# CMake Finding Dependencies

**`find_package(PackageName)` searches for an installed package and creates targets to link against**. CMake searches standard paths, `CMAKE_PREFIX_PATH`, and system paths for package metadata (typically `.cmake` files).

Basic usage:

```cmake
find_package(Eigen3 REQUIRED)

add_executable(myapp main.cpp)
target_link_libraries(myapp PRIVATE Eigen3::Eigen3)
```

This finds Eigen3 and creates the target `Eigen3::Eigen3`. If `REQUIRED` is specified, CMake fails if the package isn't found. Without it, the search is optional:

```cmake
find_package(Boost)
if(Boost_FOUND)
  target_link_libraries(myapp PRIVATE Boost::system)
else()
  message(STATUS "Boost not found, some features disabled")
endif()
```

The package sets variables like `<PackageName>_FOUND`, `<PackageName>_VERSION`, `<PackageName>_INCLUDE_DIRS`. Modern packages prefer target-based exports (cleaner, handles dependencies automatically).

Finding by version:

```cmake
find_package(OpenGL 4.6 REQUIRED)
```

Specifying search paths:

```cmake
find_package(MyLib HINTS /opt/mylib/lib/cmake)
find_package(MyLib PATHS /home/user/lib/cmake)
```

`HINTS` are checked first; `PATHS` are fallbacks.

Module vs. config mode:

```cmake
find_package(OpenGL)              # module mode: finds FindOpenGL.cmake
find_package(MyLib CONFIG)        # config mode: finds MyLibConfig.cmake
find_package(MyLib MODULE)        # module mode: finds FindMyLib.cmake
```

CMake looks in:

- `/usr/share/cmake-X.Y/Modules/` (built-in modules)
- `${CMAKE_PREFIX_PATH}/lib/cmake/` (installed packages)
- `/usr/local/lib/cmake/`
- `/usr/lib/cmake/`

For packages not installed system-wide, set `CMAKE_PREFIX_PATH`:

```bash
cmake -DCMAKE_PREFIX_PATH=/opt/mylibs ..
```

Or add to CMakeLists.txt:

```cmake
list(APPEND CMAKE_PREFIX_PATH "/opt/mylibs")
find_package(MyLib REQUIRED)
```

Use `find_package()` for all external dependencies (libraries, frameworks). Always use target-based linking (e.g., `PackageName::LibName`) rather than raw variables for portability.