CMake: Why target_include_directories() Fails with Source Directories
CMake is a powerful tool for managing build processes, but it can sometimes throw confusing errors. One such issue arises when you try to use target_include_directories() to include the source directory itself or one of its subdirectories.
Scenario: Let's imagine you have a project structured like this:
project/
|- src/
| |- main.cpp
| |- utils/
| |- helper.h
| |- helper.cpp
|- CMakeLists.txt
You might be tempted to write your CMakeLists.txt like this:
cmake_minimum_required(VERSION 3.10)
project(MyProject)
add_executable(my_executable src/main.cpp)
# This is where the error occurs:
target_include_directories(my_executable PUBLIC src)
# Additional commands for build options, etc.
However, running CMake will result in an error message like this:
CMake Error at CMakeLists.txt:7 (target_include_directories):
Cannot specify directory "src" as an include directory:
"src" is the source directory itself.
Why does this happen?
CMake, by design, aims to keep source directories separate from build directories. The target_include_directories() command is intended to inform the compiler about directories containing header files that your source code depends on. These headers should typically reside in separate locations dedicated to holding headers (often called "include" directories).
What's the solution?
The best practice is to create a dedicated include directory within your project and move the necessary headers there. For our example:
project/
|- src/
| |- main.cpp
| |- utils/
| |- helper.cpp
|- include/
| |- utils/
| |- helper.h
|- CMakeLists.txt
You would then modify your CMakeLists.txt to include this directory:
cmake_minimum_required(VERSION 3.10)
project(MyProject)
add_executable(my_executable src/main.cpp)
target_include_directories(my_executable PUBLIC include)
# Additional commands for build options, etc.
Benefits of separating headers:
- Improved organization: Having a dedicated
includedirectory helps keep your project structure clean and organized. - Reduced build times: By separating headers from source files, the compiler can quickly find the needed headers, leading to faster build times.
- Better compatibility: This approach is generally more compatible with standard project structures and build systems.
Additional considerations:
- If you have a very specific need to include files from a source subdirectory, you can use the
{{content}}lt;BUILD_INTERFACE>generator expression withintarget_include_directories()to specify a build-time directory where the header files will be copied. This should only be used as a last resort, as it can lead to more complex build setups.
By following these best practices, you can avoid the frustrating errors associated with including source directories and ensure a smooth and efficient build process with CMake.
