How Does Cmake Find A Package?

CMake uses a mechanism called "Find Modules" to search for packages on the system. These Find Modules are scripts written in the CMake language that instruct CMake on how to locate and configure a package. When CMake attempts to find a package, it will first look in its own collection of built-in Find Modules. If it does not find a suitable module, it will then search through the CMAKE_MODULE_PATH variable, which contains a list of additional directories where Find Modules may be located. CMake will execute the appropriate Find Module script for the package, which will typically check for the presence of necessary headers, libraries, and other components required for the package. If the package is found, CMake will set variables containing information on how to include and use the package in the build system.

How to tell CMake where to find a package?

There are several ways to tell CMake where to find a package:

1. Use the CMake command find_package(PackageName) to search for the package in the default locations on your system.
2. Set the CMake variable Package_NAME_DIR to the directory containing the package's configuration file (e.g. PackageConfig.cmake).
3. Use the CMake command find_package(PackageName PATHS path/to/package) to specify a specific directory where CMake should look for the package.
4. Set the CMake variable CMAKE_PREFIX_PATH to include the directory where the package is installed.
5. Use the CMake command include_directories(path/to/include) to specify additional include directories where CMake should look for header files related to the package.
6. Use the CMake command link_directories(path/to/libs) to specify additional library directories where CMake should look for libraries related to the package.

By using these methods, you can ensure that CMake can find the package you need for your project.

How does CMake know where to look for packages?

CMake uses find_package() commands in CMakeLists.txt files to search for packages. When a find_package() command is used, CMake looks in specific locations for the package. These locations can include system directories, environment variables, and directories specified by the CMAKE_PREFIX_PATH variable. Additionally, CMake provides built-in support for finding common packages such as Boost, Eigen, and Qt.

CMake also supports the use of package configuration files, which provide information about the package's location and dependencies. Package configuration files contain variables and commands that CMake uses to locate the package and its components. By using find_package() with a package configuration file, CMake can automatically find and configure the package for the project.

How does CMake find a package in a project?

CMake uses the find_package command to search for an installed package on the system. When find_package is called, CMake looks for a file called Find<Package>.cmake in specified directories (e.g., in the CMake module directory or in the directory where the project's CMakeLists.txt file is located).

If the file is found, it is executed, and this file typically sets variables that describe the location of the package's include directories, libraries, and other necessary information. These variables can then be used in the CMakeLists.txt file to configure the project to use the package.

If the Find<Package>.cmake file is not found, CMake will try to locate the package using other methods, such as searching system paths or environment variables. If the package cannot be found, CMake will generate an error.

What is the default behavior of CMake when searching for packages?

By default, CMake searches for packages in the following directories:

1. The CMake module directory (${CMAKE_ROOT}/Modules)
2. CMake's internal package registry
3. The PATH environment variable
4. The CMAKE_PREFIX_PATH variable
5. The CMAKE_MODULE_PATH variable

If a package is not found in any of these locations, an error will be thrown.