#40: Redefine IDF Functions

Have you ever wanted to modify a low level ESP-IDF call but didn’t want to create a custom branch of the repository? Maybe you want to add logging to track when certain functions are called, intercept function calls for testing, or completely replace a function’s implementation with your own optimized version.

ESP-IDF’s build system provides a powerful linker feature that lets you wrap or completely redefine any function in the framework. This approach keeps your customizations separate from the core ESP-IDF code, making updates easier and your project more maintainable.

Why Override or Wrap IDF Functions?

There are several practical scenarios where you might need this capability:

Adding Debug Logging: Wrap a function to log parameters and return values without changing the original implementation. This is invaluable for troubleshooting production issues.

Testing and Mocking: Replace hardware-dependent functions with test implementations that work in simulation or on different platforms.

Performance Optimization: Substitute a generic IDF function with a custom implementation optimized for your specific use case.

Behavioral Modification: Extend an existing function to add validation, caching, or other functionality before or after the original implementation runs.

Temporary Workarounds: Apply quick fixes to IDF bugs while waiting for official patches, without maintaining a forked version of ESP-IDF.

The beauty of this approach is that you’re working with the build system, not against it. Your customizations live in your project code, and you can easily enable or disable them as needed.

How Function Wrapping Works

The mechanism relies on the GNU linker’s --wrap feature. When you tell the linker to wrap a function, it performs a clever substitution:

• All calls to function_name get redirected to __wrap_function_name
• The original implementation becomes available as __real_function_name

This gives you complete control: you can call the original implementation (extending behavior) or ignore it entirely (replacing behavior).

Step-by-Step: Wrapping a Function

Let’s walk through wrapping an ESP-IDF function to add custom behavior. We’ll use esp_restart() as an example, adding a log message before the system reboots.

Step 1: Add the Linker Flag

In your project’s top-level CMakeLists.txt, add the linker wrap directive. This should come after the project() command:

cmake_minimum_required(VERSION 3.16)
include($ENV{IDF_PATH}/tools/cmake/project.cmake)
project(my_project)

# Tell the linker to wrap esp_restart
target_link_libraries(${COMPONENT_LIB} INTERFACE "-Wl,--wrap=esp_restart")

The format is always -Wl,--wrap=function_name where function_name is the exact name of the function you want to wrap.

Step 2: Implement Your Wrapper Function

Now create your wrapper implementation. You can define this in your main component or create a customer component. Whichever fits the style of your existing repo. You’ll define a function named __wrap_function_name:

#include "esp_system.h"
#include "esp_log.h"

static const char *TAG = "WRAPPER";

// Declare the original function
void __real_esp_restart(void);

// Implement the wrapper
void __wrap_esp_restart(void)
{
    ESP_LOGW(TAG, "System restart requested - performing cleanup...");
    
    // Add any custom pre-restart logic here
    // For example: save state, close connections, etc.
    
    // Call the original implementation
    __real_esp_restart();
}

Notice two important parts:

1. Declaration: void __real_esp_restart(void); - This declares the original function so you can call it
2. Implementation: void __wrap_esp_restart(void) - Your wrapper that gets called instead

Step 3: Build and Test

That’s it! Build your project normally:

idf.py flash monitor

Now whenever any code calls esp_restart(), your wrapper executes first, logs the message, then calls the original function.

Completely Replacing a Function

If you want to completely replace a function without calling the original, simply omit the call to __real_function_name:

void __wrap_esp_restart(void)
{
    ESP_LOGI(TAG, "Restart blocked for testing");
    // Original function is NOT called
    // Could implement entirely different behavior here
}

Wrapping Multiple Functions

You can wrap as many functions as needed by adding multiple linker flags:

target_link_libraries(${COMPONENT_LIB} INTERFACE 
    "-Wl,--wrap=esp_restart"
    "-Wl,--wrap=esp_deep_sleep_start"
    "-Wl,--wrap=nvs_flash_init"
)

Then implement a wrapper for each one in your source files.

Important Considerations

Function Signature Matching: Your wrapper must have the exact same signature as the original function - same return type, same parameters. Otherwise you’ll get confusing linker errors or runtime crashes.

Header Inclusion: Include the appropriate ESP-IDF headers so the compiler knows the function signatures. This helps catch mistakes early.

Static Functions: You cannot wrap static functions since they’re not visible to the linker. This technique only works with externally visible functions.

Real-World Example: Adding Retry Logic

Here’s a practical example - adding automatic retry logic to nvs_flash_init():

#include "nvs_flash.h"
#include "esp_log.h"

static const char *TAG = "NVS_WRAPPER";

// Original function declaration
esp_err_t __real_nvs_flash_init(void);

// Wrapper with retry logic
esp_err_t __wrap_nvs_flash_init(void)
{
    esp_err_t ret;
    int retries = 3;
    
    while (retries-- > 0) {
        ret = __real_nvs_flash_init();
        
        if (ret == ESP_OK) {
            return ret;
        }
        
        ESP_LOGW(TAG, "NVS init failed (0x%x), retries left: %d", ret, retries);
        
        if (ret == ESP_ERR_NVS_NO_FREE_PAGES || 
            ret == ESP_ERR_NVS_NEW_VERSION_FOUND) {
            ESP_LOGW(TAG, "Erasing NVS and retrying...");
            nvs_flash_erase();
        }
        
        vTaskDelay(pdMS_TO_TICKS(100));
    }
    
    ESP_LOGE(TAG, "NVS init failed after all retries");
    return ret;
}

Add the linker flag:

target_link_libraries(${COMPONENT_LIB} INTERFACE "-Wl,--wrap=nvs_flash_init")

Now your application automatically handles NVS initialization failures with retry logic, without modifying ESP-IDF.

C++ Compatibility

If you’re implementing your wrapper in a C++ file, you need to use extern "C" linkage for both the original function declaration and your wrapper implementation. This ensures the function names don’t get mangled by the C++ compiler:

#include "esp_system.h"
#include "esp_log.h"

static const char *TAG = "WRAPPER";

extern "C" {
    // Declare the original function with C linkage
    void __real_esp_restart(void);
    
    // Implement the wrapper with C linkage
    void __wrap_esp_restart(void)
    {
        ESP_LOGW(TAG, "System restart requested from C++ code");
        
        // You can use C++ features inside the wrapper
        // as long as the function signature is C-compatible
        
        __real_esp_restart();
    }
}

Without extern "C", the C++ compiler will mangle the function names, and the linker won’t be able to properly redirect calls to your wrapper. This is one of the most common issues when using function wrapping in C++ projects.

Remember that while the function signature must be C-compatible, you can still use C++ features inside the wrapper body - classes, templates, RAII, etc. Just keep the function interface as plain C.

Debugging Tips

If your wrapper isn’t being called:

1. Verify the linker flag syntax - Make sure you have the exact function name
2. Check that the flag comes after project() in CMakeLists.txt
3. Do a clean build - Sometimes cached build files cause issues: idf.py fullclean
4. Add a log message at the start of your wrapper to confirm it’s executing
5. For C++ files - Ensure you’re using extern "C" for both declarations

Video Tutorial

Wrapping Up

Function wrapping is a powerful technique that gives you explicit control over ESP-IDF behavior without forking the framework. Whether you’re debugging, testing, or optimizing, this approach keeps your customizations clean and maintainable.

The next time you find yourself wishing you could “just add a quick log statement” to an IDF function, remember: you can! And you can do it the right way, keeping your modifications separate and your ESP-IDF pristine.

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