Controlling Compiler and Linker Flags in CMake

CMake is a powerful, cross-platform build system generator. Often, you need to specify compiler and linker flags to control how your code is built – for example, to enable exception handling, enable debugging features, or specify optimization levels. This tutorial explains how to correctly and effectively manage these flags within your CMake projects.

Understanding Compiler and Linker Flags

• Compiler Flags: These flags are passed to the compiler (like g++, clang++, or MSVC) during the compilation of source code. They instruct the compiler on how to process the code, including optimization levels, warning levels, and language standard settings.
• Linker Flags: These flags are passed to the linker after the compilation stage. They instruct the linker on how to combine the compiled object files and libraries into an executable or library. Common linker flags control library paths, linking order, and optimization.

Setting Flags Globally

While possible, modifying global CMake variables directly is generally discouraged for maintainability and portability. However, for simple cases, you can set flags for all targets in your project.

The variables CMAKE_CXX_FLAGS (for C++), and CMAKE_C_FLAGS (for C) are used to set flags applied to all C++ and C source files, respectively. For linker flags, use CMAKE_EXE_LINKER_FLAGS (for executables) and CMAKE_MODULE_LINKER_FLAGS (for libraries/modules).

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -std=c++17") # Example: Enable warnings and C++17 standard
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fexceptions") # Enable exception handling

Important: Appending to existing variables (using "${CMAKE_CXX_FLAGS} ..." ) is best practice. This avoids overwriting existing flags that may be set by the system or other parts of your project.

Setting Flags Per Target (Recommended Approach)

The most flexible and maintainable approach is to set flags specifically for each target (executable or library) in your project. CMake provides several ways to achieve this.

1. target_compile_options()

This is the preferred method for setting compiler flags for a target. It clearly communicates your intent and allows for precise control.

add_executable(my_program main.cpp)

target_compile_options(my_program PRIVATE -fexceptions -g) # -g adds debugging symbols

The PRIVATE keyword means these flags only apply to the target my_program and are not inherited by other targets that link against it. Use PUBLIC if you want these flags to be propagated to dependent targets. INTERFACE will only propagate flags to dependent targets, without applying them to the target itself.

2. target_link_options()

Use this to set linker flags for a target.

add_executable(my_program main.cpp)

target_link_options(my_program PRIVATE -static) # Link statically

Like target_compile_options(), PRIVATE, PUBLIC, and INTERFACE keywords control propagation.

3. set_property() (Less Common, but Useful)

While less readable than the above methods, set_property() can be used to set properties like COMPILE_FLAGS and LINK_FLAGS directly.

add_executable(my_program main.cpp)
set_property(TARGET my_program APPEND_STRING PROPERTY LINK_FLAGS " -lm") # Link with the math library

Example: Enabling Exception Handling

As illustrated in the initial problem, enabling exception handling often requires the -fexceptions flag. The correct way to do this using the recommended approach is:

add_executable(my_program main.cpp)
target_compile_options(my_program PRIVATE -fexceptions)

This ensures that exception handling is enabled for the my_program target only.

Best Practices

• Per-Target Configuration: Always prefer setting flags per target. This improves project maintainability and reduces the risk of unexpected side effects.
• Use PRIVATE, PUBLIC, and INTERFACE: Carefully consider the visibility of your flags using these keywords.
• Avoid Global Modification: Minimize changes to global CMake variables like CMAKE_CXX_FLAGS.
• Append to Existing Flags: When modifying flags, append to the existing values rather than overwriting them.
• Documentation: Clearly document why you are using specific flags in your CMakeLists.txt file.