Ensuring C Code Compatibility: The C_STANDARD Property in CMake

What it Does

• This property influences the compiler flags used during the compilation process.
• The C_STANDARD property specifies the version of the C language standard that your target's C source code adheres to.

How it Works

• Common C_STANDARD values include C99, C11, C17 (C18), and GNU11 (for enabling GNU C extensions).
• When you set the C_STANDARD property for a target, CMake instructs the compiler to enforce the language features and limitations of the chosen standard.

Compiler Support

• The effectiveness of C_STANDARD depends on your compiler's capabilities.
  • Modern compilers typically recognize and handle various C standards well.
  • Older compilers might not support setting the standard explicitly, in which case this property might have no effect.

Setting the C_STANDARD Property

There are two primary ways to set the C_STANDARD property for a target:

1. • Use the CMAKE_C_STANDARD variable.

   set(CMAKE_C_STANDARD C11)  # Example: Set to C11 standard globally
   

2. For specific targets
   

   • Use the set_target_properties command.

   add_executable(my_program main.c)
   set_target_properties(my_program PROPERTIES C_STANDARD C99)  # Set to C99 for 'my_program' target only
   

Additional Considerations

• If targeting multiple compilers with varying C standard support, you might need to adjust the C_STANDARD property conditionally based on the detected compiler. CMake's CMAKE_CXX_COMPILER and similar variables can help with compiler identification.
• Consult your compiler's documentation for specific supported C standards and the corresponding compiler flags.

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Example 1: Setting C Standard Globally

This example sets the C_STANDARD to C11 for all targets in your project:

cmake_minimum_required(VERSION 3.1)  # Require CMake 3.1 or later (for C_STANDARD support)
project(my_c_project)

set(CMAKE_C_STANDARD C11)  # Set C standard to C11 globally

add_executable(my_program main.c)
add_library(my_library library.c)

# All targets will now use the C11 standard

Example 2: Setting C Standard for a Specific Target

This example sets the C_STANDARD to C99 for the my_legacy_program target only:

cmake_minimum_required(VERSION 3.1)
project(my_c_project)

add_executable(my_program main.c)  # Default C standard (likely C99)

add_executable(my_legacy_program legacy.c)
set_target_properties(my_legacy_program PROPERTIES C_STANDARD C99)  # Explicitly set C99 for this target

Example 3: Conditional C Standard Based on Compiler

This example (more complex) sets the C_STANDARD based on the detected compiler:

cmake_minimum_required(VERSION 3.1)
project(my_c_project)

if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu11")  # Use GNU C11 extensions for GCC
else()
  set(CMAKE_C_STANDARD C11)  # Use C11 standard for other compilers
endif()

add_executable(my_program main.c)

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Using CMAKE_C_FLAGS (or compiler-specific variables)

• This approach works universally but has drawbacks:
  • It's less portable as the flags might vary between compilers.
  • CMake won't handle verification or adjustment of flags based on compiler support.
• You can directly set compiler flags using the CMAKE_C_FLAGS variable.
  • For example: set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11") (sets C11 standard).

Using compiler-specific generators

• This is highly specific to the generator and lacks portability.
• Some CMake generators (e.g., Xcode) might have built-in mechanisms for setting the C standard.

Why C_STANDARD is preferred

• CMake generally recommends using C_STANDARD because it offers several advantages:
  • Portability
    It uses a consistent approach across different compilers.
  • Flexibility
    You can easily adjust the standard for specific targets.
  • Compiler integration
    CMake handles interacting with the compiler to ensure proper flag usage.

• Only consider alternatives if you have a specific reason for needing more granular control over compiler flags, but be aware of the trade-off in terms of portability.
• For most cases, sticking with C_STANDARD provides the best balance of portability and control.