# Undefined reference to included function **URL:** \ **Category:** Programming\ **Created:** [May 21, 2025, 11:23pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428 "2025-05-21T23:23:37Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![tucanae47](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/tucanae47/32/1128475_2.png) [@tucanae47](https://forum.arduino.cc/u/tucanae47)\ **Post date:** [May 21, 2025, 11:23pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/1 "2025-05-21T23:23:37Z")
I'm trying to learn about library creation and have gotten stuck on what I suspect is a really simple error. My sketch includes a pair of simple files in their own subdirectory: led/blinkfile.h and led/blinkfile.cpp, but the compiler is complaining about the lone function inside them. Here's my main sketch code: ```arduino #include "led/blinkfile.h" bool ledState = true; void setup() { pinMode(13, OUTPUT); } void loop() { ledState = !ledState; setLed(ledState); delay (200); } ``` My header file ("led/blinkfile.h"): ```arduino #include void setLed(bool); ``` My C++ file ("led/blinkfile.cpp"): ```arduino void setLed(bool lState) { digitalWrite(13, lState); } ``` If I move the contents of "led/blinkfile.cpp" to the end of "led/blinkfile.h", the sketch compiles and functions as intended, but my goal is to learn how to create separate header and source files properly. Any help would be greatly appreciated! ---
**Author:** ![sonofcy](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/sonofcy/32/1283617_2.png) [@sonofcy](https://forum.arduino.cc/u/sonofcy)\ **Post date:** [May 21, 2025, 11:35pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/2 "2025-05-21T23:35:08Z")
Show the verbose error log in code tags. Without seeing that, where is the blinkfile.cpp? What I mean is, is it in the source folder, or one of the library folders? ---
**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\ **Post date:** [May 21, 2025, 11:41pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/3 "2025-05-21T23:41:04Z")
> [@tucanae47](#): > > My sketch includes a pair of simple files _in their own subdirectory_ And therein lies the problem. The Adrduino IDE knows nothing of .cpp files in subdirectories. Move the .h and .cpp files to the sketch directory. Change the include to remove the subdirectory reference. Add `#include ` to your .cpp file (otherwise it knows nothing of the Arduino functions, like `digitalWrite`). Compiles like a charm. ---
**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\ **Post date:** [May 21, 2025, 11:47pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/4 "2025-05-21T23:47:23Z")
i renamed your files - blinkfile.cpp to tuc.cpp - blinkfile.h to tuc.h, and - change the #include your .ino accordingly. with just these changes, `digitalWrite()` is undeclared until i moved the `#include "Arduino.h";` to tuc.cpp. the code compiles and the LED flashes ---
**Author:** ![grb](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/grb/32/1201129_2.png) [@grb](https://forum.arduino.cc/u/grb)\ **Post date:** [May 21, 2025, 11:49pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/5 "2025-05-21T23:49:36Z")
> [@van\_der\_decken](#): > > Move the .h and .cpp files to the sketch directory. Shouldn't a subdirectory structure like `src/led/blinkfile.h` (within the sketch root folder) also work? ---
**Author:** ![embeddedkiddie](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/embeddedkiddie/32/1081520_2.png) [@embeddedkiddie](https://forum.arduino.cc/u/embeddedkiddie)\ **Post date:** [May 22, 2025, 12:53am UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/6 "2025-05-22T00:53:02Z")
[https://docs.arduino.cc/arduino-cli/sketch-specification/#src-subfolder](https://docs.arduino.cc/arduino-cli/sketch-specification/#src-subfolder) : > `src` **subfolder** > > The contents of the `src` subfolder are compiled **recursively**. Unlike the code files in the sketch root folder, these files are not shown as tabs in the IDEs. ---
**Author:** ![tucanae47](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/tucanae47/32/1128475_2.png) [@tucanae47](https://forum.arduino.cc/u/tucanae47)\ **Post date:** [May 22, 2025, 2:20pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/7 "2025-05-22T14:20:46Z")
I neglected to specify that the subdirectory containing my .h and .cpp files is located in my sketch's directory. Here's the error log: ```arduino FQBN: arduino:avr:uno Using board 'uno' from platform in folder: /home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6 Using core 'arduino' from platform in folder: /home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6 Detecting libraries used... /home/bt/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6/variants/standard /home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD/sketch/basic_library_test.ino.cpp -o /dev/null Generating function prototypes... /home/bt/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6/variants/standard /home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD/sketch/basic_library_test.ino.cpp -o /tmp/3941653225/sketch_merged.cpp /home/bt/.arduino15/packages/builtin/tools/ctags/5.8-arduino11/ctags -u --language-force=c++ -f - --c++-kinds=svpf --fields=KSTtzns --line-directives /tmp/3941653225/sketch_merged.cpp Compiling sketch... /home/bt/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -Wall -Wextra -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/home/bt/.arduino15/packages/arduino/hardware/avr/1.8.6/variants/standard /home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD/sketch/basic_library_test.ino.cpp -o /home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD/sketch/basic_library_test.ino.cpp.o Compiling libraries... Compiling core... Using precompiled core: /home/bt/.cache/arduino/cores/arduino_avr_uno_050da7a295295d30c066a7c49d0955b0/core.a Linking everything together... /home/bt/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-gcc -Wall -Wextra -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections -mmcu=atmega328p -o /home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD/basic_library_test.ino.elf /home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD/sketch/basic_library_test.ino.cpp.o /home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD/../../cores/arduino_avr_uno_050da7a295295d30c066a7c49d0955b0/core.a -L/home/bt/.cache/arduino/sketches/C3B25C32EC53E7491701D920D6F16CDD -lm /tmp/ccog6V6j.ltrans0.ltrans.o: In function `loop': /home/bt/Arduino/tests/basic_library_test/basic_library_test.ino:14: undefined reference to `setLed(bool)' collect2: error: ld returned 1 exit status exit status 1 Compilation error: exit status 1 ``` ---
**Author:** ![MicroBahner](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/microbahner/32/238923_2.png) [@MicroBahner](https://forum.arduino.cc/u/MicroBahner)\ **Post date:** [May 22, 2025, 2:31pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/8 "2025-05-22T14:31:03Z")
> [@tucanae47](#): > > I neglected to specify that the subdirectory containing my .h and .cpp files is located in my sketch's directory. The problem is that your blinkfile.cpp does not get compiled. The Arduino IDE does not compile files in arbitrary subdirectories of the sketch directory. All files you want to be compiled must be located in a subdirectory (tree) namend 'src' within your sktech directory. ---
**Author:** ![tucanae47](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/tucanae47/32/1128475_2.png) [@tucanae47](https://forum.arduino.cc/u/tucanae47)\ **Post date:** [May 22, 2025, 2:31pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/9 "2025-05-22T14:31:08Z")
This is what I was trying to achieve, a modular library that it self-contained within the sketch. Thanks to your reply, I just discovered my error: I had the "led" directory right there in the sketch's root directory. I just now added a "src" directory, moved the "led" directory into it, and changed the path in my sketch to "src/led/blinkfile.h" and it works! ---
**Author:** ![tucanae47](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/tucanae47/32/1128475_2.png) [@tucanae47](https://forum.arduino.cc/u/tucanae47)\ **Post date:** [May 22, 2025, 2:33pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/10 "2025-05-22T14:33:06Z")
Exactly! That's what I just learned. I knew I had to be overlooking something simple. 😄 Thank you to everyone who replied! ---
**Author:** ![grb](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/grb/32/1201129_2.png) [@grb](https://forum.arduino.cc/u/grb)\ **Post date:** [May 22, 2025, 2:33pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/11 "2025-05-22T14:33:49Z")
Glad I could help. If your problem has been solved, please mark the corresponding answer as the thread "Solution", by clicking the ✅ below the comment. ---
**Author:** ![alto777](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/alto777/32/317385_2.png) [@alto777](https://forum.arduino.cc/u/alto777)\ **Post date:** [May 22, 2025, 2:38pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/12 "2025-05-22T14:38:42Z")
> [@van\_der\_decken](#): > > Add `#include ` to your .cpp file Or include the header for the cpp in the cpp file, not uncommon. It has the Arduino.h aready. a7 ---
**Author:** ![tucanae47](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/tucanae47/32/1128475_2.png) [@tucanae47](https://forum.arduino.cc/u/tucanae47)\ **Post date:** [May 22, 2025, 2:45pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/13 "2025-05-22T14:45:19Z")
Will do. Everyone's reply provided good information, but the reply from MicroBahner concisely describes exactly what I had to do and why, so I would like to mark that one as the solution. ---
**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\ **Post date:** [November 18, 2025, 2:45pm UTC](https://forum.arduino.cc/t/undefined-reference-to-included-function/1383428/14 "2025-11-18T14:45:55Z")
This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.