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How to Build And Use an External Library With Cmake?

5 minutes read

To build and use an external library with cmake, you first need to download the source code of the library you want to use and place it in a directory within your project folder.


Next, create a CMakeLists.txt file in the same directory as the library source code. In this file, you will need to define the build rules for the library using CMake commands like add_library(), target_include_directories(), and target_link_libraries().


After setting up the CMakeLists.txt file for the library, include it in the CMakeLists.txt file at the root of your project using the add_subdirectory() command. This will tell cmake to build the library when you build your project.


In your project's CMakeLists.txt file, you will also need to link the library using the target_link_libraries() command. This will ensure that your project can access the functions and classes provided by the external library.


Finally, configure and generate your build system using cmake, then build your project. CMake will handle building the external library along with your project, allowing you to use the functionality provided by the external library in your code.


What is the CMake find_package command?

The find_package command in CMake is used to locate and load external libraries or packages that are required for a CMake project. It searches for configuration files provided by the packages and sets up variables accordingly to link against the package in the project. This command is typically used to find and include packages such as Boost, OpenCV, Qt, and many others.


How to create a CMake library target?

To create a CMake library target, follow these steps:

  1. Create a new CMakeLists.txt file in the root directory of your project, if you don't already have one.
  2. Add the following code to the CMakeLists.txt file to define a library target:
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# Specify the sources for the library
set(SOURCES 
    source1.cpp
    source2.cpp
    ...
)

# Create the library target
add_library(my_library ${SOURCES})


  1. Replace source1.cpp, source2.cpp, etc. with the actual source files of your library.
  2. Customize the name of the library target by replacing my_library with a suitable name for your project.
  3. Optionally, you can specify additional properties for the library target, such as include directories or compile flags, by using the target_include_directories and target_compile_options commands.
  4. Save the CMakeLists.txt file and run CMake to generate the build files for your project. You should now have a library target that can be linked against other targets in your project.


What is CMakeLists.txt?

CMakeLists.txt is a file used by the CMake build system to define the structure of a project and how it should be built. This file contains instructions on how to compile, link, and install the project's source code, as well as any additional dependencies or configurations that are needed. It is typically located in the root directory of a project and is used to generate the build files for the project using CMake.


What is the CMake option() command used for?

The option() command in CMake is used to define an option that can be used to control the behavior of the build system. It allows developers to specify whether a certain feature or behavior should be enabled or disabled during the build process. The option() command takes three arguments: the name of the option, a short description of the option, and an initial value (ON or OFF). Developers can then use the value of the option to conditionally include or exclude certain parts of the build configuration.


How to add external sources to a CMake project?

To add external sources to a CMake project, you can use the add_subdirectory() function in your CMakeLists.txt file.


Here's an example of how you can add an external source directory to your CMake project:

  1. Create a directory for the external source code (e.g., external_library).
  2. Place the external source code in the external_library directory.
  3. In your main CMakeLists.txt file, use the add_subdirectory() function to include the external source code:
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add_subdirectory(external_library)


  1. Create a CMakeLists.txt file in the external_library directory to build the external source code. This file will typically contain the necessary configuration and build commands for the external source:
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cmake_minimum_required(VERSION 3.10)

# Add source files to the project
add_library(external_library
  source_file1.cpp
  source_file2.cpp
)

# Include directories for external sources
target_include_directories(external_library PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)


  1. Make sure to link the external library to your main project if needed:
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target_link_libraries(your_project external_library)


By following these steps, you can successfully add external sources to your CMake project.


How to use the CMake find_package command to locate a library?

To use the find_package command in CMake to locate a library, you need to follow these steps:

  1. Add the find_package command in your CMakeLists.txt file. For example, if you want to find the Boost library, you can add the following line:
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find_package(Boost REQUIRED)


  1. Specify the package name as the first argument of the find_package command. You can also specify additional components that you want to find by providing them as subsequent arguments. For example, if you want to find the headers and filesystem component of the Boost library, you can specify it like this:
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find_package(Boost REQUIRED COMPONENTS headers filesystem)


  1. The REQUIRED keyword indicates that the package is mandatory for building the project. If the package is not found, CMake will generate an error and stop the configuration process.
  2. Once the package is found, CMake will set the variables that you can use in your CMakeLists.txt file to link the library and include its headers. For example, you can use the Boost_INCLUDE_DIRS variable to include the Boost headers in your project: 
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include_directories(${Boost_INCLUDE_DIRS})


  1. You can also use the Boost_LIBRARIES variable to link against the Boost library:
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target_link_libraries(your_target_name ${Boost_LIBRARIES})


By following these steps, you can use the find_package command in CMake to locate and use a library in your project.

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