08. find_package() Works in CMake

How find_package() Works in CMake (Step-by-Step)

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Prerequisites

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1. Step-by-Step Guide

This post explains what actually happens when you call find_package() in CMake, including how .cmake files are located and executed.

2. Executable

2-1. Starting Point

find_package(OpenCV REQUIRED)

This single line triggers a multi-step process inside CMake.

2-2. Determine Search Mode

CMake tries two modes:

1) Config Mode (Preferred)

Search for:

OpenCVConfig.cmake
opencv-config.cmake

2) Module Mode (Fallback)

Search for:

FindOpenCV.cmake

Modern libraries (like OpenCV, fmt) use Config Mode

2-3. Search Paths

CMake searches in the following order:

1. OpenCV_DIR (if manually set)
2. CMAKE_PREFIX_PATH
3. System install locations

Example:

set(OpenCV_DIR "C:/opencv/install/lib/cmake/opencv4")

2-4. Load the Config File

Once found:

include(OpenCVConfig.cmake)

This is the most important step. find_package() is essentially:

"find + include"

2-5. Config File Execution

The .cmake file is executed as code, not just read.

Inside OpenCVConfig.cmake:

1) Environment Detection

set(OpenCV_ARCH x64)
set(OpenCV_RUNTIME vc17)

Detect platform, compiler, architecture

2) Locate Actual Library Path

check_one_config(OpenCV_LIB_PATH)

Finds correct directory like:

x64/vc17/lib

3 Include Secondary Config

include("${OpenCV_LIB_PATH}/OpenCVConfig.cmake")

Delegates to the real config file

2-6. Define Variables

Inside the final config file:

set(OpenCV_INCLUDE_DIRS ...)
set(OpenCV_LIBS ...)
set(OpenCV_VERSION ...)

These become available in your project

2-7. Create Imported Targets

Modern CMake defines targets:

add_library(OpenCV::OpenCV IMPORTED)

And sets properties:

INTERFACE_INCLUDE_DIRECTORIES
IMPORTED_LOCATION

2-8. Dependency Propagation

target_link_libraries(OpenCV::OpenCV INTERFACE opencv_core opencv_imgproc)

👉 Dependencies are automatically passed to your target

2-9. Your Code Uses It

target_link_libraries(myapp OpenCV::OpenCV)

• adds include directories
• links libraries
• propagates dependencies

3. Full Flow Summary

find_package(OpenCV)
        ↓
search OpenCVConfig.cmake
        ↓
include(OpenCVConfig.cmake)
        ↓
detect platform (x64, vc17, etc.)
        ↓
resolve actual library path
        ↓
include secondary config
        ↓
define variables + targets
        ↓
ready to use in your project