If you are seeing this error after exporting your UI from SquareLine Studio and rebuilding your project in ESP-IDF v5.x, you are not alone.

The build suddenly fails with something like:

build/config/sdkconfig.h:939:39: error: expected identifier or '(' before string constant
#define CONFIG_LV_FONT_CUSTOM_DECLARE ""

Sometimes the error trace continues into internal LVGL files such as:

lv_disp.c

The confusing part? You never modified those files.

Why This Error Is So Confusing

The compiler points to:

build/config/sdkconfig.h

Or even deeper inside:

components/lvgl__lvgl/src/core/lv_disp.c

But remember:

Error location does not always mean root cause.

Understanding LVGL Configuration in ESP-IDF v5.x

When LVGL is integrated as a component in ESP-IDF, configuration is handled via Kconfig.

You configure it using:

idf.py menuconfig

The settings are stored in:

sdkconfig

And macros are auto-generated into:

sdkconfig.h

In ESP-IDF integration, Kconfig overrides lv_conf.h.

Root Cause Analysis

This is the problematic macro:

#define CONFIG_LV_FONT_CUSTOM_DECLARE ""

Inside LVGL internal config:

#define LV_FONT_CUSTOM_DECLARE CONFIG_LV_FONT_CUSTOM_DECLARE

After macro expansion, the compiler effectively sees:

#define LV_FONT_CUSTOM_DECLARE ""

But LVGL expects something like:

extern lv_font_t ui_font_MyFont;

A string literal is not a valid C declaration — therefore the compiler throws the error.

Proper Fix (Recommended)

Run:

idf.py menuconfig

Navigate to:

Component config
→ LVGL configuration
→ Custom font declaration

If unused, disable it.

Then clean and rebuild:

idf.py fullclean
idf.py build

Quick Debug Fix (Temporary)

Edit:

sdkconfig

Change:

CONFIG_LV_FONT_CUSTOM_DECLARE=""

To:

# CONFIG_LV_FONT_CUSTOM_DECLARE is not set

Then rebuild.

Key Takeaway

ESP-IDF is not Arduino.

Configuration is controlled by Kconfig.

Understanding how sdkconfig generates macros can save hours of debugging.

In embedded systems, fixing the error is easy. Understanding why it happens is what makes you an engineer.

 

Fix CONFIG_LV_FONT_CUSTOM_DECLARE Error via ESP-IDF PowerShell (Step-by-Step)

If you are seeing this error:

build/config/sdkconfig.h:939:39: error: expected identifier or '(' before string constant
#define CONFIG_LV_FONT_CUSTOM_DECLARE ""

The safest and most reliable way to fix it is through ESP-IDF PowerShell using menuconfig.

Step 1 — Open ESP-IDF PowerShell

Click the Start Menu and open:

ESP-IDF 5.3 PowerShell

Step 2 — Navigate to Project Directory

Use the cd command to open your project folder:

cd C:\Users\arto\Desktop\keyboard_test\keyboard_test

Step 3 — Run menuconfig

Type the following command:

idf.py menuconfig

This will open the ESP-IDF configuration interface.

Step 4 — Enter Component Config

Inside the menu, select:

Component config

Step 5 — Open LVGL Configuration

Scroll down and choose:

LVGL configuration

Step 6 — Go to Font Usage

Select:

Font usage

Step 7 — Disable Custom Font

Scroll down until you find:

Enable the custom font

Make sure it is unchecked.

If this option is enabled, ESP-IDF generates:

CONFIG_LV_FONT_CUSTOM_DECLARE=""

 

disable like this :

Step 8 — Save Configuration

Press:

S

To save the configuration.

Then exit menuconfig.

Step 9 — Clean and Rebuild

Run the following commands:

idf.py fullclean
idf.py build

Why This Works

This error is caused by a Kconfig-generated macro:

#define CONFIG_LV_FONT_CUSTOM_DECLARE ""

By disabling custom font support, ESP-IDF stops generating the invalid macro, and the build system regenerates a valid sdkconfig.h.

Always fix configuration-layer errors at the configuration layer — not inside source files.

 

Read Also : How to add / register Waveshare ESP32-S3 7-inch board into SquareLine Studio as a custom board configuration