espressif--esp-idf/examples/build_system/cmakev2/features/import_lib_direct

Supported Targets	ESP32	ESP32-C2	ESP32-C3	ESP32-C5	ESP32-C6	ESP32-C61	ESP32-H2	ESP32-H21	ESP32-H4	ESP32-P4	ESP32-S2	ESP32-S3	ESP32-S31

Import External C Library Directly (cmakev2)

This example demonstrates importing an external C library that is a pure CMake project. It is downloaded at configure time, built by its own CMake, and linked directly to the IDF app without wrapping it as an IDF component.

Overview

In Build System v1, integrating third-party CMake libraries required wrapping them as IDF components (see the import_lib example). With cmakev2, CMake's FetchContent module can be used to fetch and build an external CMake library (lwjson) and link the library to an IDF component using target_link_libraries.

This example uses lwjson, a lightweight JSON parser for embedded systems.

Project Structure

import_lib_direct/
├── CMakeLists.txt          # Initialize the IDF project
├── README.md
└── main/
    ├── CMakeLists.txt      # FetchContent for lwjson, register component, link
    └── main.c              # Uses lwjson API

How It Works

1. The main component's CMakeLists.txt uses FetchContent_Declare() and FetchContent_MakeAvailable() to download and build lwjson at configure time. The fetched sources land in ./build/_deps/.

2. After registering the main omponent with idf_component_register, it links the fetched library:

   target_link_libraries(${COMPONENT_LIB} PUBLIC lwjson)
   

3. main.c uses the lwjson API (lwjson_init, lwjson_parse, lwjson_find, etc.) to parse a sample JSON string and print device name, cores, features, and specs.

How to Use

Build and flash the example:

idf.py set-target <target>
idf.py build
idf.py flash monitor

Expected Output

I (275) import_lib_direct: lwjson library imported directly (downloaded, built, linked) without IDF component wrapper
I (275) import_lib_direct: Parsing JSON string...
I (285) import_lib_direct: Device name: ESP32
I (285) import_lib_direct: Number of cores: 2
I (285) import_lib_direct: Features:
I (295) import_lib_direct:   - WiFi
I (295) import_lib_direct:   - Bluetooth
I (295) import_lib_direct:   - GPIO
I (305) import_lib_direct: Specifications:
I (305) import_lib_direct:   Flash: 4MB
I (305) import_lib_direct:   RAM: 520KB
I (315) import_lib_direct: Example complete!