*C++11.*")

if(HAS_CXX11_COMPILER)
  message(STATUS "C++ compiler supports C++11.")
else()
  message(STATUS "C++ compiler does not support C++11. You might need to adjust your code or use a different compiler.")
endif()

Checking for a Specific Library

This example checks if the pthread library (commonly used for threading) is available:

set(C_CODE "
#include <pthread.h>

int main() {
  pthread_t thread;
  pthread_create(&thread, NULL, NULL, NULL);
  pthread_join(thread, NULL);
  return 0;
}
")

CHECK_C_SOURCE_COMPILES("${C_CODE}" HAS_PTHREAD_LIB FAIL_REGEX "fatal error: pthread.h: No such file or directory")

if(HAS_PTHREAD_LIB)
  message(STATUS "pthread library found and can be linked.")
else()
  message(STATUS "pthread library not found. Your code might not work as expected on this system.")
endif()

Using CMAKE_REQUIRED_FLAGS

This example shows how to use CMAKE_REQUIRED_FLAGS to pass additional compilation flags:

set(C_CODE "
int main() {
  printf(\"Hello, world!\\n\");
  return 0;
}
")

set(CMAKE_REQUIRED_FLAGS "-Wall -Wextra")  # Enable warnings

CHECK_C_SOURCE_COMPILES("${C_CODE}" HAS_C_COMPILER_WITH_WARNINGS)

unset(CMAKE_REQUIRED_FLAGS)  # Reset the flag

if(HAS_C_COMPILER_WITH_WARNINGS)
  message(STATUS "C compiler found and code compiles with warnings enabled.")
else()
  message(STATUS "C compiler not found or code compilation failed with warnings.")
endif()

---

try_compile() Macro

• Provides finer-grained control over the build process and can be used for more complex checks.
• Allows you to specify the source code, compiler flags, include directories, libraries, and target type (executable or library).
• Offers more control over the compilation and linking process.

Example

set(C_CODE "
int main() {
  printf(\"Hello, world!\\n\");
  return 0;
}
")

try_compile(HAS_C_COMPILER
  SOURCE_FILES "${C_CODE}"
  COMPILER_FLAGS "-Wall -Wextra")

if(HAS_C_COMPILER)
  message(STATUS "C compiler found and code compiles successfully.")
else()
  message(STATUS "C compiler not found or code compilation failed.")
endif()

Creating a Simple Executable

• Allows you to directly test functionality beyond simple compilation.
• Ideal for situations where you need to build and test a small piece of code that interacts with system libraries or headers.

Example

add_executable(test_c_code main.c)
target_link_libraries(test_c_code pthread)  # Example for linking with a library

if(TARGET_TEST_C_CODE)
  message(STATUS "C executable built successfully.")
else()
  message(STATUS "Failed to build C executable.")
endif()

Custom CMake Modules

• This approach requires more advanced CMake knowledge and development effort.
• You can create a module with custom functions tailored to your specific needs.
• Useful for complex checks requiring specialized logic.

• Consider custom modules for highly specialized checks that existing commands can't handle.
• Go with a simple executable for testing functionality beyond basic compilation.
• Use try_compile() when you need more granular control over the build process or want to test code interaction with libraries and headers.
• Use CheckCSourceCompiles for simple checks to verify the presence of a C compiler and basic compilation capabilities.