I have written a library that contains a statically allocated buffer (f.g. struct taskList proc[TBL_SIZE];)

TBL_SIZE is a macro currently defined in my .h file.

I have heard that there's a way to set or reset the define statement inside the .ino file to customize this setting for some libraries, but I don't know how to do this myself, and by the time I realized that this was a problem I had forgotten what library allowed this setting.

This would be extremely useful as currently I have to edit my library every time I build a new project (or worse, am working on several at once with different settings).

My guess was a variant on an include guard, like this

#ifndef TBL_SIZE
#define TBL_SIZE 16 /*(or whatever)*/

block, but that failed to work. I tried it during a final project, so my memory is fuzzy from lack of sleep (and time, now), but I'm fairly sure I got an "undefined" error.

I don't know what this is called to search for it otherwise, so I'm hoping someone can help.

Here is more information about my specific issue:

In my .h (at the top, after the regular include guard):

#ifndef TBL_SIZE
#define TBL_SIZE 16

(Note that everything works if I just set this directly without the defines)

Further down the .h, inside my class:

struct task processes[TBL_SIZE];

In the main .ino file before the library includes:

#define TBL_SIZE 6
#include <CoRTOS.h>

Debug output of the build (I know from experience it built with 16 instead of 6 entries in that table from the memory size. The low memory warning is another big clue.)

Linking everything together...
"C:\\Users\\rdrag\\AppData\\Local\\Arduino15\\packages\\arduino\\tools\\avr-gcc\\7.3.0-atmel3.6.1-arduino7/bin/avr-gcc" -w -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections -mmcu=atmega328p -o "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761/ECE554_ASP.ino.elf" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\sketch\\ECE554_ASP.ino.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\CoRTOS\\CoRTOS.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_GFX_Library\\glcdfont.c.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_GFX_Library\\Adafruit_GFX.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_GFX_Library\\Adafruit_GrayOLED.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_GFX_Library\\Adafruit_SPITFT.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_BusIO\\Adafruit_BusIO_Register.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_BusIO\\Adafruit_I2CDevice.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_BusIO\\Adafruit_SPIDevice.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Wire\\Wire.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Wire\\utility\\twi.c.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\Adafruit_SSD1306\\Adafruit_SSD1306.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\SPI\\SPI.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761\\libraries\\SparkFun_BME280_Arduino_Library-master\\SparkFunBME280.cpp.o" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761/core\\core.a" "-LC:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761" -lm
C:\Users\rdrag\Documents\Arduino\libraries\CoRTOS\src/CoRTOS.h:38:7: note: array types have different bounds
 class cortos_class {
"C:\\Users\\rdrag\\AppData\\Local\\Arduino15\\packages\\arduino\\tools\\avr-gcc\\7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy" -O ihex -j .eeprom --set-section-flags=.eeprom=alloc,load --no-change-warnings --change-section-lma .eeprom=0 "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761/ECE554_ASP.ino.elf" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761/ECE554_ASP.ino.eep"
"C:\\Users\\rdrag\\AppData\\Local\\Arduino15\\packages\\arduino\\tools\\avr-gcc\\7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy" -O ihex -R .eeprom "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761/ECE554_ASP.ino.elf" "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761/ECE554_ASP.ino.hex"
Multiple libraries were found for "Adafruit_GFX.h"
 Used: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_GFX_Library
 Not used: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit-GFX-Library-master
Multiple libraries were found for "Adafruit_I2CDevice.h"
 Used: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_BusIO
 Not used: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_BusIO-master
Multiple libraries were found for "Adafruit_SSD1306.h"
 Used: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_SSD1306
 Not used: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_SSD1306-master
Using library CoRTOS at version 0.2.0 in folder: C:\Users\rdrag\Documents\Arduino\libraries\CoRTOS
Using library Adafruit_GFX_Library at version 1.10.7 in folder: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_GFX_Library
Using library Adafruit_BusIO at version 1.7.3 in folder: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_BusIO
Using library Wire at version 1.0 in folder: C:\Users\rdrag\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.3\libraries\Wire
Using library Adafruit_SSD1306 at version 2.4.5 in folder: C:\Users\rdrag\Documents\Arduino\libraries\Adafruit_SSD1306
Using library SPI at version 1.0 in folder: C:\Users\rdrag\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.3\libraries\SPI
Using library SparkFun_BME280_Arduino_Library-master at version 2.0.9 in folder: C:\Users\rdrag\Documents\Arduino\libraries\SparkFun_BME280_Arduino_Library-master
"C:\\Users\\rdrag\\AppData\\Local\\Arduino15\\packages\\arduino\\tools\\avr-gcc\\7.3.0-atmel3.6.1-arduino7/bin/avr-size" -A "C:\\Users\\rdrag\\AppData\\Local\\Temp\\arduino_build_258761/ECE554_ASP.ino.elf"
Sketch uses 28172 bytes (91%) of program storage space. Maximum is 30720 bytes.
Global variables use 1609 bytes (78%) of dynamic memory, leaving 439 bytes for local variables. Maximum is 2048 bytes.
Low memory available, stability problems may occur.
  • Define "failed to work". If the preprocessor sees a #define LISTSIZE before your file is included it would skip over your #define because it's been defined. May 26 at 22:54
  • That was the goal. I'd want the earlier one (from the .ino) to override the one in the file. May 28 at 2:00
  • I've added information. "Fails to work" means that it won't customize the array size. I misremembered the warning/error. Some sort of array mismatch complaint, but there isn't actually anything remotely array-related on that line... May 28 at 2:14
  • 1
    the #define in ino will apply only for .h included in ino, not for the same .h included in other file. the #include directive makes the whole content of .h to be copied into the file with the directive
    – Juraj
    May 28 at 5:12
  • I was under the impression that the #define in the .ino file would override the .h file's default define entry for all items that included it, since it was the first use of it or something. That doesn't work? May 29 at 3:08

I have heard that there's a way to set or reset the define statement inside the .ino file to customize this setting for some libraries, but I don't know how to do this myself

There are many ways to achieve what you want, you might not even need the #ifndef macro, use your library class construct to allow user to pass in the list size as an argument and but set the argument with a default value.


class YourLibrary
  size_t _userSize;
  YourLibrary::YourLibrary(size_t listSize = 16) {
    _userSize = listSize;

User's sketch .ino

#include <YourLibrary.h>

#define LIST_SIZE 32;

YourLibrary instance(LIST_SIZE);  // instance() would set the _listSize to 16

setup() {


loop() {

  • Doesn't that require allocating the item on the heap, though? It's setting the size of an array. May 28 at 2:01
  • 1
    No it does not. Unless you use String object.
    – hcheung
    May 28 at 2:22
  • 1
    BTW, I read your updated post. There is no error as you mentioned regarding #ifndef, so what exactly is the problem and your question?
    – hcheung
    May 28 at 2:32
  • The problem listed in my post is that adding the #define in the root .ino file does not actually have any effect on the array size in the library. RE your answer, how does setting the variable inside the class as in your example create the array I need? I'm really trying not to dynamically allocate this table, so was that what you meant or is there some sort of magic I'm missing here? May 29 at 3:10

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