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1. Overview of CMake Build Types

In CMake, “build type” refers to a set of predefined settings and options used to control the compilation and linking process. It is essentially a variable <span>CMAKE_BUILD_TYPE</span> that can be explicitly set or determined by CMake based on default rules.

The build type is not just a simple switch, but a collection of various behaviors that control the compiler and linker, including:

• Optimization level: For example, whether to enable compiler optimizations (<span>-O2</span>, <span>-O3</span>, etc.), and to what extent.
• Whether to generate debug symbols (<span>-g</span>) so that debuggers (like GDB) can trace code execution and display variable values.
• Defining or undefining certain preprocessor macros (like <span>NDEBUG</span>), which can affect the behavior of the code (e.g., disabling assertions).
• Other compiler options: For example, whether to enable certain warnings or use specific language standards.
• Linker options: For example, whether to use static or dynamic linking.

In simple terms, the build type can be seen as a “profile” or “preset” for compilation and linking. It configures the build process for different purposes (e.g., development, testing, release) without manually changing a large number of compiler and linker options.

CMake allows for the configuration of different build types, such as: Debug, Release, RelWithDebInfo, and MinSizeRel. These build types define different compilation and linking options, allowing the build process to be optimized for various purposes (e.g., development, debugging, release). When configuring a project, options or properties related to each build type can be set, such as compiler and linker flags, optimization levels, and debug information generation.

CMake uses the configuration variable <span>CMAKE_BUILD_TYPE</span> to control the build system generation, which defaults to empty. If no build type is explicitly specified, CMake will use default compilation options. It is recommended to always explicitly set <span>CMAKE_BUILD_TYPE</span>.

CMake can recognize the following common build types:

Build Type	Main Purpose	Characteristics
<span>Debug</span>	Development and debugging	Disables or uses lower compiler optimizations. Enables the generation of debug symbols (debug information). May include additional debug code or assertions. Build speed is slower, but easier to debug. Compiler options: <span>-g</span>, <span>-O0</span> (disable optimizations).
<span>Release</span>	Final release	Enables compiler optimizations for improved performance. Disables the generation of debug symbols to reduce file size. Build speed is faster, but debugging is more difficult. Compiler options: <span>-O3</span> (highest level of optimization), <span>-DNDEBUG</span> (disable assertions).
<span>RelWithDebInfo</span>	Balances optimization and debugging	Uses a lower optimization level. Enables the generation of debug symbols. Useful when performance optimization is needed, but debugging capability is still required. Compiler options: <span>-O2</span> (moderate optimization), <span>-g</span> (generate debug information)
<span>MinSizeRel</span>	Minimizes release version size	Uses optimizations to reduce code size. Disables the generation of debug symbols. Suitable for resource-constrained environments, such as embedded systems or mobile devices. Compiler options: <span>-Os</span> (optimize for code size), <span>-DNDEBUG</span>

Main Functions of Build Types:

1. Adjust the build process for specific use cases. For example, during development, convenience for debugging is prioritized, while during release, performance and size of the program are more important.
2. CMake build types provide a standardized way to configure the build process without manually managing a large number of compiler and linker options.
3. By selecting the appropriate build type, unnecessary compilation and linking operations can be avoided, thus improving build speed.
4. Build types can affect the behavior of the program by defining or undefining preprocessor macros. For example, in the Release build type, the <span>NDEBUG</span> macro is typically defined, which disables assertion statements, thereby improving program performance.
5. Different build types will affect the generation of debug information. The <span>Debug</span> type generates a lot of debug information, making it easier for programmers to debug; while the <span>Release</span> type usually removes debug information, but the <span>RelWithDebInfo</span> type retains both optimization and debug information, facilitating performance analysis and troubleshooting.

The <span>CMAKE_BUILD_TYPE</span> can be set in several ways:

• Command line argument (recommended): When configuring a CMake project, use the <span>-DCMAKE_BUILD_TYPE=<build type></span> command line argument. For example:

cmake -DCMAKE_BUILD_TYPE=Release ..

• Set in <span>CMakeLists.txt</span>: You can use the <span>set</span> command in the <span>CMakeLists.txt</span> file to set the <span>CMAKE_BUILD_TYPE</span> variable. For example:<span>set(CMAKE_BUILD_TYPE Release)</span>Note: This method is not recommended as it hardcodes the build type into the <span>CMakeLists.txt</span> file, reducing flexibility.

<span>CMAKE_BUILD_TYPE</span> switches the configuration for generating the build system: https://cmake.org/cmake/help/v3.5/variable/CMAKE_BUILD_TYPE.html.

2. Example of Configuring Build Types

This example demonstrates how to set the build type for a CMake project and observe the compiler flags corresponding to different build types. It covers setting a default build type, overriding the default from the command line, and checking the compiler flags for different build types.

First, create a <span>CMakeLists.txt</span> file with the following content:

cmake_minimum_required(VERSION 3.10 FATAL_ERROR)
project(recipe-07 LANGUAGES C CXX)

# Set the default build type (if not already set), this variable is set as a cache variable and can be edited through the cache
if(NOT CMAKE_BUILD_TYPE)
    set(CMAKE_BUILD_TYPE Release CACHE STRING
        "Choose the type of build, options are: Debug Release RelWithDebInfo MinSizeRel."
        FORCE)
endif()

message(STATUS "Build type: ${CMAKE_BUILD_TYPE}")

# Print the C compiler flags for different build types
message(STATUS "C flags, Debug configuration: ${CMAKE_C_FLAGS_DEBUG}")
message(STATUS "C flags, Release configuration: ${CMAKE_C_FLAGS_RELEASE}")
message(STATUS "C flags, Release configuration with Debug info: ${CMAKE_C_FLAGS_RELWITHDEBINFO}")
message(STATUS "C flags, minimal Release configuration: ${CMAKE_C_FLAGS_MINSIZEREL}")

# Print the C++ compiler flags for different build types
message(STATUS "C++ flags, Debug configuration: ${CMAKE_CXX_FLAGS_DEBUG}")
message(STATUS "C++ flags, Release configuration: ${CMAKE_CXX_FLAGS_RELEASE}")
message(STATUS "C++ flags, Release configuration with Debug info: ${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
message(STATUS "C++ flags, minimal Release configuration: ${CMAKE_CXX_FLAGS_MINSIZEREL}")

Create a build directory and run CMake:

mkdir -p build
cd build
cmake ..

The output should be similar to:

-- The C compiler identification is GNU 11.4.0
-- The CXX compiler identification is GNU 11.4.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Build type: Release
-- C flags, Debug configuration: -g
-- C flags, Release configuration: -O3 -DNDEBUG
-- C flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C flags, minimal Release configuration: -Os -DNDEBUG
-- C++ flags, Debug configuration: -g
-- C++ flags, Release configuration: -O3 -DNDEBUG
-- C++ flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C++ flags, minimal Release configuration: -Os -DNDEBUG
-- Configuring done
-- Generating done

Switching Build Types can be done using the <span>-D</span> option to set the <span>CMAKE_BUILD_TYPE</span> variable from the command line:

cmake -D CMAKE_BUILD_TYPE=Debug ..

After running <span>cmake</span> again, the output should be similar to:

-- Build type: Debug
-- C flags, Debug configuration: -g
-- C flags, Release configuration: -O3 -DNDEBUG
-- C flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C flags, minimal Release configuration: -Os -DNDEBUG
-- C++ flags, Debug configuration: -g
-- C++ flags, Release configuration: -O3 -DNDEBUG
-- C++ flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C++ flags, minimal Release configuration: -Os -DNDEBUG
-- Configuring done
-- Generating done

Note: Although the <span>CMAKE_BUILD_TYPE</span> has changed, the displayed C and C++ flags remain unchanged because these variables define the default values corresponding to that build type. Their values can be modified in the toolchain file or overridden in the <span>CMakeLists.txt</span> using the <span>set</span> command.

What Did You Learn from the Example?

• How to use the <span>CMAKE_BUILD_TYPE</span> variable to control the build type of the project.
• Learn how to set a default build type and how to override it from the command line. Flexibly choose to use optimizations or disable them and enable debugging.
• See the compiler flags used for different build types. These flags mainly depend on the selected compiler. By default, CMake sets some common compiler flags based on the build type, such as <span>-g</span> (for debugging) and <span>-O3</span> (for optimization).
• By explicitly printing the flags when running CMake, understand the default combination of the current platform, default compiler, and language.
• You can also use the <span>cmake --system-information</span> command to get detailed information about the current system, including the compiler, toolchain, and default flags. The output of this command is very verbose and should be viewed using <span>less</span> or another text viewer.

3. Knowledge Expansion

For more detailed information about <span>CMAKE_BUILD_TYPE</span>, please refer to the official CMake documentation:https://cmake.org/cmake/help/v3.5/variable/CMAKE_BUILD_TYPE.html.

CMake supports two main types of generators:

• Single-configuration generators: These generators can only generate one build configuration at a time. Common single-configuration generators include: Unix Makefile, MSYS Makefile, Ninja. When using a single-configuration generator, if you need to switch between different build configurations (e.g., from Debug to Release), you need to rerun CMake and specify a new <span>CMAKE_BUILD_TYPE</span> value. This means reconfiguring and regenerating the entire project, which can be time-consuming.
• Multi-configuration generators: These generators can generate multiple build configurations simultaneously. Common dual and multi-configuration generators include: Visual Studio (Visual Studio IDE), Xcode (Xcode IDE). When using a multi-configuration generator, CMake generates a project file that includes all supported build configurations. You can then use the IDE or command line tools to select which configuration to build.

For multi-configuration generators, you can use the <span>CMAKE_CONFIGURATION_TYPES</span> variable to specify the configuration types to generate. This variable accepts a semicolon-separated list of values. For example, to generate Release and Debug configurations, you can set the <span>CMAKE_CONFIGURATION_TYPES</span> variable to <span>"Release;Debug"</span>.

For more information about <span>CMAKE_CONFIGURATION_TYPES</span>, please refer to the official CMake documentation:https://cmake.org/cmake/help/v3.5/variable/CMAKE_CONFIGURATION_TYPES.html.

Here is an example of using CMake to generate Release and Debug build configurations for Visual Studio:

mkdir -p build
cd build
cmake .. -G"Visual Studio 17 2022 Win64" -D CMAKE_CONFIGURATION_TYPES="Release;Debug"

• <span>-G"Visual Studio 17 2022 Win64"</span> specifies using the 64-bit generator for Visual Studio 17 2022.
• <span>-D CMAKE_CONFIGURATION_TYPES="Release;Debug"</span> specifies generating Release and Debug build configurations.

After executing the above commands, CMake will generate a Visual Studio solution file (.sln file). You can open this solution file in Visual Studio and select which configuration to build (Release or Debug).

You can also use the command line to build a specific configuration:

cmake --build . --config Release

This will build the Release configuration. To build the Debug configuration, use <span>--config Debug</span>.

When developing code with single-configuration generators, it is recommended to create separate build directories for Release and Debug builds. This can avoid reconfiguring the entire project when switching build configurations. For example, you can create two directories:<span>build_release</span> and <span>build_debug</span>.

mkdir build_release
mkdir build_debug

cd build_release
cmake .. -D CMAKE_BUILD_TYPE=Release
cd ..

cd build_debug
cmake .. -D CMAKE_BUILD_TYPE=Debug
cd ..

Then, you can build the Release and Debug versions in their respective directories:

cd build_release
make
cd ..

cd build_debug
make
cd ..

This way, you can quickly switch between Release and Debug versions without reconfiguring and recompiling the entire project. This is very useful for developing and debugging large projects.

4. Conclusion

CMake build types are a set of predefined settings used to control the compilation and linking process, configured through the <span>CMAKE_BUILD_TYPE</span> variable. Common build types include Debug, Release, RelWithDebInfo, and MinSizeRel, each suitable for different scenarios such as development debugging, final release, balancing optimization and debugging, and minimizing version size. By selecting the appropriate build type, you can optimize compilation options, debug information generation, preprocessor macro definitions, and more, thereby improving build speed, optimizing program performance, and facilitating troubleshooting.<span>CMAKE_BUILD_TYPE</span> can be set via the command line or in <span>CMakeLists.txt</span>, with the command line method recommended. CMake supports single-configuration and multi-configuration generators, with multi-configuration generators allowing for simultaneous generation of multiple configurations, while single-configuration generators are advised to create separate directories for different build types.

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